Gliese 581 is an utterly maddening star, one that continues to tantalize us with potential habitability. The case of Gl 581g, examined here yesterday, is only the latest wrinkle, but it’s in some ways the most frustrating. We’re studying planets we cannot actually see, inferring their presence through the tiniest of variations in starlight caused by the planets’ gravitational effect on their star. Adjust planetary orbits here in the direction of eccentricity and you can make Gl 581g disappear. Assume circular orbits and you can produce a habitable zone super-Earth.
I think we’ll still be arguing about this one for some time, but in the interim the question will lose a lot of its force. Remember, the reason we’re so excited about Gl 581g is that it would become the closest planet with the possibility of liquid water at the surface (possibly joined by Gl 581d in the same system). But Jean Schneider’s Exoplanet.eu catalog shows 777 confirmed planets this morning, and Kepler has pulled in 2,321 planetary candidates, 246 of which are 1.25 times Earth’s radius or less. In short, habitable planets are not going to be uncommon for long.
Remember, too, that while Kepler has found 46 candidates orbiting within the habitable zones of their stars, we’re soon going to be seeing truly Earth-class planets, worlds that orbit at about our planet’s distance from a star like the Sun. It takes three transits to flag one of these, but as the Kepler mission winds on, the focus will shift at least for a time from habitability around problematic red dwarf stars and on to planets so like ours that they will seize the imagination. And who knows, we may yet find something interesting around the Alpha Centauri stars, close to home indeed. Three ground-based radial velocity studies continue to probe these stars.
So let the Gl 581g controversy have its moment in the press. We should be thinking ahead to the possibilities of missions like the Transiting Exoplanet Survey Satellite (TESS), which could go to work on stars closer to the Sun than those in the Kepler or CoRoT catalogs. We have to run missions like TESS and the European Space Agency’s PLATO (PLAnetary Transits and Oscillations of stars) on the cheap in an era when the big-ticket planet hunters like Darwin and Terrestrial Planet Finder have been placed on hold, but they could at least help us fill in the nearby exoplanet catalog while we wait for better budgets and missions that will be able to detect biomarkers.
Image: Comparison of the estimated relative size and orbits of the five possible exoplanets around Gliese 581. The green shade represents the size of the habitable zone, or the orbital region where an Earth-size planet could have surface liquid water. Planets e, b, and c are too hot for liquid water and life but g and d are in the habitable zone. Planet g is specially in the right spot for Earth-like conditions while d is marginally within these limits, and colder. This is the first case of a stellar system with two potential habitable exoplanets orbiting the same star, assuming Gl 581g exists. Credit: Planetary Habitability Laboratory/University of Puerto Rico at Arecibo.
While we wait, consider that the five possibly habitable exoplanets shown in the illustration for yesterday’s post are all fairly large. The disputed Gl 581g, if it exists, is at least twice as massive as the Earth, while its sister world Gl 581d is seven times as massive and, as the image above shows, orbits at the extreme outer edge of the classical habitable zone. Kepler-22b — the first known transiting world to orbit inside the habitable zone — looks to be about 2.4 times as wide as the Earth with a mass only roughly estimated at something like Neptune’s.
The above are among the five planets chosen by the Planetary Habitability Laboratory of the University of Puerto Rico at Arecibo as ‘the best five objects of interest for Earth-like exoplanets,’ a list that within months will be undergoing convulsions as new discoveries are announced. About Gl 581g, PHL’s Abel Méndez says “The controversy around Gliese 581g will continue and we decided to include it to our main catalog based on the new significant evidence presented, and until more is known about the architecture of this interesting stellar system.”
Fair enough, and PHL also adds two other interesting worlds. HD 85512b is a super-Earth about 3.6 times as massive as the Earth some 35 light years out, while Gl 667Cc is 4.5 times as massive as the Earth, orbiting a red dwarf some 22 light years out that is part of a triple-star system. Those distances are right in the neighborhood compared to Kepler-22b’s 600 light year distance, but we can expect even closer worlds as our studies continue. After all, as Kepler chief scientist William Borucki told The Economist in a recent article:
“The data so far suggest that most stars have planets, and we see quite a few of those in the habitable zone. So I suspect there’s nothing particularly special about planets at such distances.”
