As I decompress from the Tennessee Valley Interstellar Workshop (and review my notes for next week’s report), I have the pleasure of bringing you Andrew LePage’s incisive essay into a key exoplanet question. Are some of the planets now considered potentially habitable actually unlikely to support life? Recent work gives us some hard numbers on just how large and massive a planet can be before it is more likely to be closer to Neptune than the Earth in composition. The transition from rocky to non-rocky planets is particularly important now, when our instruments are just becoming able to detect planets small enough to qualify as habitable. LePage, who writes the excellent Drew ex Machina, remains optimistic about habitable planets in the galaxy, but so far the case for many of those identified as such may be weaker than we had thought. A prolific writer, Drew is also a Senior Project Scientist at Visidyne, Inc., where he specializes in the processing and analysis of remote sensing data.
by Andrew LePage
For much of the modern era, astronomy has benefitted greatly from the efforts of amateur scientists. But while amateur astronomers equipped with telescopes have certainly filled many important niches left by the far less numerous professionals in the field, others interested in astronomy equipped with nothing more than a computer and an Internet connection are capable of making important contributions as well. One project taking advantage of this resource is Planet Hunters.
The Planet Hunters project was originally started four years ago by the Zooinverse citizen science program to enlist the public’s help in searching through the huge photometric database of NASA’s Kepler mission looking for transits caused by extrasolar planets. While automated systems have been able to uncover thousands of candidate planets, they are limited to finding only what their programmers designed them to find – multiple, well defined transits occurring at regular intervals. The much more adaptable human brain is able to spot patterns in the changes in the brightness of stars that a computer program might miss but could still indicate the presence of an extrasolar planet. Currently in Version 2.0, the Planet Hunters project has uncovered 60 planet candidates to date through the efforts of 300,000 volunteers worldwide.
A paper by a team of astronomers with Joseph Schmitt (Yale University) as the lead author was just published in The Astrophysical Journal which describes the latest find by Planet Hunters. The target of interest for this paper is a billion year old, Sun-like star called Kepler 289 located about 2,300 light years away. Automated searches of the Kepler data had earlier found two planets orbiting this distant star: a large super-Earth with a radius 2.2 times that of the Earth (or RE) in a 34.5-day orbit originally designated Kepler 289b (called PH3 b in the new paper) and a gas giant with a radius of 11.6 RE in 125.8-day orbit, Kepler 289c (now also known as PH3 d). The new planet, PH3 c, has a radius of 2.7 RE and a mean orbital period of 66.1 days. With a mean stellar flux about 11 times that of Earth, this planet is highly unlikely to be habitable but its properties have profound implications for assessing the potential habitability of other extrasolar planets.
The planet had been missed by earlier automated searches because its orbital period varies regularly by 10.5 hours over the course of ten orbits due to its strong interactions with the other two planets, especially PH3 d. Because of this strong dynamical interaction, it was possible for Schmitt et al. to use the Transit Timing Variations or TTVs observed in the Kepler data to compute the masses of these three planets much more precisely than could be done using precision radial velocity measurements. The mass of the outer planet, PH3 d, was found to be 132±17 times that of Earth (or ME) or approximately equivalent to that of Saturn. The mass of the inner planet, PH3 b, was poorly constrained with a value of 7.3±6.8 ME. The newest discovery, PH3 c, was found to have a mass of 4.0±0.9 ME which, when combined with the radius determined using Kepler data, yields a mean density of 1.2±0.3 g/cm3 or only about one-fifth that of the Earth. Models indicate that this density is consistent with PH3 c possessing a deep, hot atmosphere of hydrogen and helium making up about half of its radius or around 2% of its total mass.
PH3 c is yet another example of a growing list of known low-density planets with masses just a few times that of the Earth that are obviously not terrestrial or rocky in composition. Before the Kepler mission, such planets were thought to exist but their exact properties were unknown because none are present in our solar system. As a result, the position in parameter space of the transition from rocky to non-rocky planets and the characteristics of this transition were unknown. So when astronomers were developing size-related nomenclature to categorize the planets they expected to find using Kepler, they somewhat arbitrarily defined “super-Earth” to be any planet with a radius in the 1.25 to 2.0 RE range regardless of its actual composition. Planets in the 2.0 to 4.0 RE range were dubbed “Neptune-size”. This has generated some confusion over the term “super-Earth” and has led to claims about the potential habitability of these planets being made in the total absence of an understanding of the true nature of these planets. Now that Kepler has found planets in this size range, astronomers have started to examine the mass-radius relationship of super-Earths.
The first hints about the characteristics of this transition from rocky to non-rocky planets were discussed in a series of papers published earlier this year. Using planetary radii determined from Kepler data and masses found by precision radial velocity measurements and analysis of TTVs, it was found that the density of super-Earths tended to rise with increasing radius as would be expected of rocky planets. But somewhere around the 1.5 to 2.0 RE range, a transition is passed where larger planets tended to become less dense instead. The interpretation of this result is that planets with radii greater than about 1.5 RE are increasingly likely to have substantial envelopes of various volatiles such as water (including high pressure forms of ice at high temperatures) and thick atmospheres rich in hydrogen and helium that decrease a planet’s bulk density. As a result, these planets can no longer be considered terrestrial or rocky planets like the Earth but would be classified as mini-Neptunes or gas dwarfs depending on the exact ratios of rock, water and gas.
Image: It now appears that many of the fanciful artist depictions of super-Earths are wrong and that most of these planets are more like Neptune than the Earth (NASA Ames/JPL-Caltech).
A detailed statistical study of this transition was submitted for publication this past July by Leslie Rogers (a Hubble Fellow at the California Institute of Technology) who is also one of the coauthors of the PH3 c discovery paper. In her study, Rogers confined her analysis to transiting planets with radii less than 4 RE whose masses had been constrained by precision radial velocity measurements. She excluded planets with masses determined by TTV analysis since this sample may be affected by selection biases that favor low-density planets (for a planet of a given mass, a large low-density planet is more likely to produce a detectable transit event than a smaller high-density planet). Rogers then determined the probability that each of the 47 planets in her Kepler-derived sample were rocky planets by comparing the properties of those planets and the associated measurement uncertainties to models of planets with various compositions. Next, she performed a statistical analysis to assess three different models for the mass-radius distribution for the sample of planets. One model assumed an abrupt, step-wise transition from rocky to non-rocky planets while the other two models assumed different types of gradual transitions where some fraction of the population of planets of a given radius were rocky while the balance were non-rocky.
