The smallest super-earth?
Published on May 31, 2008 at 1:34 pm.
7 Comments.
Filed under extrasolar planets.
Recently Christophe Lovis of Switzerland’s Geneva Observatory presented preliminary findings of work done with the European Southern Observatory’s High Accuracy Radial velocity Planet Searcher (HARPS) in which he reported several dozen extrasolar planet candidates. One extremely interesting point in this report is that these planets were all much smaller than Jupiter. The way that these planets are detected is through monitoring the motion of their star as they orbit. HARPS is able to measure the radial motion of the star (back and forth along our line of sight). From that, knowing the type of star, we can infer the mass of the star, and thus the mass of the planet making it wobble back and forth. However, the mass that we get is really only accurate if our line of sight to the star is very nearly in the plane of the planet’s orbit. That is not really known. If we are looking from an angle out of the plane of the orbit, then the mass that we compute is too small. So, the masses Lovis presented may be too small in some cases. However, it would be expected that some of them may be right on, or very nearly so. In that case, then some of these planets may indeed be only a few times the mass of Earth. Planets that size have come to be called super-earths. Since the results are a work in progress and the findings need to still be verified, they are to be regarded as tentative. So, to avoid embarrassment his team isn’t releasing the final details until they can verify the results. This is how good science is done. You don’t release unverified findings that have to be retracted later.
One further thing that I found interesting was that one of these possible super-earths had a mass that was reported to be as low as only 4 Earth masses. Now, I thought that was interesting because that is getting small enough to consider habitability. The giant Jupiter-sized planets, which are the easiest to detect, are not likely to be something that could support life as we know it. Smaller planets, Neptune-sized worlds, such as were found a few years ago, are also unlikely to be very habitable. But, rocky worlds that are Earth-sized are a much more likely candidate. Apparently none of the planets that Lovis found are at the right distance from their respective stars to be habitable. But, his findings suggest that worlds of this size are quite common, with super-earths possibly outnumbering Jupiter-sized planets by a factor of 3 to 1. If that is true, then some of the as-yet-undiscovered ones are undoubtedly located in the habitable zone of some planets.
In our own Solar System, we see that smaller bodies are more common than larger ones. There are two giant worlds made mostly of hydrogen and helium: Jupiter and Saturn. There are two other very large worlds that are water rich, with atmospheres of hydrogen and helium: Neptune and Uranus. And, there are four small rocky worlds: Mercury, Mars, Venus, and Earth. And, there are a host of small rocky bodies with some ice (asteroids) and icy bodies with some rock (comets and Kuiper belt objects). So, if our Solar System is even somewhat typical (still a debatable point), then we may expect similar distributions of planets elsewhere. So, it may be that there would be an occasional planet like Earth or a super-earth in the habitable zone of a star.
But, is a planet 4 times more massive than Earth really a candidate for habitability? One could argue that a planet too large might hold onto too much atmosphere, making for too great of a greenhouse effect. Once could also argue that too much gravity would impede the development of higher life forms. Really, these arguments can only go so far, because we simply don’t know enough about these matters. We really don’t even know what sort of composition these Earth-sized or super-earths might really have. But, still, we can have a bit of fun. If we assume that a planet has the same density as Earth, then if it had four times the mass, you can do a back-of-the-envelope calculation showing that it would have about 60% higher gravity. Now, that is something that I find interesting. You could make an argument that a planet with four times Earth’s gravity would have significant differences to Earth. It would make the atmosphere thicker (simply from compression due to the gravity), it would make tall plants unlikely, and it would make pumping blood difficult for higher life forms. Those factors are not insurmountable to life, perhaps. Life is very adaptable. But, how much can life adapt? We don’t know. But, a gravity only 60% higher than Earth’s would perhaps not be very different from Earth. It would seem to me that many Earth life forms would likely be able to survive in 1.6g gravity. Of course, I’m an astronomer, not a biologist. So, I could be wrong. But, as I said, I am just having fun with a very simplistic calculation here.