Dr. Borucki is surely right, and we should also keep in mind that we’re getting better and better at finding smaller planets. A few years from now, our five top prospects for habitable worlds are all going to look a lot more like the Earth.
Meanwhile, we enter the weekend with all eyes on a search for habitability much closer to home. Gale Crater on Mars has the advantage of being accessible — assuming the Curiosity rover’s elaborate landing system delivers it safely — and ideal for analysis by the rover’s high-tech laser and drill, along with its instruments for studying atmospheric methane. Finding solid evidence of life, past or present, on Mars is not crucial for the exoplanet hunt, but I suspect that its discovery would give a lift to those seeking habitable planets around other stars. Show us a nearby ‘second genesis’ and our thoughts will soar to the billions of habitable worlds that fill the galaxy.
Actually the number of 777 planets is a bit outdated, since the Extrasolar Planets Encyclopaedia has not included new planets for a while due to website restructuring.
Nice article! It would be a great tool if you’d enabled a “Share this in facebook” or twitter button.
Best regards!
I wonder why nobody talk about the closest planets in the habitable zone that we know, might because they aren’t terrestrial planets, i don’t know if it’s only me but i really believe in habitable exomoons.
The closest star system known and the closest to have planets in habitable zone too is Gliese 876 at “only” 15,3 light-years away, Gliese 581 is the second closest planetary system known.
About Gliese 876:
Star:
Mass: 0,33 Radius: 0,36
Temperature: 3.350 K (+-300 K)
Planet ‘d’
Mass: 6,4 to 7,2 mass of Earth
Orbit: 0,0208 UA (1,937 days)
Ecc: 0,20?
Planet ‘c’
Mass: 0,71 mass of Jupiter
Orbit: 0,13 UA (30 days)
Ecc: 0,25? (0,10 to 0,16 UA)
Planet ‘b’
Mass: 2,27 mass of Jupiter
Orbit: 0,21 UA (61 days)
Ecc: 0,03?
Planet ‘e’
Mass: 14,5 mass of Earth
Orbit: 0,33 UA (124 days)
Ecc: 0,06?
Planet ‘c’ lie in the hot edge of the habitable zone and planet ‘b’ lie in the cold edge, i like planet ‘b’ more because the huge mass, i think that means more chance for big moons of the size of Earth or at least the size of Mars.
Fernando, it’s very easy to share this on Facebook just by pasting the link into a posting there, and ljk frequently does so.
Evidence of life on Mars may not prove a second genesis, unless the biochemistry is different from that on Earth. It’s frustrating to realise that 36 years after the Vikings landed we still cannot yet either rule in or rule out life on Mars. Or indeed anywhere else in the Solar System (Venus, Europa, Enceladus, etc.). Need to remember we still have a lot of homework to do here first.
Stephen
Oxford, UK
Stephen is right, Fernando. Larry Klaes always reposts Centauri Dreams posts in the Interstellar Travel page, or you can feel free to connect with me — I’m on Facebook under my name rather than as ‘Centauri Dreams.’ On Twitter, I’m @centauri_dreams.
@Rangel
Epsilon Eridani is the closest exoplanet hosting star. Beats GJ 876 by a few light years.
@Thomas Tarrants: that of course depends on how much you believe the data obtained for Epsilon Eridani. There are definitely issues with stellar activity, the case for planets in this system may be rather less secure than one would like. See for instance Anglada-Escudé and Butler (2012).
I know this a repetitive and plaintive “are we there yet?” question from the backseat, but…..
What is the best guess for when the three Alpha Centauri searches will report their first positive or negative results? Might we hear something this year? Two more years? Three? Thanks for any informed guesses!
I’m still trying to wrap my head around the idea of a “complex” extrasolar system smaller than Mercury’s orbit around the sun.
Wouldn’t all these planets be tidally locked though?
And how does that affect an interstellar probe’s mission plan?