Rogers’ analysis clearly showed that a transition took place between rocky and non-rocky planets at 1.5 RE with a sudden step-wise transition being mildly favored over more gradual ones. Taking into account the uncertainties in her analysis, Rogers found that the transition from rocky to non-rocky planets takes place at no greater than about 1.6 RE at a 95% confidence level. Assuming a simple linear transition in the proportions of rocky and non-rocky planets, no more than 5% of planets with radii of about 2.6 RE will have densities compatible with a rocky composition to a 95% confidence level. PH3 c, with a radius of 2.7 RE, exceeds the threshold found by Rogers and, based on its density, is clearly not a terrestrial planet.
An obvious potential counterexample to Rogers’ maximum rocky planet size threshold is the case of Kepler 10c, which made the news early this year. Kepler 10c, with a radius of 2.35 RE determined by Kepler measurements and a Neptune-like mass of 17 ME determined by radial velocity measurements, was found to have a density of 7.1±1.0 g/cm3. While this density, which is greater than Earth’s, might lead some to conclude that Kepler 10c is a solid, predominantly rocky planet, Rogers counters that its density is in fact inconsistent with a rocky composition by more than one-sigma. Comparing the measured properties of this planet with various models, she finds that there is only about a 10% probability that Kepler 10c is in fact predominantly rocky in composition. It is much more likely that it possesses a substantial volatile envelope albeit smaller than Neptune’s given its higher density.
While much more work remains to be done to better characterize the planetary mass-radius function and the transition from rocky to non-rocky planets, one of the immediate impacts of this work is on the assessment of the potential habitability of extrasolar planets. About nine planets found to date in the Kepler data have been claimed by some to be potentially habitable. Unfortunately, all but two of these planets, Kepler 62f and 186f, have radii greater than 1.6 RE and it is therefore improbable that they are terrestrial planets, never mind potentially habitable planets.
This still leaves about a dozen planets that have been frequently cited as being potentially habitable that were discovered by precision radial velocity surveys whose radii are not known. However, we do know their MPsini values where MP is the planet’s actual mass and i is the inclination of the orbit to our line of sight. Since this angle cannot be derived from radial velocity measurements alone, only the minimum mass of the planet can be determined or the probability that the actual mass is in some range. Despite this limitation, the MPsini values can serve as a useful proxy for radius.
Rogers optimistically estimates that her 1.6 RE threshold corresponds to a planet with a mass of about 6 ME assuming an Earth-like composition (which is still ~50% larger than the measured mass of PH3 c, which is now known to be a non-rocky planet). About half of the planets that some have claimed to be potentially habitable have minimum masses that exceed this optimistic 6 ME threshold while the rest have better than even odds of their actual masses exceeding this threshold. If the threshold for the transition from rocky to non-rocky planets is closer to the 4 ME mass of PH3 c, the odds of any of these planets being terrestrial planets are worse still. The unfortunate conclusion is that none of the planets discovered so far by precision radial velocity surveys are likely to be terrestrial planets and are therefore poor candidates for being potentially habitable.
Please do not get me wrong: I have always been a firm believer that the galaxy is filled with habitable terrestrial planets (and moons, too!). But in the rush to find such planets, it now seems that too many overly optimistic claims have been made about too many planets before enough information was available to properly gauge their bulk properties. Preliminary results of the planetary mass-radius relationship now hints that the maximum size of a terrestrial planet is probably about 1½ times the radius of the Earth or around 4 to 6 times Earth’s mass. Any potentially habitable planet, in addition to having to be inside the habitable zone of the star it orbits, must also be smaller than this. Unfortunately, while recent work suggests that planets of this size might be common, our technology is only just able to detect them at this time. With luck, over the coming years as more data come in, we will finally have a more realistic list of potentially habitable planet candidates that will bear up better under close scrutiny.
The discovery paper for PH3 c by Schmitt et al., “Planet Hunters VII: Discovery of a New Low-Mass, Low Density Planet (PH3 c) Orbiting Kepler-289 with Mass Measurements of Two Additional Planets (PH3 b and d)”, The Astrophysical Journal, Vol. 795, No. 2, ID 167 (November 10, 2014) can be found here. The paper by Leslie Rogers submitted to The Astrophysical Journal, “Most 1.6 Earth-Radius Planets are not Rocky”, can be found here.
For a fuller discussion of how Rogers’ work impacts the most promising planets thought by many to be potentially habitable, please refer to Habitable Planet Reality Check: Terrestrial Planet Size Limit on my website Drew Ex Machina.
Great article! I’d just like to point out the small unit correction needed at
“all but two of these planets, Kepler 62f and 186f, have radii greater than 1.6 ME”
Thanks for catching that, Thomas! I just fixed it in the text.
Do we have an understanding of the different compositions with size from rocky worlds, water worlds and mini-Neptunes? Also, if the transition at ~1.5 Re is almost stepwise, and worlds < X Re lose their atmospheres, doesn't this mean Earth-like worlds with continents and an atmosphere are likely to be very rare? (even before we deal with the HZ issue)
I suspected that there was too much wishful thinking in some estimates
of habitability. It’s only natural to hedge things to a positive conclusion.
But will the General Science magazines emphasize this study, or will
they bury it so interest in new planet findings wlll continue to be put on their covers or as lead stories.
This website and others that look into Planet hunting in more detail
causes me awkward moments in conversation with those who are informed via regular media channels. Very few people that view science stories from network news, know that Kepler due to stellar surface noise, cannot spot an Earth sized planet in the Habitability Zone of their central primary, and that this was one of it’s primary goals. I must sound like the Grinch to them telling them the realities of some of these breathless “announcements”
What about the other side of the coin, For a smaller planet than the Earth
what is the minimum limit for habitability. What size planet can retain
an atmosphere for more at least 3 billion years in a stars’ HZ. and also
have a chance at plate tectonics. Sounds like something close to 66% of
earth size to me.
Regarding Kepler-10c, the paper where the mass measurement was published noted that they had to include a water mass fraction of 5–20% by mass to model the mass and radius. This is higher than the terrestrial planets by at least 2 orders of magnitude. This was entirely omitted from the press material, which gave the impression the planet is a giant rock, complete with artist’s image showing a rocky surface beneath a thin cloud layer.