I will find it interesting to hear about the final report from Lovis’ team. I know that the people who are doing research in astrobiology are doing all sorts of theoretical studies about what life would be like in various conditions. Only a few years ago, the only planets that we knew were those in our Solar System. Then, we found some other planets, but they were gas giants, not Earth-like planets. Those are the easiest to find. But, out technology is getting to the point that we are almost able to find planets almost like Earth. These are exciting times in planetary astronomy.
-Astroprof
Sili on May 31, 2008 at 4:49 pm: 1
Is the method that looks at the dimming of stars when a planet passes complementary to the radial method?
If not, can that perhaps give a better handle on the angling of the orbit?
Michael Welford on June 1, 2008 at 3:02 pm: 2
I don’t think your remarks about the mass distribution of planets in the solar system are quite on the mark. Mars and Mercury are too light to go into the same mass bin as Earth and Venus. I hadn’t thought of this before, but if we ignore moons or better regard them is contributing to the mass of their planetary system, we get 5 quite evenly spaced “planet mass pairs” with Eris and Pluto forming the 5th pair.
There is a factor of about 17 between the pairs. Saturn and to a lesser extent Pluto and Eris are somewhat below their assigned masess. Mars is somewhat above. Below Pluto the mass distribution gets much more crowded. Please get some log-ruled paper and head to Wikipedia and check my results.
I don’t assign much signicance to this “5 pair model”. For one thing I expect the PS1 telescope to find plenty of worlds in the far outer solar system with masses greater then Pluto. Perhaps a lot greater.
Anyway, very good post overall. It gave me plenty to think about. I look forward to hearing more from the Lovis team.
Astroprof on June 2, 2008 at 2:33 pm: 3
Sili, the transit method is definitely complementary. However, it only works if the orbit is exactly lined up correctly, so it won’t help a whole lot.
Astroprof on June 2, 2008 at 2:40 pm: 4
Michael, I understand what you are saying. Mercury and Mars are definitely much smaller than Earth and Venus. However, if you plot Solar System body sizes (excluding satellites) on a number line, you see a near continuum of bodies from tiny through Eris. But, the other eight bodies are clearly separated. That is particularly obvious in a log plot. Eris and Pluto make a nice pair, but so would Pluto and several other of the bodies out there that are just a shade smaller than Pluto. But, indications are that Mercury and Mars formed in a similar manner to Venus and Earth, so in that sense they could still be lumped together. But, pairing Earth and Venus and Mars and Mercury has something to say for it, too.
Michael Welford on June 3, 2008 at 4:57 am: 5
Many years ago I defined a”relative giant” to be a solar system object that outmasses all of the known objects in the solar system less massive than itself. The phrase “known objects” is defined in a way that lets me ignore the unknown mass of comet reservours. The smallest object whose status was obvious was Venus. Venus might remain a relative giant even if PS1 discovers half a dozen objects with masses in the Mercury-Mars range. This is why I put Mercury and Mars into their own separate pair.
“Exomoons” are undectable apart from their primaries with present exoplanet search methods, except maybe sometimes for occultation. So your discussion of relatively small explanets led me to the concept of “relative giant orbital subsystem” where moon masses are added in with mass of their primaries. Now Mars and Mercury get to be giants after a fashion. But not Earth, which is too close to the mass of smaller sister Venus. *sigh*
Now I’ll have to work out what objects are relative giants. It’s a lot easier to get the necessary data in the age of the internet than it was when I was a teenager. But I’m not going tell anyone my results. Why spoil everybody elses fun?
Sili on June 3, 2008 at 4:54 pm: 6
Thanks. I just thought that it might be possible to get a handle on the angle of the planet’s plain relative to us by looking for evidence of a transit. If there’s not dip in the light the angle’s likely to be large - or so my reasoning went.
Astroprof’s Page » An even smaller super-earth on June 6, 2008 at 2:33 pm: 7
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