Paul, I’ve got two questions for you. If you would please what was the name of a previous book that you were reading and had posted the cover of prior to the one that you are going through now. Secondly you mentioned the following ‘Larry Klaes always reposts Centauri Dreams posts in the Interstellar Travel page’; what is the Interstellar Travel page that you are referring to? I don’t believe I’m familiar with that.
may be a little ot, but how was the 48. joint propulsion conference??
https://www.aiaa.org/JPC2012/
was hopping to find at least an article about it here??
may b u guys are just typing it :)
i hope so, keep up the great work and thanks for centauri-dreams.org!
Exomoons in habitable zones is a concept that I became aware of only in the last few years.
Detection of Exomoons was a topic at the American Astronomical Society meeting in Austin last January.
It’s easier to explain planets without moons , Mercury and Venus (solar tidal effects) than to understand the origin of planets with moons. Now we have dwarf planets with moons, and asteroids that are binaries and even asteroids that ‘kind of’ have moons.
Planets and planetary systems now fill the galaxy…. so must their small fry partners.
Habitability of Exomoons now seems a important concept to explore, there are papers , I expect to see more.
It’s odd… no well known science fiction prose story jumps to my mind using Exomoons…. tho someone may have a story from the 1940’s using one!
I can think of stories where there is Terraforming of solar system moons.
The idea for Avatar’s Pandora must of had it’s origin in SF prose somewhere.
william writes:
The previous book was Kelvin Long’s Deep Space Propulsion (Springer, 2011), about which more next week. The Interstellar Travel page is on Facebook:
http://www.facebook.com/groups/47450538997/
and please feel free to drop by.
Eric writes:
It’s still only a guess, but I wouldn’t be surprised if something came out later this year. It could be longer — Alpha Cen is a very, very tough nut to crack!
advances space propeller writes:
We had several people there and I’ll pass along any news as it becomes available.
Raz Ramli writes:
Right now getting there is the overwhelming problem, coupled with the huge issue of deceleration. If we can manage those feats, actually investigating tidally locked planets will be fairly straightforward by comparison.
And finally, Al Jackson writes:
Let me dig around on this. As you know, I’m a great fan of science fiction from this era (1940s and 50s), and maybe I can track something down. Some of the readers may also want to weigh in on this. Of the top of my head, I’m recalling an exomoon setting in one of the early Science Wonder Stories with a Frank Paul cover (very early ’30s), but let me ponder which one it was.
@ AA Jackson. I’m fairly certain that McDevitt has an exomoon location in one of his stories – either the Alex Benedict or Priscilla Hutchison series.
Centauri Dreams just had a post on exomoons a few months ago in regards to a group searching for them via Kepler data right here:
https://centauri-dreams.org/?p=22193
Perhaps the most famous exomoon of recent years outside of Pandora from Avatar (read known to most of the general public) is the forest moon of Endor from the Star Wars universe as seen in “Episode Six: Return of the Jedi”.
http://starwars.wikia.com/wiki/Endor
Apparently this Earthlike satellite was once circling a gas giant planet, which somehow exploded and in the process not only left no traces of itself but also managed not to wreck Endor or its life forms on it. Or this may have happened a really long time ago and did harm that world but it recovered – which then begs the question how do the present residents of the Star Wars galaxy know it was once a moon? Or does any of this really matter?
Two real moons were featured in Clarke’s 2010 novel from 1982 and film from 1984, Jupiter’s Io and Europa, with the latter having some native life forms that the Monolith ETI were big on nurturing and protecting.
http://en.wikipedia.org/wiki/2010_(film)
A lot of old SF stories featured life on the larger moons of the giant planets. A lot of old bad-SF had life just about everywhere. My fave, for old & bad SF, is 1940s “Buck Rogers” comic strips, which featured characters getting castaway on meteoroids possessing breathable atmospheres.
While it isn’t an exomoon, H.G. Wells’ 1901 novel “The First Men in the Moon” dealt with the Earth explorers’ adventures and misadventures with the Selenites who inhabited our satellite. Also:
“When Worlds Collide” (first published in 1932 as a magazine serial and in 1933 as a novel, co-written by Philip Wylie and Edwin Balmer) featured the rogue giant planet Bronson Alpha and its terrestrial planet-sized moon Bronson Beta, which was colonized by refugees who escaped from Earth before it was destroyed. After their arrival, they discovered traces of a native civilization that had been destroyed millions of years before, when the moon and its giant planet primary were torn away from their parent star.