Another one to add to the long sad list of extrasolar planet related press releases that don’t match the science.
The question of whether the ice layer in such a planet would actually be in the form of solid high pressure ice is another matter. It is unlikely to be pure water there, which could perhaps change things from the naïve 2-layer model.
@Alex Tolley November 14, 2014 at 11:38
There are several groups that have generated theoretical models of the properties various types of planets – terrestrial, water-worlds, mini-Neptunes, gas dwarfs – over the past several years in part to make sense of the Kepler observations. It is by comparing Kepler observations and independent mass determinations to these models that it is becoming apparent that planets with radii greater than ~1.5 Re are likely to have deep, dense atmospheres and thick layers of hot, high-pressure ices (i.e. they are mini-Neptunes or gas dwarfs and not terrestrial planets like the Earth). This is NOT the same as saying that planets with radii less than 1.5 Re are airless. They can easily have atmospheres with densities within a couple of orders of magnitude of Earth’s. It will require more than just information on the bulk density to determine the characteristics of those atmospheres.
@RobFlores November 14, 2014 at 12:42
I think that your assessment pretty well hits the mark: It is the hype surrounding the announcement of potentially habitable planets (fueled by the desire of some teams of astronomers to claim finding the first habitable planet and writers to be first to report it) that is typically of interest to the media. And there seems to be very little follow up by the press to look past the hype and objectively examine claims of potential habitability. That is one of the reasons I have written the “Habitable Planet Reality Check” series for my own web site (see http://www.drewexmachina.com/category/astrobiology/planetary-habitability/)
As for your question about the MINIMUM size of habitable worlds, this has been examined especially in connection with the possibility of habitable moons (see https://centauri-dreams.org/?p=31557). While the minimum size is still a matter of debate it seems that a planet or moon as small around 0.12 to 0.25 Me has prospects of being habitable (at least “habitable” like the Earth is) depending on the precise circumstances. So that means terrestrial worlds as small as maybe ~0.12 Me to as large as around ~6 Me orbiting inside the HZ of their suns could potentially be habitable in that they have conditions that allows for liquid water on their surfaces and could support life like the Earth. Although Kepler can barely detect the largest of these worlds, indications are that there could be a lot of them in our galaxy.
Once we have a spectroscopic analysis of their atmospheres we will know for sure but even if rocky planets are limited to 1.6 Earth diameter or smaller how many such world exist in our galaxy? It’s probably a great number when you consider how many planets our in the galaxy sine all or most stars have planets.
This is a slightly off topic post, but this problem has not only shown itself in this post but has been apparent in many previous posts and discussions.
When we are talking about Earth analogs, there is a tendency to conflate Earth-like, Life Bearing, Habitable and Colonizable. What we need to do is devise and use a clear and consistent terminology. A precise terminology will illuminate the underlying assumptions we make when we talk about the topic, one of which is the tendency to assume that because a planet has certain similar properties to Earth, it will be like Earth.
Just because a planet is similar in size to Earth and insolation does not mean it’s going to be habitable or ever life bearing. Earth would not be classed a habitable planet for most of its life.
For instance, what we are looking at in today’s post is a class of planets that is not too large that it collects a massive atmosphere, not too small that over time it looses its atmosphere, and not too hot that runaway greenhouse leads to a thick atmosphere over a solid surface.
We need a term that encompasses this category of planets: I came up with Terroid. Suggestions welcome.
Within this category, there will be numerous types of worlds-subcatergories. We need a term to differentiate a planet with oceans of water and land masses with an oceanic planet. (Terran, Giaean?)
We also have to consider how changes in planetary disk chemistry will change an Earth mass and insolation planet. We know that a lot of exoplanetary systems have a higher Carbon to Oxygen ratio. (Our system appears to be at the high end of the Oxygen ratio to Carbon) This may produce planets with different surface constituents-probably with oceans of Hydrocarbons.
Also the term life-bearing does not necessarily overlap with the previous definitions. It may apply to a Terriod planet or it may apply to an Icy moon like Io. Earth, of course was a life-bearing Terriod planet, subclass Terran, for most of it’s existence, but it was still not habitable in the strictest sense.
What is a habitable planet? Some people liken it to a planet that is colonizable, but given enough money and technology, you can just about colonize anything. There may be a lot of planets out there that are fine except the Oxygen level is to low, so all you need when you go out is an Oxygen mask, but is this habitable? I’m going to leave the arguments over what is a colonizable planet for another day, and assume that when we talk about habitability we are talking about the subset that you can walk out of the airlock without a spacesuit on. This particular defined class I would call Doyle Planets after the author of Habitable Planets For Man. The term habitable is just too vague to be of much use.
If we are to accurately discuss and analyze exoplanets, we need a set of terminology that is precise. One of the first things done in science when it comes across a complicated new phenomena is to develop a universally recognized system of classification so every knows exactly what everyone is talking about. This field needs it. Somebody or some group needs to come up with a dictionary of terms we can all refer to. We have the Extrasolar Planetary Encycopedia. We need the Extrasolar Planetary Dictionary.
The mention of the lower limit of potential living planet mass for atmospheric retention, brings up the debate over how Mars (0.107 ME) lost most of its atmosphere (It seems meteorite impact erosion is one of the best explanations and these vary greatly per system and location within it). Also note that a tidally locked planet with the Martian level of insolation will receive 40% more sunlight at its subsolar point than the tropics of Earth.
Another thought might be the signs that there is life on Titan. If life really is ever found there, then we would have two examples of living worlds, one with only 2% our of our mass yet ten times as much atmosphere per unit area!!
RobFlores, when you point out “that Kepler due to stellar surface noise, cannot spot an Earth sized planet in the Habitability Zone of their central primary” I think you may be being a bigger Grinch than you can imagine.
Ever since Copernicus, we have imagined our sun a typical star, now Kepler finds that it is much much to quiet… or is it abnormal? Are we viewing it at an typical time or… Those ice cores do seem to show that the last ten thousand years of our rise to civilisation have been freakish in their stability, but then we would never have been here to talk about them if they hadn’t… what about those climate models that we can never force into and out of ice ages, as if some factor is missing from them… and it seems so hard to explain the apparent erosion of the Martian atmosphere using the current levels of solar activity.
