In a sense the planets discovered around the Sun-like star HD 10180 are no surprise. We’ve long assumed that planetary systems with numerous planets were common. We lacked the evidence, it’s true, but that could be put down to the limitations of the commonly used radial velocity method, which favors massive worlds close to their stars. But we’re getting much better at radial velocity work and, using instruments like the HARPS spectrograph at the European Southern Observatory’s La Silla (Chile) telescope, we’re teasing out ever more exquisite signals from distant systems. More and more multiple-planet scenarios are in our future.
Noting that high-precision radial velocity surveys are now able to detect planets down to roughly 1.9 Earth masses, the paper on the HD 10180 work frames the situation this way:
Preliminary results from the HARPS survey are hinting at a large population of Neptune-like objects and super-Earths within ?0.5 AU of solar-type stars (Lovis et al. 2009). Moreover, hundreds of small radius candidate planets have been announced by the Kepler Team (Borucki & the Kepler Team 2010). Clearly, the exploration of the low-mass planet population has now fully started, and will become the main focus of the field in the coming years.
But five planets at one go is still an eye-opener, especially when you consider that two others are also possible here. It took six years of study of this star, some 127 light years away in the constellation Hydrus, to bag the five leading signals, representing planets like Neptune in being between 13 and 25 Earth masses. These worlds circle their star in orbits that range from six to 600 days at distances between 0.06 and 1.4 AU. Some accounts are citing similarities with our Solar System because of the number of worlds, but we might just as well note the differences, including the crowding of the inner system and the presence of massive planets there.
Image: This wide-field image shows the sky around the star HD 10180, which appears as a fairly bright star just below the centre. The picture was created from photographs taken through red and blue filters and forming part of the Digitized Sky Survey 2. The field of view is approximately three degrees across. The coloured halos around the stars are artifacts of the photographic process and are not real. The remarkable planetary system around this star is far too faint and close in to be visible in this image. Credit: ESO, Digitized Sky Survey 2. Acknowledgement: Davide De Martin.
The paper notes that systems like this open up new realms of study:
It is expected that the characterization of planetary system architectures, taking into account all objects from gas giants to Earth-like planets, will greatly improve our understanding of their formation and evolution. It will also allow us to eventually put our Solar System into a broader context and determine how typical it is in the vastly diverse world of planetary systems. The characterization of a significant sample of low-mass objects, through their mean density and some basic atmospheric properties, is also at hand and will bring much desired insights into their composition and the physical processes at play during planet formation.
Those two additional worlds, whose existence Christophe Lovis (Observatoire de Genève), lead author of the cited paper, says is supported by solid evidence, include a 65 Earth-mass gas giant in a 2200-day orbit and a world that, if confirmed, would be the least massive exoplanet yet discovered, with a mass of about 1.4 times that of the Earth. This one is not exactly a candidate for astrobiology, though, orbiting the host star at a distance so close (0.02 AU) that a planetary year lasts a mere 1.18 Earth days. This ESO news release likens the possible world to the rocky inferno CoRoT-7b.
If there were a gas giant like Jupiter in this system, we should have evidence of it. And note that the orbits of all these planets seem to be almost circular. Says Lovis:
“Systems of low-mass planets like the one around HD 10180 appear to be quite common, but their formation history remains a puzzle.”
Although HD 10180 presents us with one of fifteen planetary systems known to have at least three worlds, that number will grow quickly. The new planets were found in a radial velocity survey of about 400 bright FGK stars in the solar neighborhood using HARPS, and the paper notes that ‘many new systems are about to be published.’ We’re homing in on the ability to derive statistical properties of the low-mass planet population, a new phase in the exoplanet hunt, one that focuses on complex planetary systems rather than individual planets.
From the paper:
The HD 10180 system shows the ability of the RV technique to study complex multi-planet systems around nearby solar-type stars, with detection limits reaching rocky/icy objects within habitable zones. Future instruments like VLT-ESPRESSO will build on the successful HARPS experience and carry out a complete census of these low-mass systems in the solar neighborhood, pushing towards planets of a few Earth masses at 1 AU.
The paper, submitted to Astronomy & Astrophysics, is Lovis et al., “The HARPS search for southern extra-solar planets. XXVII. Up to seven planets orbiting HD 10180: probing the architecture of low-mass planetary systems” (full text).
Fascinating discovery!
From basic data that I could find, the star HD 10180, a G1V solar type, has a Mv (absolute magnitiude) of 4.35 – 4.37, corresponding to a luminosity of 1.53 – 1.56 times solar, or comparable to (just slightly more than) Alpha Centauri A.
This means that the (liquid water based) habitable zone extends roughly from 1.2 to 1.5 AU (conservative estimate, based on a HZ in our own solar system from 0.95 – 1.2 AU).