In the “Star Trek” universe, the home world of the Andorians (one of the member races of the United Federation of Planets) was a habitable but cold Earth-like moon called Andoria (also called Andor), which orbited a ringed gas giant planet. These particular details of the Andorians’ home world may not date back to the beginning of the “Star Trek” franchise, however.
Assuming superjovians do form large enough moons to be “habitable” by which I mean in this context the ability to retain substantive atmosphere and liquid water on the surface, the problem with these exomoons around M stars is that they’re SO close to the star that huge tidal effects occur as the exomoon orbits the planet. Imagine oceanic or large lake tides from tens to hundreds of meters, never mind the volcanic implications as the internals get sloshed around. Hey, they’d be great places to film Hollywood disaster movies though.
And don’t forget, from the same Scifi universe, the densely forested Yavin IV.
Though its originator didn’t consider his material to be SF, Immanuel Velikovsky’s Mythological Cosmogony featured Earth as a former moon of either Jupiter or Saturn. Saturn then experienced a small nova event and Earth was thrown out of orbit… or something to that effect. Velikovsky didn’t consider physical possibility to be as significant as vague mythological references when deciding on what had happened in prehistory.
Regarding exomoons in science fiction (specifically habitable ones):
1) The “War World” series, written by various authors and set in Jerry Pournelle’s CoDominium/Empire of Man series, is set on a harshly habitable exomoon called Haven. (My impression is that the stories vary in quality; I’ve only read a few.) See: http://www.goodreads.com/series/57881-war-world
2) A series of stories by various authors have been written about an exomoon called Medea, which orbits a superjovian planet of a red dwarf. (I think a similar description applies to Haven, but there I’m not sure about the red dwarf part.) See: http://en.wikipedia.org/wiki/Medea:_Harlan's_World
I’m pretty sure I’ve read other stories featuring habitable exomoons, but these are all that I can think of right now.
Oops, a few more examples of SF stories featuring habitable exomoons (biggies — a couple classics plus a recent major-seller):
– The Dispossessed by Ursula K. LeGuin
– Orphans of the Sky by Robert Heinlein (per Wikipedia, the ultimate destination of the world-ship)
– Coyote by Allen Steele
Adam Crowl said on August 4, 2012 at 19:22:
“A lot of old SF stories featured life on the larger moons of the giant planets. A lot of old bad-SF had life just about everywhere. My fave, for old & bad SF, is 1940s “Buck Rogers” comic strips, which featured characters getting castaway on meteoroids possessing breathable atmospheres.”
The Twilight Zone was guilty of having habitable planetoids as locations in a handful of its episodes. And in another episode Earth was just 11 million miles from an alien planet in another solar system. Rod Serling was a great writer but he never got too wrapped up with the scientific details. Folks may have been a wee bit less concerned about those kinds of things back in the day, but now they are just glaring, at least to me.
philw1776 – There seems to be a limit to the size of a moon – slightly smaller than Mars in mass. A Mars sized moon that forms in the outer reaches of a solar system and then migrates with its planet into the star’s habitable zone will be a lot like the Monty Python parrot sketch: an ex-Mars sized moon.
Warm up an icy ball the size of Mars to room temperature, and it’ll quickly evaporate into a much smaller and less interesting world.
Even our own rocky Mars is not massive enough to retain a substantial atmosphere where it orbits now. Warm it up by moving it further into the HZ, and it’ll be just another big rocky, airless world.
The only way you’re going to get an Earth-size, habitable moon is via capture of an already existing planet, when the gas giant is migrating inwards. You then have to hope that the dynamics of the capture don’t disrupt the new moon and that the new moon is protected by the gas giant’s magnetosphere.
It’ll be interesting to look at the hot (or warm) Jupiters and see how many have captured a massive moon; they’re the low hanging fruit since the periods are very short. My guess is that the number will be very close to 0.
Speaking for myself, I am really happy to see that assortment of intrusive buttons gone. They were slowing down pageloads, and had a nasty way of popping up when not needed.