To me it is no wonder that most don’t want to talk about Kepler’s difficulties, least the topic turns to when the Sun might return to its normal state again.
@Dave Moore November 14, 2014 at 20:18
I don’t think your comment is that off topic and in fact I am glad you brought it up! One of the key parts of science is establishing definitions for categorizing objects. For quite some time there was a fairly well established “use” (I hesitate to use the term “definition” because it never was that formally established) of the term “habitable” – it referred to a terrestrial body whose conditions were such that it could possess liquid water on its surface and possibly support life like the Earth. This is still the definition I prefer to use but, as our studies of other worlds have shown (e.g. Europa, Enceladus), such “habitable” worlds probably represent a subset of all worlds that can support life. And of course, contrary to the public’s understanding of the term, the majority of “habitable” worlds could NOT support terrestrial animal life. Because of the carbonate-silicate cycle, most “habitable” worlds would have high level of CO2 in their atmosphere to maintain their surface temperatures above freezing – levels that would be toxic to us but not to any native life forms. Only a tiny subset of “habitable” worlds close to the inner edge of the habitable zone would have CO2 levels that wouldn’t kill us or other terrestrial animal life. And since it is dangerous to assume that oxygen-producing forms of autotrophism like photosynthesis is inevitable, only a tiny subset of “habitable” planets with low levels of CO2 would probably have O2 levels high enough to support us (i.e. worlds that we could colonize without any sort of breathing aid).
In any case, I agree that the scientific community needs to more clearly define such terms as “habitable” or “biocompatible” (a term used by Martin Fogg a couple of decades ago where “habitable planets” were a subset of “biocompatible planets”) or maybe some of the terms that you suggest. As it stands now, too many scientists are using the term “habitable” to mean too many different things which differ significantly from the public’s understanding of these terms. This just further confuses the topic of the potential habitability of other worlds.
I guessed this would be the case with planets larger than Earth. If you take the planets of our solar system and plot them on a graph of size on one axis and density on the other, density decreases with size. This finding confirms this trend. The planets larger than 1.5 Earth radii are likely Nini-Neptunes with their dense atmospheres and high-pressure ices. I would reckon that planets of say 1.2 Earth radii are waterworlds, with atmospheres of say 10-20 bar. Not a place you would want to live. Planets smaller than Earth are likely to be more Mars-like, just how Mars-like to be determined.
It is also worth noting that the Earth is the densest planet in our solar system with Mercury being a rather distant 2nd place. I wonder if that means anything.
Does anyone have an answer to the plate tectonics question? Is a large moon-forming impact necessary to realize plate tectonics, or not?
There are several factors that go into determining what type of planet is discovered- Not only the mass/radius relationship (the greater the radius for the mass determines whether it’s rocky, liquid or gaseous– Mass alone is NOT the determining factor!) but the solar temperature (what the planet gets from its sun) and solar wind pressure- aka “blast factor”- which would determine how much gas is retained at the planet. (There is still a misconception out there that mass value is the same as surface gravity, which it isn’t! A planet of ~6Me would most likely only have a gravity of something like 1.3Ge, which is near the upper limit of human-tolerability.) Mass does not determine radius, either, and that needs to be made clear- You need to have both mass and radius to determine density, and it’s the density that determines whether it’s a Neptune-type or Terrestrial-type world. Mass alone won’t tell you. :)
d.m.f.
@Rob Flores,
“that Kepler due to stellar surface noise, cannot spot an Earth sized planet in the Habitability Zone of their central primary”
Maybe. However, what about 2 Re ? Or 4 Re ?
If one looks at this diagram on Systemic:
http://oklo.org/2012/11/10/the-mmen/
If noise prevented the detection of 1 Re planets in the HZ, then one would
expect a lot of bigger ones, just like they appear for shorter periods.
Instead the graph is empty, at lest when compared to the short period ones.
The most common planets in the HZ seem to be gas giants.
It might be a bit outdated but I don’t think it has changed that much.
Are G stars really that noisy that even Neptunes cannot be detected ?
“Are G stars really that noisy that even Neptunes cannot be detected ?”
I mean detected in the HZ, they are definitely detected closer to the star.
@Andrew LePage November 15, 2014 at 3:53
I’m glad that I’m not the only person that sees this problem. Is there any possibility that at a conference there could be presentations and some sort of a working group hash out some neutral definitions?
We are getting a lot of papers deducing the frequency of Earth-like planets, but just how Earth like are your Earth-like planets?
Bad terminology leads to faulty assumptions. The problem with “habitable,” aside from the fact it is not well defined, is that it carries with the assumption that you can march down the ramp of your space craft and have a picnic on the planet’s surface.
Another piece of terminology that probably needs replacement is the concept of “Habitable Zone.” This term was coined on the assumption that if we have an approximately Earth mass body with Earth’s insolation we get Earth, but now we know what the term actually means is “there-is-a-faint-chance-we-will-get-a-habitable-planet (whatever that means)-in-this-zone.”
When we talk about a Tropical Zone, we talk about a place where we get tropical weather patterns and tropical vegetation. In a mountain zone we expect mountains. Our definition of “Habitable Zone” is like pointing to a plain and saying we call this the Mountain Zone because there maybe a mountain out there–somewhere.
I have no expectations that the press will do anything other than make a sloppy hash of the terminology, but it would be nice if the field had some standardized, precise and neutral definitions to work with.
@d.m.falk November 15, 2014 at 13:38
You have missed the point about the discussion of the observed mass-radius relationship of extrasolar planets. Of course the average density of any planet is determined by its mass and radius. And the observed density of planets at any given mass does display a fair amount a variation indicating some range of compositions. But the types of planets that are actually observed to occur in nature show that planets with radii less than ~1.6 Re or masses less than ~4 to 6 Me are going to tend to be denser, rockier planets like the Earth while those that have larger radii or greater masses are going to tend to be less dense, non-rocky planets like Neptune. Apparently, Neptune-like planets with masses similar to the Earth or rocky planets with masses similar to Neptune do not exist or are at very least appear to be uncommon. This is telling us something important about the formation and physics of planets. The same thing occurs with main sequence stars which have their own mass-radius relationship: only certain combinations of mass and radius occur within a fairly narrow range and this tells us something about the physics of stars.