This means that planet HD 10180 g is well within this star’s HZ.
I don’t expect a Neptune/Uranus class, 20 earth mass, planet to be habitable in the terrestrial planet sense, nor would I expect such a planet to have a very large, terrestrial planet like, moon.
But fascinating all the same.
It’s gratifying to read these updates which display the continuing advances in astronomy studying other solar systems. What an era of exploration this is: we live in the dawn period of exoplanet discovery. It ranks with Herschel’s discovery of the seventh planet.
This is a big development! I was excited when I saw this on the online news this morning. Finding solar systems with numerous planets around other stars definitely has big implications for the exoplanet hunt – namely that we can’t assume that our kind of solar system is rare.
A terrestrial exoplanet only slightly larger than earth is also of great interest, even if it’s hotter than mercury. Once again, the evidence indicates that it’ll only be a matter of time before we find another earthlike world out there.
Once again, the radial velocity people conveniently forget the 0.020 Earth masses (=1.6 lunar masses) planet orbiting PSR B1257+12. These people should stop making these kind of claims, or at least remember to qualify them (and yes, the claim that this is the least massive exoplanet is right there in the discovery paper, available at ESO here). Sorry folks, you still have almost two orders of magnitude to go…
AMAIZING. It´s like being back into the 17th century, when enormous and fundamental scientific discoveries where made one after another. We´re building up the foundations for a brand new line of investigation wich may last for hundreds of years. (Sorry for my bad english)
The paper, submitted to Astronomy & Astrophysics, is Lovis et al., “The HARPS search for southern extra-solar planets. XXVII. Up to seven planets orbiting HD 10180: probing the architecture of low-mass planetary systems.” I’ll link to the preprint as soon as it is available./
The preprint is already available at the ESO website: http://www.eso.org/public/archives/releases/sciencepapers/eso1035/eso1035.pdf
Thanks for the link, Dunkleosteus — I’ve added it to the post.
Well, Neptune has Triton (although I suppose it is an oddball as these things go).
Triton has less than a third of the mass of our moon, and one of the easiest possible astronomical observations will show you what a moon-mass world located in the habitable zone of a G-type star looks like…
If the Nepture g was created by minor planet colissions like Earth-Moon it could have a moon of the size of Earth. With enough mass, dense atmosphere and magnetosphere it could be habitable.
I’m guessing if we put the Neptune system in Earth’s orbit, Triton would look very different from our moon. Also I’m trying to imagine what a moon like Europa / Titan / Enceladus would look like if they were closer / warmer.
Touche!
>I don’t expect a Neptune/Uranus class, 20 earth mass,
>planet to be habitable in the terrestrial planet sense
Are you thinking gas giant? What if its a rocky world.
Is there any research on what a 20 earth mass “earth” might be like?
Would its larger mass help it maintain a low and stable tilt ?
(I’m guessing all the other large planets around it wouldn’t help this)
In the paper they mention that a stable orbit exists between planets f and g, that its in the habitable zone and that they wouldn’t be able to detect anything under 10 earth masses in that orbit.
I’m waiting on the device that comes after the VLT-ESPRESSO.
We really need to be able to detect a .80 earth mass planet out to 2 AUs.
Of course, that now raises the question: What is the smallest mass a body can have and still keep an appreciable atmosphere in the habitable zone? Could we reasonably expect any gas giant moon to be sizeable enough, or do gas giants occupying a habitable zone rule out habitable moons (except for non-solar heated worlds)?
I’ve read that something >0.5 Earth mass is required not to lose most of the atmosphere and oceans at the Earth’s point in the HZ. Clearly, lunar masses (1/80th Earth mass) can’t hold volitiles in the HZ and Mars masses even further out lose them over less than a gigayear.
Tulse: “What is the smallest mass a body can have and still keep an appreciable atmosphere in the habitable zone?”
From what I have read: probably somewhere around 0.25 – 0.35 earth mass, assuming roughly earthly density. That is, to keep an appreciable atmosphere for a period of time long enough to allow for long-term evolution (>= 3 gy). Besides, such a minimum mass is not only required to hold on to an atmosphere, but also to maintain long-term geological activity (i.e. keep a ‘living’ planet).
Mars, for instance, at 0.11 Me, has not only lost most of its atmosphere, but is also geologically (almost) dead.
If, as is often quoted, the largest moon of a gas giant is indeed about 1/5000th part of its mass, then, to get a minimal sufficiently large terrestrial moon, a gas giant of about 3 Jupiter masses is required.
I have also wondered (as Michael above) why a habitable planet could not be much larger than Earth. Why not have a Neptune sized rocky planet in the HZ?