Rangel:
Gl 876 b is well outside the HZ (from 0.10 to 0.16 AU) , even if this is based on total bolometric luminosity (0.012 * solar) and assuming an outer HZ boundary at 1.5 AU in our solar system. In other words: total illumination for b would be less than for Mars (about 60%), rather on the cold side.
Planet c would be smack in the middle of the HZ. Too bad it is between Saturn and Jupiter size (closer to Jupiter).
Since largest giant planet moons seem to be only about 1/5000 of their mother planet, a truly gigantic giant planet would be required to get a minimum earthlike planet (about 1/3 Me), something like at least 3 Mj or more. The disruptive tidal effects and radiation belts of such a giant planet could be huge.
See FrankH’s comment on earthlike planets captured as neo-moons by inward migrating giant planets.
And see my comment under the previous post (on Gl 581) about other giant planets in their (solar type) star’s HZ. A few interesting ones within 70 ly are: 55 Cnc f, HD 147513 b, Ups And d, HD 176051 b, Mu Arae b, HD 210277 b, 16 Cyg B b.
Ronald and FrankH, the problem with assuming that a Mars sized planet is too small to hold an atmosphere is that it is an educated guess based on the limited data from our system. If we took Mars away, we would have worked from the example of Titan, and guessed that if a cold body one fiftieth the mass of Earth can hold an atmosphere denser than Earths, a planet one tenth Earth’s mass should be fine for a 1au equivalent. My point is that if we base it only on fundamental principles, a Mars sized planet is more than heavy enough to hold an atmosphere.
And Ronald, models seem to consistently spit out sever limits to the size of natural moons around gas giants so you would be on better grounds here, but for one thing. Captured moons should be rare, but Tritans retrograde orbit implies that we must have at least one such capture in our own system.
@Rob Henry,
I think we’ve gone over this before.
It’s not an assumption that a Mars sized planet is too small to hold an atmosphere; it’s just basic physics. If a gas is hot enough so that the average molecule is moving close to 1/6 (or faster) of the planet’s escape velocity then most of those gas molecules will eventually escape (eventually = millions but << billions of years).
Escape velocity is determined by a planet's mass and radius. Those are the variables.
At Mars' distance from the Sun, it's just cool enough to hold on to a tenuous atmosphere. Not having a strong magnetic field means that charged particles from the Sun can warm up its upper atmosphere directly, leading to more loss.
Move Mars to 1AU and it would be just a bigger, airless version of our Moon.
You example of Titan is a perfect example of the physics involved. Titan can retain a substantial atmosphere ONLY because it's far away from the Sun. Move it a little closer, say to Jupiter, and it would be just another icy moon like Ganymede with a pretend atmosphere of few gas molecules between its surface and hard vacuum. Move it to 1AU and it would quickly stop being a large icy moon and would end up as a much smaller, rocky, airless moon.
Again, this handy interactive web page offers a simplified but still accurate way of determining if a world will retain an atmosphere:
http://astro.unl.edu/naap/atmosphere/animations/gasRetentionPlot.html
Yes FrankH we have gone over it before, and you know it just as well as me. Yet I never seem to see how that knowledge fits your argument.
Non-thermal atmospheric loss mechanisms are so complex to model that they are poorly understood. Thermal loss mechanisms provide the only solid background with which to work and as you stated, an escape velocity 6 times average molecular velocity (at the exobase) equates to an effective permanent retention. Actually, just 5x it results in retention for about a hundred million years.
For Earth this permanent thermal retention velocity is 1900m/s, for Mars 840m/s and for Titan 440m/s if calculated from their surfaces. This means that the heaviest gas that each could retain at Earths average surface temperature of 287K has a molecular weight of 1.3, 6 and 25 respectively.
See how we can only rule out hydrogen and helium retention for a warm Mars sized planet. Even for Titan, diatomic nitrogen and oxygen are retained. Of cause, for a better approximation of thermal loss we have to work out the temperature profile of the atmosphere all the way to the exobase, and this is tricky – if not impossible – except from direct measurements. From direct measurements, we know that Earth easily retains helium against this type of loss.
