“Habitable” planet? Maybe not!
Published on Apr 25, 2007 at 6:01 pm.
11 Comments.
Filed under extrasolar planets.
The news media have been all over the discovery of a “habitable” planet around a nearby star. I have heard all sorts of adjectives tossed around about the planet, and most make it sound like a twin of Earth. If you didn’t know any better, you’d think that this newly discovered planet was an idealistic park-like setting complete with trees, birds, and ET’s. It sounds like one of the planets that the starship Enterprise visits on Star Trek: Earth-like. Well, I just wanted to put the record straight. This planet is NOT going to be particularly Earth-like any more than Mars and Venus are Earth-like.
So, why do I say this? Well, first of all, let me say a bit about the planet. Recently, a team of astronomers led by Stephane Udry and Michel Mayor (Geneva Observatory) announced the discovery of two new planets orbiting the star Gliese 581. Already, a larger planet, having over 15 times the Earth’s mass, had been found orbiting very close to the star. But, there were additional wobbles in the star that the team found can be explained by two more, smaller, planets of masses about 5 Earth masses (Gliese 581c) and about 8 Earth masses (Gliese 581d).
Now, finding those two new planets would be a good deal, anyway, since they are among the smallest extrasolar planets found to date (most have been gas giants). However, there have been at least two other extrasolar planets of similar size: Gliese 876d and OGLE-2005-GLB-390Lb (Don’t blame me for the name of that second one!). But, what makes the smaller of the two new planets (Gliese 581c) special is where it is located. The other smaller (possibly rocky) planets are either too close or too far from their star to support life. Gliese 876d is so close to its star that it likely has a surface temperature nearly high enough to melt lead on the daytime side. The average temperature is probably over 300° C. And OGLE-2005-GLB-390Lb is so far from its star that it is perpetually frozen: nearly as cold as Pluto, with even oxygen and nitrogen (if present) frozen solid on its surface. But, Gliese 581c is at just the right distance that Udry et al suggest that the average temperature may fall within the range of -3° C to 40° C. The presumption is that if it is somewhere in the middle of that range, then it may be able to support liquid water, as on Earth. Their paper even makes the claim that “Gl 581c is probably the most Earth-like of all known exoplanets.” (Udry, et al, 2007).
Now, that may be true, but let’s not get carried away here. There is no need to sensationalize this news! While this planet may be rather Earth-like in general characteristics, it is probably not all Earth-like the way that most people are thinking. For one thing, we don’t know the composition of the planet. The parent star, Gliese 581, is a red dwarf star, quite smaller than the Sun, and with a composition that is less metal rich than the Sun (that is astrophysicist talk that means less of anything other than hydrogen or helium). That might suggest that this planet may have a lower percentage of the rocky material that make up the terrestrial planets in our Solar System. Water is a very common substance, so there may be more water in this planet. But, we really don’t know how planets form around red dwarf stars. These very small stars are very slow to form, so one might presume that planets around them have longer in which to form than planets around more massive stars. If so, then there may be time to accumulate more of the metals to make a larger planet than a similar composition proplyd would make around a more massive star. It is hard to say. So, that means that we can’t say much of anything about the composition of the planet. So, how can we call it Earth-like?
If the planet is a planet with consistency similar to that of Earth, the Udry team suggest that it may have a diameter about 1.5 times that of Earth. If that is the case, then with a mass about 5 time Earth’s, the acceleration due to gravity on the surface of the planet will be about twice what it is on Earth. That would likely affect the atmospheric composition. And, certainly, it would make the planet not very comfortable to life forms like us.
But, there is a more serious concern. The parent star, Gliese 581, is a red dwarf. Red dwarf stars are cool and dim. Gliese 581 is an M3 star. That means that it has a temperature of about 3500K (compared with 5800K for the Sun) and is only about 4% as bright as the Sun. Thus a planet must be very close to the star in order to be warm enough (the so-called habitable zone). This planet, Gliese 581c, is indeed close: about 11 million kilometers from the star. It takes the planet only about 12.9 days to orbit the star. At that distance, it may well be that the planet is tidally locked to the star. That would mean that one side of the planet perpetually faces the star, much like one side of the Moon perpetually faces Earth. In that case, habitability becomes problematic. The side facing the star will be baked. The star will never set. The temperature will rise higher and higher. My back-of-the-envelope calculations suggest a sub-stellar temperature possibly up to 200° C (possibly even much hotter), but certainly well above the boiling point of water. The nighttime temperature would be always well below freezing. Only a tiny zone near the terminator would have any hope of habitability, but even that would be tenuous, and it would be likely wracked by storms of unbelievable intensity. Another possibility that has been suggested would be if a planet had some other spin orbit coupling than the 1:1 coupling of the Earth and Moon. Mercury, for example, has a 3:2 coupling (rotating three times for every two orbits). However, Mercury is able to have the 3:2 spin orbit coupling because of its terribly elliptical orbit. There is no indication that this planet’s orbit is anywhere near that elliptical. The other possibility might be a coupling with the much larger planet that orbits just a little closer to the star than Gliese 581c. If it were to couple to that planet, then it might rotate fast enough that some hope of habitability might exist. Of course, it still would likely have nothing approximating an Earth climate. But, really, any speculation as to climate and temperature is quite difficult, since, as I said before, we don’t really even know the composition of the planet! Even slight changes in atmospheric composition (if it has an atmosphere) would radically change climate conditions. This planet might be in a runaway greenhouse situation like Venus (not too hard to fathom given the likely hot temperatures on the daytime side).