I was under the impression that Mars main problem was that it’s magnetosphere turned off at some point allowing solar winds to strip the atmosphere / water. During the first billion or so years it had a working magnetosphere, oceans and an atmosphere.
Why would the largest moon of a gas giant be 1/5000th of its mass? Where did you read that? I’m hoping for binary Jovian planets.
Also, Epsilon Indi has a binary dwarf pair, with a separation of 2.1 AU. That’s awfully far apart, but I wonder how extensive their collection of moons would be.
stephen: the source of that appears to be this 2006 paper by Canup and Ward, who suggest that the total mass in a gas giant’s satellite system should be a few 1/10000ths of the mass of the planet. They link the origin of that ratio to satellite migration within the circumplanetary disc: more massive satellites migrate inwards faster and hence rapidly fall into the planet.
As for the scenario of satellite capture, you need to transfer the energy and momentum of the captured satellite elsewhere to cause it to end up on a bound orbit. One fairly easy way to do this is to have something of comparable mass already in orbit around the giant planet, which suggests that captured satellites will have masses comparable to the mass of the original satellite system. Note that Triton has a mass 0.0002 times that of Neptune, fitting the scaling rule despite the evidence that it is a captured satellite.
How long a world keeps its core dynamo going, and thus generating a magnetosphere, is dependent on size. Not necessarily in a straightforward way. It’s theorized that Mars dynamo shutdown because of asteroid impacts during the heavy bombardment, rather than due to cooling. But a larger planet is less susceptible to that, and a better chance of recovering, than a small world.
This area of planetology is still largely theory and models, Venus is not too much smaller than Earth, but its dynamo has shutdown too, and we don’t have a good explanation for why.
Bounty:
LarryD:
I am confused. If Mars lost its atmosphere because of dynamo shutdown, why does Venus still have such a dense one? Something is not right…
It seems the dynamo is not necessary for an atmosphere, and perhaps not for life? Radiation can’t be it, because life is likely to have originated deep in the ocean, where no radiation will reach. I think there are a lot of supposed preconditions for life floating around that are conjectures more than facts. Dynamo, plate tectonics, large moon, etc. Some of these seem to me like attempts to interpret anything that seems unusual about Earth as necessary for life, which is not valid reasoning. Is there a good compilation of “preconditions for life” somewhere with the reasoning and evidence behind them explained?
I’m not sure one can come up with a reasonable list of “preconditions for life” based on a sample of one (especially since we are continually finding life on earth in spots that we thought it couldn’t exist). I’d argue that, until we actually work out the processes involved in generating synthetic life from scratch in the lab, we won’t have a good idea as to the constraints involved (e.g., what chemical processes need to be available; how much energy in the environment is required; etc.).
Indeed. It has always irked me when people assume that the requirements for life are an approximation of earthlike conditions. Too often, I’ve heard people argue for the rarity of extraterrestrial life by pointing out unique conditions of earth, such as a large moon, magnetosphere, plate tectonics, presence of Jupiter/gas giant, a certain climate, and so on.
Life, if nothing else, is versatile. There’s no reason why life can’t develop in different conditions, whether they be hot or cold, greater or lesser gravity, no magnetosphere, etc. Right here on earth we have extremophiles that exist in the most extreme conditions such as hot springs, arctic ice, nuclear waste, and so on. Based on the great variety of earth life, we must keep an open mind about ET life. It is a reasonable assumption that alien life is at least as fit as earth life. For example, the problem of radiation could be solved by living underwater or in caves. There are so many other workarounds, as well as natural adaptations. Possibilities remain open.
I believe Venus lost its hydrogen to space due to the lack of a magnetosphere. Other than that, I can’t comment.
I am pretty sure that all Earth sized planets lose their hydrogen quickly, with or without magnetosphere, simply because the Maxwell distribution for hydrogen still has a significant density at escape velocity.
Tulse: I agree with you. I was more thinking about a list of preconditions that have been proposed (rather than actual ones), with the rationale and evidence (or lack thereof) given for each. That way, it would be easier to cut a swath through the thicket that seems to exist in this area of astrobiology. The “Rare Earth” book might be a good place to start collecting material for such a list. Is there perhaps one in it?
Big Dan: Thanks for confirming my suspicion. I think you are completely right, besides the Venus hydrogen nitpick.
[blockquote]Big Dan: Thanks for confirming my suspicion. I think you are completely right, besides the Venus hydrogen nitpick.[/blockquote]
Indeed. I’m not too well versed in that particular area, so I can’t discuss it in depth. I just remember reading that the hydrogen on Venus was swept away by solar winds. I’d have to study more about why the hydrogen on Earth got bound up into water while this didnt happen on Venus. I assumed the magnetosphere played a role, but the cooler climate of earth couldve been just as important. I’ll have to withhold until further knowledge.