I’ve noticed that there’s a tendency towards grandstanding and excessive optimism in science news. it’s not that bad, but it does create misconceptions. In addition, it damages reliability when statements are retracted or changed.
as I’ve pointed out, our technology for detecting exo-earths is still in its frontier phase. as detection improves, I’m sure we’ll haul in many more. in the meantime, it’s not surprising that many of these findings are in fact neptunes.
@Enzo November 15, 2014 at 15:58
“If noise prevented the detection of 1 Re planets in the HZ, then one would
expect a lot of bigger ones, just like they appear for shorter periods.”
First of all, the figure you cite is misleading since it just shows just the raw numbers of extrasolar planets discovered to date (presumably using precision RV measurements since the units are in mass). Noise in the data is certainly an issue with finding Earth-size planets with orbits that have periods on the order of hundreds of days. In the case of precision RV measurements, the natural “jitter” in the RV measurements from stellar activity and the ability of astronomers to model this accurately means that only planets with several times the mass of the Earth can be detected in Earth-like orbits assuming that the star’s natural jitter is low enough and the star is observed long enough. But not all stars are quiet enough or have necessarily been observed long enough to detect these super-Earth size planets. Most RV surveys are performed at lower precision with less data looking for easy-to-detect planets which just compounds the already strong selection bias for finding larger planets and planets in shorter period orbits which the plot of raw-discovery numbers does not correct for. The dearth of Earth-size planets in Earth-size orbits in this plot of raw discovery numbers can easily be the result of the fact that such planets are nearly impossible to detect in the surveys that have been conducted to date (and are likely to be undetectable using precision RV measurements). In other words, the lack of such planets is an illusion caused by a combination of selection bias and the natural limits of the detection method.
Kepler data using the detection of transits of extrasolar planets has its own set of selection biases that similarly affect the raw numbers. A planet with an orbital period of 3 days is 22 times more likely to have its orbit aligned to produce an observable transit than a planet with an orbital period of 300 days. And assuming that the noise in the photometric data is white noise, a detection threshold corresponding to a 1 Re for a planet in a 3-day orbit is equivalent to 3.2 Re for a 300-day orbit. And, as ROb Flores points out, it turns out that Sun-like stars are a bit noisier than expected making the detection of small planets all the more difficult. As with the RV method, there is a selection bias at work here that creates an illusion that small planets in long-period orbits are rare when looking at the raw numbers alone.
It should also be remembered that the analysis of the Kepler data is still ongoing and the difficult task of performing careful statistical analyses of the data that take into account selection biases has only just started. It is still too far too early to come to any conclusions about the prevalence of Earth-size planets in Earth-size orbits around Sun-like stars.
@Andrew Le Page,
“presumably using precision RV measurements since the units are in mass”
I believe that that graph includes all Kepler planets known to that date, not
just RV ones. You can see it from the last graph in this post :
http://oklo.org/2012/02/19/regular-systems-of-satellites/
Greg Laughlin makes a rough conversion from diameters to mass to put them
on a diagram. For the newer plot, I believe that the gray dots are Kepler’s
planets. They really are diameters, at least for Kepler’s data.
“And assuming that the noise in the photometric data is white noise, a detection threshold corresponding to a 1 Re for a planet in a 3-day orbit is equivalent to 3.2 Re for a 300-day orbit.”
So, according to what you are saying, since Kepler had no problem finding
various earth size planets for longer periods (15+ days), then it should have no problems finding planets > 3.2 Re at 300 days and longer.
Especially considering that its own data indicates that > 3.2 Re planets are
amongst the most common. Instead no, it hardly finds anything.
In other words :
1) Detection threshold corresponding to a 1 Re for a planet in a 3-day orbit is equivalent to 3.2 Re for a 300-day orbit
2) Kepler has no problem finding various earth size planets with longer
period than 3 days (presumably harder than 1) ).
3) Neptune and sub-Neptunes are one of the most common planet in the
Kepler data
Therefore , if Neptune and sub-Neptunes were as common in the HZ, then
Kepler should have no problem detecting them. That is not what it finds.
According to what you are saying, the drop in Neptunes and sub-Neptunes
with longer orbital periods is real.
Maybe that’s not true for earth size planets but such drop doesn’t bode well
for plenty of earth size planets in the HZ of G stars.
Instead, it is consistent with the concept of “compact” systems being very
common, with examples in our own solar system.
@Enzo November 16, 2014 at 18:48
The figures I cite for the detection limits for Kepler are just hypothetical examples for illustrative purposes and were not meant to be taken as actual published detection thresholds. The actual detection thresholds will vary from star to star depending on the the star’s brightness and the natural variations in its brightness. These are all details that further complicate the statistical analysis of the Kepler results. But these are the points I am trying to make:
1) RV and transit methods have very strong selection biases that limit their ability to detect Earth-size planets in Earth-like orbits around Sun-like stars. Earth-size planets and, in some cases, even Neptune-size planets are near the detection limits of many of these surveys. These two factor combine to produce a woefully incomplete picture of the prevalence of such planets in long period orbits.
2) The analysis of Kepler data is still going on and only the “easy” planets have been found so far. There are many more planets to be found in the data including Earth-size, super-Earth-size, and Neptune-size planets in the HZ of Sun-like stars .
The bottom line is that it is still far too early to be making any definitive statements one way or the other about the prevalence of Earth-size planets in general or the size distribution of planets to be found in the HZ of Sun-like stars. The published survey data to date are simply insufficient to draw any statistically meaningful conclusions. As more data come in from increasingly more sensitive RV surveys of larger numbers of stars, continuing analysis of the Kepler data (not to mention the new data coming in from the extended K2 mission) as well as new data from TESS, astrometric surveys from Gaia and other mission, we will slowly get a better handle on these important questions.
I would highly recommend the following fully-referenced review article on the current state of knowledge of extrasolar planetary systems: Joshua N. Winn and Daniel C. Fabrycky, “The Occurrence and Architecture of Exoplanetary Systems”, arXiv: 1410.4199 (submitted to The Annual Review of Astronomy & Astrophysics), posted October 15, 2014 http://arxiv.org/abs/1410.4199
It is frustrating that the gap in the Kepler data, leaves us with only
the ability to make Inferences regarding Twin earths in HZ of other stars.
While there is good reason to believe that those planets do in fact exist,
one thing gives me pause. An HZ around a star is by definition the point
where H2O transitions from gas to liquid for those planets close to RE = 1.0
Could this fact create a natural bias by: making 2 outcomes in planet formation vastly more likely
1) No planets form there
2) massive planets (RE>2) form there.