Non thermal loss mechanisms are even worse and no one has even explained how Earth manages to loose sufficient radiogenic helium to match our current atmospheric level. This is why I see the confidant statement that Mars is too small to hold an atmosphere as being closer to a wild guess than a demonstrated situation.
Oh, how I hate the paradigm approach to science!
As I pointed out the last time you posted that link, it is fairly misleading. The relevant temperature to consider is the temperature at the exobase, which for Earth is far higher than it is for Venus. So while it may give results that seem somewhat plausible, the underlying physics is not on a firm footing.
The situation with Earth’s helium seems to be fairly clearly reliant on non-thermal processes, e.g. the polar wind (see for example this conference abstract), though the full picture still needs to be worked out. Unfortunately it is not a particularly easy subject to find out about, a typical Google search will be polluted by the blatherings of young-Earth creationists who have seized on this as one of the key pieces of “evidence” for their beliefs.
@Rob,
Atmospheric escape doesn’t happen at the surface; it happens at the top of the atmosphere. What’s lost is going to be replenished from below. When that’s lost, you’re done.
There’s no way that Titan, as currently configured, would survive at 280K for the duration of the solar system, or even a few hundred million years. You can argue variations in the loss rate, but over 3 or 4 billion years it doesn’t matter.
Please find and cite a source that refutes Jeans theory (the 5x – 6x ratio) or the Maxwell-Boltzmann distribution curve for atmospheric loss.
Helium in the Earth’s atmosphere is mostly from radioactive decay. The rates of production is pretty well known and the loss rate can be calculated from the Maxwell-Boltzmann distribution used in the atmospheric loss calculator, and they match observation… unless you’re a Creationist, in which case… aliens.
Andy, although I also implied that Earth’s helium loss had to be via non-thermal mechanisms, and used the lack of any model fitting the data well as an example of how poorly understood such processes were, I have this worry…
Our consensus here just reinforces that paradigm approach that I hate. From what I have read, the loss could be entirely thermal if the exobase was 300K hotter than at present at least 1% of the time. In that regard, the *noise* from the Kepler data is worrying, since if our star is really average, it has been abnormally quite through the last few centauries. Could it be that if we were viewing it in a more typical period, we would see helium loss also as a product of thermal loss and figure that non-thermal mechanisms were negligible? Please keep that thought at the back of your mind, while we are placing non-thermal explanations at the front.
FrankH, Indeed atmospheric escape does not happen direct from ground level. The difference for Mars and Earth is small, but for Titan it is rather important, so this simplification for my calculation was one that I had to state.
You persist in finding relevant models without calculating results from them or referencing calculated results. Further discussion would be more fruitful if you started doing so.
It’s odd… no well known science fiction prose story jumps to my mind using Exomoons…
Poul Anderson’s “The Longest Voyage” is set on an Earth-sized satellite of a jovian exoplanet. It’s fairly well known — it won the Hugo Award for Best Short Story in 1961.
Stagnant Interiors Suppress Chances of Life on Super-Earths
by Staff Writers
Madrid, Spain (SPX) Sep 28, 2012
The team has found the propensity of plate tectonics to rather decline with planetary mass. But they also find that water in the lithosphere can easily buffer these effects. Hence plate tectonics on super-Earths is not inevitable, but rather depends on a set of unknown planetary characteristics, which can’t be observed on exoplanets in the near future.
Exoplanet hunters estimate that there could be billions of super-Earths – planets with a mass of up to ten times that of Earth – orbiting stars in the Milky Way alone.
But do super-Earths really deserve their name and would they be capable of hosting life? A study of the thermal evolution of rocky super-Earths suggests that they may bear very little resemblance to our home planet. Dr. Vlada Stamenkovic will present the results at the European Planetary Science Congress on Wednesday 26th September.
“We are discovering planets orbiting distant stars that are similar to Earth in composition but more massive than Earth. The major question is: are they just scaled-up versions of Earth, or are they fundamentally different?
Full article here:
http://www.spacedaily.com/reports/Stagnant_Interiors_Suppress_Chances_of_Life_on_Super_Earths_999.html