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGF-4731BRD-67&_user=10&_coverDate=06%2F30%2F1988&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=e732e7a9d7e8ad148141ec4a1c2a9f1b
I believe this means that Venus’s extreme greenhouse effect and the lack of magnetosphere means the hydrogen in the evaporated water was pulled away by the solar wind. So I’m thinking a working magnetosphere protects your hydrogen, therefore water, which is a good thing. Having a nice thick carbon dioxide atmosphere w/o water isn’t my first choice for a living planet. Having said that, I think the magnetosphere from a gas giant might protect a moon from getting it’s hydrogen stripped?
I think I understand now: While molecular hydrogen can easily escape without help from the solar wind, the solar wind can dissociate water molecules and free the bound hydrogen. Thus, a magnetosphere may protect the water in the atmosphere from being dissociated, and keep the hydrogen bound in that water from escaping. Neither Mars nor Venus have much water, so this role of the magnetosphere in preserving water makes some sense.
With regard to Venus and its lacking magnetosphere, internal dynamo;
I understood that the main reason for this is considered to be the lack of convection in the core and mantle, also resulting in the shutting down of plate tectonics.
This in turn is often thought to be the result of the dryness of the mantle, i.e. lack of water. So water itself may play an important role in the continuation of plate tectonics and the internal convection.
The heat and lack of a protective magnetosphere resulted in the loss of water through dissociation of H2O (and loss of H2).
Venus seems to have possessed much more water in its youth, but most of that water evaporated (and dissociated) due to the heat. I am not sure whether that was principally due to an initial smaller amount of water than on earth, or due to Venus’s proximity to the sun (about twice as much irradiation as on earth).
So there seems to be a kind of vicious circle: the initial lack of water through evaporation and dissociation, resulting in lack of plate tectonics and magnetosphere, resulting in more loss of water and preservation of heat, etc.
The root cause of it all, in comparison with the earth, is probably simply Venus’s proximity to the sun.
The gravity on a Neptune-like world would very likely be far too strong to allow the development of anything larger than a few cells thick. While life might thrive there, we are not hoping to communicate with microbes.
On Venus the temperatures are sufficiently high that water can get into the stratosphere, where it can easily be photodissociated by ultraviolet and rapidly escape to space. On Earth there is very little water vapour above the thermopause, thus preventing significant water loss.
Billsey: you might want to check out what the surface gravities are for the giant planets in our solar system, you might be in for a surprise… That being said, there are many other potential issues regarding gas giants as potential homes for life.
Thanks for the info on Venus, interesting…
Indeed.. the equatorial surface gravity of the gas giants are as follows (from wiki):
jupiter: 2.528 g
saturn: 1.065 g
uranus: .886 g
neptune: 1.14 g
As you see, only Jupiter has significantly more gravity than earth. Uranus has less, Neptune has slightly more, and saturn is almost the same.
Certainly gravity plays a large role in what sort of life develops. However, I wouldn’t be surprised if life could exist even in a high gravity environment, much like creatures at the bottom of the ocean who survive enormous water pressure.
As has been pointed out, surface gravity increases only mildly with the mass of the planet. Specifically: With the third root, assuming constant density. The maximum size of an object before it collapses is proportional to the inverse of gravity. A planet with 1000 Earth masses and Earth-like density will have a 10 g surface gravity and will support animals one tenth the size of a dinosaur, which is still quite large.
In water, the effect of gravity is almost neutralized, and on the microbial size scale it becomes irrelevant. Even 10,000 g would not interfere significantly with microbial life.
So, how about that Neptune-massed but Earth-like habitable rocky planet, is there a reason it should not exist?
Eniac, from what i gather a planet above 2 earth mass collects too much gas in the planet froming process leading to a thick atmosphere miles thick like neptune. No sunlight reaches the surface etc. Bad news for complex life like ours.
Further to Eniac and yeti101: I read somewhere (forgot exactly where) that the threshhold for a runaway thick atmosphere (also holding large amounts of helium, ammoniak, methane) is somewhere around 3 – 3.5 earth mass. But I wouldn’s mind being amended here.
I really enjoy the discussions here of various characteristics needed for an earth-like world, and why they are needed (e.g., magnetosphere prevents water dissociation; > 3-3.5 earth mass causes runaway think atmosphere; < .25 earth mass fails to keep atmosphere). Is there anywhere where all such information on habitability criteria (or at least "earth-like" criteria) are collected in one spot and explained?
yeti101, Ronald: Thanks for the explanation. That makes sense.
I am thinking that the threshold may depend on the relative amounts of dust vs. gas in the protoplanetary disc, or, in other words, on the metallicity of the system. Perhaps as stars are becoming more metallic, the size range of rocky planets increases?