Do we have evidence from proto planetary disks that H20 is present in most
stars where it’s HZ will be? Do all proto planetary disks have the same
composition? On the Earth it appears that most of the H2O arrived from
comets when is was closer to it’s final size. What if the building blocks of
the planitesimals that would latter aggregate into a proto planets have large
amounts of H20 on their surfaces , would that affect the evolution of the proto-planet to it’s final destiny. Has this been modeled?
If my math is right,a 1.5 Re planet with 6 Me should have a surface gravity of 2.67 times that of Earth. Quite a load on the back and legs!
@Rob Flores November 17, 2014 at 1:57
I understand your frustration. And, unfortunately, the advertised abilities of Kepler were a bit misleading. On paper, Kepler was design to be able to detect the transit of an Earth-size planet in an Earth-like orbit around a Sun-like star with an apparent magnitude of 12. Unfortunately given the sample size of stars in Kepler’s field of view and the low probability of any given Earth-twin having its orbit aligned to produce a transit, Kepler was never going to detect more than a few Earth-size planets in Earth-like orbits around Sun-like stars assuming that our solar system is typical. Certainly never enough to have a statistically significant sample to make any claims about the prevalence of Earth-twins orbiting Sun-twins. And given Kepler’s limited 3.5 year primary mission, it was never going to confirm the presence of planets with orbital periods longer than ~14 months. That corresponds to a distance of only ~1.1 AU around a Sun-twin so Kepler was never going to probe deeply into the HZ of such stars which extends out over twice as far.
The latest published estimates predict that Kepler might find only about 9 “Earth-analogs” in its data (which is a bit misleading since “Earth analogs” are defined as planets with a radius between 1 and 2 times that of the Earth orbiting a G-type dwarf star with a period of 200 to 400 days – Venus would be an “Earth analog” as well as mini-Neptune with radii >1.5 Re while potentially habitable planets in the outer HZ are not even being considered). I discuss this in more detail in the following article:
http://www.drewexmachina.com/2014/06/25/abundance-of-earth-analogs/
As impressive as Kepler has been and continues to be, it is but the first step in figuring out the true prevalence of Earth-like habitable planets.
This also suggests that most habitable planets would have more acidic oceans than the Earth does, so any life there would probably not evolve calcium carbonate shells as they would not be stable under such conditions. The evolution of calcium carbonate shells has had significant geological consequences on the Earth, which would presumably not occur on the majority of Earth-type habitable worlds.
@andy November 17, 2014 at 16:02
The high CO2 levels one would expect in the outer parts of the HZ as a natural consequence of the carbonate-silicate cycle would NOT result in ocean acidification since one of the key steps is the cycle is the weathering of silicate minerals in the crust by the carbonic acid produced by the CO2 dissolved in water to produce carbonates and essentially pH neutral water. The details of this cycle were originally described over two decades ago by Dr. James Kasting and is colleagues and it is the basis for our current understanding of planetary habitability. While detailed descriptions can be found throughout the professional literature on the topic, a quick explanation can be found on a page in Dr. Kasting’s web site (http://www3.geosc.psu.edu/~jfk4/PersonalPage/ResInt2.htm). If you want more details, a very readable explanation of the carbonate-silicate cycle and how it affects long-term climate stability can be found in the book “How to Find a Habitable Planet” by Dr. Kasting (published by Princeton University Press in 2010… and no, I have no connection with the author or the publisher).
I could see how that would affect the water on the continents but most of the ocean water is not in direct contact with the land. So how does the carbonate-silicate cycle stop atmospheric carbon dioxide dissolving directly into the ocean?
@andy November 18, 2014 at 3:25
The oceans are in contact with the silicate crust that makes up the ocean basins and are typically filled suspended silicate particulates transported by air and water. There is plenty of material available to buffer the pH of the oceans.
Alien Life Could Thrive on ‘Supercritical’ CO2 Instead of Water
by Charles Q. Choi, Space.com Contributor | November 16, 2014 09:50 am ET
Alien life might flourish on an exotic kind of carbon dioxide, researchers say. This “supercritical” carbon dioxide, which has features of both liquids and gases, could be key to extraterrestrial organisms much as water is to biology on Earth.
Most familiar as a greenhouse gas that traps heat, helping warm the planet, carbon dioxide is exhaled by animals and used by plants in photosynthesis. While it can exist as a solid, liquid and gas, past a critical point of combined temperature and pressure, carbon dioxide can enter a “supercritical” state. Such a supercritical fluid has properties of both liquids and gases. For example, it can dissolve materials like a liquid, but flow like a gas.
Full article here:
http://www.space.com/27777-alien-life-supercritical-carbon-dioxide.html
On the other hand, that link does not discuss the dependence of ocean acidity on the location in the habitable zone.
I tried to find something to confirm your assertion that the ocean acidity should be independent of atmospheric carbon dioxide on worlds mediated by the silicate carbonate cycle, and came across Abbot, Cowan and Ciesla (2012) which states:
Where ? represents the atmospheric carbon dioxide concentration (original text used a tilde on the ? but I figure I’m probably dicing around with compatibility issues using the Unicode subscript/superscript characters without messing with combining diacritics on Greek letters as well). This suggests that the ocean acidity should indeed be dependent on atmospheric CO? concentrations.
@andy November 18, 2014 at 19:24
OK, let’s assume for the moment I am an utter idiot when it comes to chemistry: Where is there an equation in this paper you cite (which addresses issues surrounding the ability of the carbonate-silicate cycle on water-worlds to regulate global temperature) that supports your assertion of a relationship between the partial pressure of CO2 and ocean pH? I’ve read the paper twice and I can not find anything that does NOT back the common made assertion that the ocean floor will buffer an ocean’s pH and keep it from becoming too acidic. Please cite a specific equation number in the paper or please provide a derivation that links the information you have cited above to an equation of pH as a function of partial pressure of atmospheric CO2 because I just do not see the connection.
@andy November 18, 2014 at 19:24
After sleeping on it, please let me rephrase my response to your citation: What you have cited is one small piece of a much larger description of the details of model being used determine the weathering rates on terrestrial planets as a function of continent fraction. The the availability of H+ in the ocean as a function of atmospheric CO2 concentration is just one small piece of a model that could be used to determine the pH of a global ocean under equilibrium conditions. There are many other factors that would ultimately contribute to the calculation of an ocean’s pH.
To be honest, I am a physicist by profession, not a chemist so I won’t pretend to understand all the details of the weathering models (if you want to talk about the details of radiative transfer models and how atmospheric CO2 fraction, clouds and aerosols affect equilibrium temperature, I’m your man given the appropriate forum). However, your original conjecture at the beginning of this thread that CO2 concentrations on a habitable world could render an ocean acidic enough to dissolve carbonate shells simply can not happen. A key part of the carbonate-silicate cycle is the precipitation of carbonates into climatically inert deposits. If the pH of the ocean were too acidic for this to occur (the equivalent of being so high that it would dissolve carbonate shells), then the paper you cites shows that weathering rates of oceanic crust would increase to scrub more CO2 out of the atmosphere decreasing the ocean acidity in the process until those deposits could form. The bottom line of these various models is that the equilibrium acidity of a habitable world’s oceans can NEVER get too high to dissolve carbonate shells or or inhibit the formation of carbonate deposits regardless of the partial pressure of CO2 in their atmospheres.
I do not pretend to understand all the intimate details of the chemistry involved but logically this must be true based on the models developed by people who do know more about this than I do and my faith in the peer-review system. If somehow this explanation is inadequate, I do apologize but can only suggest that you direct your question to one of the authors of the paper you cite or elsewhere to get the answers that will satisfy your curiosity (just as I have done many times over the past decades for my work in the field :-) ).
@ljk – obviously the author is not a biologist. CO2 won’t substitute for water for biochemistry.
@Andrew LePage
Your point may be true at equilibrium, but right now on Earth the problem of ocean acidification is real, and there is evidence that organisms are being stressed in terms of their ability to build carbonate shells. This may well disappear in the long term as equilibrium returns, but in the short term…
@Alex Tolley November 19, 2014 at 13:38
I do not wish to start a “debate” on man-made climate change and its effects on the environment here or anywhere else for that matter (and I will not participate in such a discussion in case someone wants to take issue with the overwhelming consensus of the scientific community!!!), but I wholeheartedly agree with your statement.
@Alex Tolley (@ljk) – that article covers the important bases such as enzyme stability and specificity and substrate solubility. Even though CO2 is nonpolar – the super critical state should be able to suspend much that normal liquid CO2 can not dissolve. The only problems I saw that might turn out to be of fundamental importance was what replaced condensation reactions as a means of concatenating subunits, and cellular partition from the environment (by semipermiable membrane). Do you see any others?
@Rob – without being specific, the physical and chemical properties of water are so fundamental to life (as we know it) that I have a hard time believing that supercritical CO2 could replace it.
It isn’t even clear what they tested, e.g.
Are they saying that the bacteria lost all their water and replaced it with CO2, or that they survived in CO2? Because the latter interpretation wouldn’t support the water to CO2 5replacement.
Even the idea that enzymes are more stable in CO2 than water is surprising, as the polarity of water is important in protein folding. I’d want to see proteins unfolded and then refolded in CO2 before I believe in the replacement idea. We would need to look at a lot of biochemistry and molecular biology and see which are still viable in CO2 to determine if a theoretical carbon based life form could work without water. Just the condensation reactions alone, that you mention, would seem to rule it out. There would need to be some analog that condenses out CO2, and that CO2 can also break in order to fulfill the basics of growth and metabolism.
This to me is a speculative, half-baked idea that is similar to the idea of silicon (or at least carbon-silicon) based life, or even arsenic as an alternative to phosphorus.
Ah Alex now I see. It seems you read such passages very differently than me. Long experience has always told me that science writers always take the shortest and simplest sounding quote – the one that is almost devoid of the important information that was furnished earlier. Yes, that precis didn’t make biochemical sense, but what was left out that made it so? I put my money on the bacteria+CO2 test being purely to see if the membrane dissolved away (and that it didn’t). I also guessed that those enzymes were tested separately for activity. Note how it would then suddenly make far more sense.
As for your unfolding and refolding test – where do you get your CO2 specific chaperone – because that’s what we should be talking of here.
@Rob – Chaperones might make it even more complex. I was thinking of proteins that can fold and unfold without chaperones. This is as simple a test as I can think of, using the protein’s functionality as an assay. If the protein can refold correctly in CO2, this supports the enzyme finding in te article. But if the enzyme cannot refold correctly, we should see a diminished activity. Since we know that folding has to happen during translation, failure to refold in the experiment is suggestive that CO2 won’t work for our biology, although it doesn’t rule out a different carbon biology.
Another simple test is whether viral protein coats can assemble correctly in CO2. This is similar in concept to other protein-protein interactions, including chaperones.
If water can coexist with supercritical CO2, then maybe the biology becomes easier – we still have it being water based, but the environment is predominantly high pressure CO2.
Apologies for the slight delay in response, I would suggest that if you are using Windows 8.1 that you treat the update KB3000850 with some degree of caution. (Reinstalling everything is *FUN*)
Those are some good points with regard to the carbonate in the ocean. (The relevant equation is the one at the end of the part I quoted: the ? is the partial pressure of carbon dioxide, the [H?] is the hydrogen ion concentration which is related to pH by pH = -log?? [H?]) It does unfortunately seem that the ocean chemistry typically gets rather neglected in habitability studies as compared to the atmosphere (though in terms of remote detection prospects this is perhaps understandable). I will do some more looking into this, if I find out more I’ll post stuff.
@andy November 20, 2014 at 18:41
And my point is that what you have cited is taken out of the context of a larger model meant to estimate weathering rates and that there is more to determining the pH of the ocean than just the partial pressure of CO2 in the atmosphere. For example, assuming for the moment that the units of phi is in bars (it is unclear what the units should be but it is implied to be bars) and we take the Earth’s CO2 concentration (~4e-4 bars) then -log(phi^(2/3)) means that the pH of Earth’s oceans is 2.3 which is no where near its actual pH is around 8.2. Like I said, what you cite is taken out of context and atmospheric CO2 content is just one component of many needed to properly model the pH of the ocean.
About that ocean pH at very high CO2. Why would the carbonate-silicate cycle be important for temperature regulation at these huge partial pressures? As to that ocean acidity, even some of our bacteria can cope with pH0,and carbonate shells are not the only type used by higher life – even on our planet. Our oceans are highly limited in their fertility due to a shortage of vital mineral ions (the most limiting one is currently iron). In my naivety I thought acid oceans might help that cation shortage in an alien environment. What am I missing?
Actually phi is scaled to the Earth values, so the model certainly does not imply a pH for the oceans of 2.3. The definitions (use of the tilde and the zero subscript) are near the start of section 2, just beneath equation 2. It is probably best to read through the section in order rather than skipping around in it before arguing that the equations lead to ridiculous conclusions about the state of the environment on the Earth.
@andy November 21, 2014 at 18:38
I have read the paper and I stand by my original statements: The pH of a habitable world’s oceans is dependent on more than just the partial pressure of atmospheric CO2 as the equation you quote out of context implies. In addition, the equilibrium pH of a habitable world’s can never get so high that it would dissolve carbonate shells as you originally speculated because that is the equivalent of inhibiting the deposition of carbonates which is a key part of the carbonate-silicate cycle. Right there is section 2 of the paper you cite it clearly states:
“The reaction product CaCO3 is an example carbonate and silicon dioxide (SiO2) is often called silica. These reactions occur in aqueous solution and lead to a net decrease in atmospheric CO2 if the resulting CaCO3 is eventually buried in ocean sediment, ultimately to be subducted into the mantle. CaCO3 is generally produced in the ocean by biological precipitation, but if there were no biology weathering fluxes into the ocean would drive up carbonate saturation until abiotic CaCO3 precipitation were possible.”
You can argue the point ad infinitum but the equilibrium pH of a habitable world’s ocean can never get so high as to inhibit the precipitation of carbonates. If it did, a key part of the cycle that regulates the temperature of habitable planets would no longer operate. Your original conjecture that high atmospheric CO2 content would dissolve carbonate shells can not happen on a habitable world under equilibrium conditions and your interpretation of the information in the paper you cite are incorrect. For me, this matter is closed.
Another thing that may confound simple models being extrapolated to very high CO2 worlds at Earth-like temperature, is that a liquid CO2 layer becomes stable beneath a water ocean (particularly a deep circulating one which I would expect to have a cooler base) This is because a CO2 becomes denser than water in few atmospheres pressure, especially at cool temperature.
Very interesting post and discussion.
“Assuming a simple linear transition in the proportions of rocky and non-rocky planets, no more than 5% of planets with radii of about 2.6 RE will have densities compatible with a rocky composition to a 95% confidence level.”
Shouldn´t that be 1.6 instead of 2.6, in view of the preceding sentence and the whole argumentation about 1.6 Re as a limit?
“Because of the carbonate-silicate cycle, most “habitable” worlds would have high level of CO2 in their atmosphere to maintain their surface temperatures above freezing – levels that would be toxic to us but not to any native life forms. Only a tiny subset of “habitable” worlds close to the inner edge of the habitable zone would have CO2 levels that wouldn’t kill us or other terrestrial animal life.”
Well, is that really true? CO2 is not the only greenhouse gas and not even the most important one: water vapour is probably much more important. I tend to be much more optimistic with regard to habitable planets (meaning: habitable to earthlike biology) and the HZ, they can probably exist in a large part of the HZ, depending on their host star, size and overall atmospheric composition.
So what is out of context about that relation between pH and atmospheric pressure? It is being used as the basis for the habitability model in terms of determination of the rate of seafloor weathering. Are you arguing that the entire concept of seafloor modelling is wrong?
Ok sure I am very likely wrong about the calcium carbonate precipitation being inhibeted (not sure if you could precipitate magnesium carbonate instead but never mind), but the correlation between pH and atmospheric carbon dioxide does seem likely to be a real thing. Your quote from the article doesn’t really imply anything about ocean acidity (and really, if you are arguing that my quote is out of context, the same goes for yours), in fact it suggests that you end up with higher carbonate concentration in the ocean which then re-enables precipitation. In fact, this idea seems to be supported also by Berner, Lasaga and Garrels (1983)who note that the higher atmospheric partial pressure of carbon dioxide 100 million years ago is linked to the decreased pH of the oceans (remember, lower pH means more acidic), though this does not affect the saturation of CaCO? due to the correspondingly higher levels of Ca²? and HCO??. (See page 674).
So I think that while I was probably wrong about calcium carbonate becoming unstable throughout the water column, you are incorrect in asserting that the atmospheric partial pressure of carbon dioxide has no effect on the ocean pH. For me, this matter is closed unless you can demonstrate a more convincing argument than just asserting that anything I say on the matter is “out of context”.
No Ronald. Notice this is a “95% confidence THAT 95%” situation”. This is thus not what the data set indicates as likely, its the limit to how far it the the true population could be from this sample under this model. Let me put it another way – you need extreme overkill – way way past the point where you have found no rocky planets at all to get that level in a sample this small.
@Ronald November 24, 2014 at 8:57
The statement is almost correct. Upon checking the original paper by Rogers http://arxiv.org/abs/1407.4457), I discovered a typo and that the correct figure is 2.46 Re not 2.6 Re (apologies!). This figure comes from end of the 5th paragraph in Section 4.2 of the paper on the discussion of the linear transition model where it states “We similarly set upper limits on the planet radius at which no more than 5% are dense enough to be rocky of 2.15 Re (68.27%), 2.46 Re (95.45%), and 2.78 Re (99.73)%”. In the linear transition model (which is just slightly less favored in the analysis than the abrupt step-wise model), the 1.6 Re value represents the maximum value where there would be a 50-50 split between rocky and non-rocky planets to a confidence level of 95% (with this confidence level set by the statistical uncertainties in the analysis resulting primarily from uncertainties in the measured properties of the planets in the sample). In the linear transition model, it is possible to have some fraction of planets with a rocky composition at larger radii. The quoted statement gives the limits where <5% of planets are expected to have a rocky composition to a 1-, 2-, and 3-sigma confidence levels to reflect the uncertainties of the analysis. In a step-wise model with the transition at 1.6 Re, 0% of planets with these radii would be rocky (which is consistent with <5%). As Rogers states in her paper, more data are needed to improve the accuracy of the results and better determine the character of the transition (i.e. step-wise or gradual)
As for you second question, es, there are more greenhouse gases than just CO2 with H2O being the most important. This fact is taken into account in the models of planetary habitability.