So, I think that it is really premature to call this planet “habitable” and “Earth-like.” The real significance, though, is that it probably does have more of the characteristics that we look for in a habitable planet than any other extrasolar planet detected to date, and that brings hope of finding others that are even more of match for what we expect a habitable planet to be like.
-Astroprof
Image courtesy ESA
cephyn on April 25, 2007 at 6:16 pm: 1
haven’t there been some models run that show that a tidally locked planet with a sufficient atmosphere could still have enough heat transfer around the planet to make temperatures sensible?
Astroprof on April 25, 2007 at 6:23 pm: 2
I think that there have indeed been some models that suggest that, but if I recall, those models also call for a significant greenhouse effect, meaning that the planet would need to be a bit further out in order for the temperatures to be reasonable. So, this planet might turn out to be nice after all, but I tend to think that it probably has a rather extreme climate. The big point here, though, is that we need a LOT more data before we start talking about it being a “habitable” planet!
Darnell Clayton on April 25, 2007 at 9:57 pm: 3
Astroprof is right in this regards. A tidally locked world would not be habitable at all.
A movie best displaying this case is the chronicles of Riddick (although the planet did rotate, but you get the general idea).
Although I was also excited, one thing we do not know about is whether or not this world has a magnetic field.
If not, then setting up shop may be useless due to intense radiation. Remember, even Europa has lots of water (although frozen) although it’s surface is too radioactive (thanks to Jupiter) for habitation.
Red dwarfs may have habitable worlds, but I think our best chances lie in searching yellow stars (although red dwarfs are not a bad place to look).
cephyn on April 26, 2007 at 1:05 am: 4
Thanks for addressing my question astroprof - upon considering it, it makes sense. I think you’re right about the studies, my memory is refreshening. Personally, I would tend to agree that this planet probably has some wild weather. Which would be fascinating to study, really. And calling it “habitable” is really jumping the gun - if by “habitable” we mean “for people” - but even with extreme weather, high temps, etc - we’ve seen here on earth that some life can exist in ridiculous settings. I don’t see why it’s implausible that this planet is habitable - for something. Just not us with mai tais on the beach.
But I have to totally disagree with darnell - there’s no reason to say that a tidally locked world is certainly uninhabitable. It might be difficult and limited, but there’s the possibility that parts would be habitable - even for us.
And the surface of europa would be pretty unpleasant, but all that ice provides pretty darn good insulation.
So very, very close | cephyn on April 26, 2007 at 1:21 am: 5
[…] All very exciting. But before we get ahead of ourselves - this planet really isn’t a great bet for life. Maybe only the most extreme of life. But it being tidally locked is a pretty big problem. One side of the planet would bake eternally, while the other would be stuck in eternal night. This would totally hose the chance for any climate reasonable to us. And while the mainstream press (idiots when it comes to reporting science) goes on about it being habitable, unfortunately those in the know have to rain on that parade. But even so - studying this planet would be fascinating if it does indeed have an atmosphere. We have no idea what a tidally locked planet with a significant atmosphere would be like. How would it distribute heat around? Would there be constant horrible hurricanes around the terminator line? Would the weather be super-predictable, or completely chaotic? And that’s before even considering the effects of geography. […]
andy on May 1, 2007 at 2:38 pm: 6
Never mind the effects of tidal locking (which could be mitigated by suitable atmospheric superrotation and greenhouse gas content), a quick calculation suggests not habitable:
Orbital radius of planet = 0.073 AU
Luminosity of star = 0.013*solar
(these values from the discovery paper).
Energy flux (power per unit area) is proportional to luminosity, and inversely proportional to the distance squared.
So this planet receives 0.013/0.073^2=2.44 times as much energy flux as Earth does.
For comparison, Venus, not exactly known for its pleasant climate, at 0.723 AU from our Sun, receives 1/0.723^2=1.91 times as much.
In fact, the third planet of Gliese 581 seems like a far better candidate for liquid water oceans, particularly if its atmosphere contains enough greenhouse gases to prevent freezeout on the darkside.
graywyvern on May 1, 2007 at 3:24 pm: 7
0.16 isn’t eccentric?
m.
joseph on May 14, 2007 at 6:49 pm: 8
How long would it take for a rocket to get to this earthlike planet which is 20 light years away?
Astroprof on May 15, 2007 at 3:00 am: 9
It would take tens of thousands of years for us to get there.
Astroprof’s Page » Gliese 581d on June 16, 2007 at 11:39 pm: 10
[…] Back in April, I posted about the star Gliese 581, and in particular about one of its planets, Gliese 581c. In that post, I reported on Udry et al’s announcement of a potentially habitable planet, Gliese 581c. In my previous post, I presented my doubts as to the habitability of this planet. Now, I find that Manfred Cuntz, an astronomer at the University of Texas at Arlington, along with colleagues from the Potzdam Institute for Climate Impact Research, has submitted a paper to Astronomy and Astrophysics with mathematical analysis supporting my impression of this planet. The problem, you see, is that planetary atmospheres tend to have a greenhouse effect. Our own Earth does, in fact. Without the Earth’s atmospheric greenhouse effect, Earth would have an average temperature well below the freezing point of water. According to Cuntz and his colleagues’ paper, Gliese 581c turns out to be far too hot once you include the effect of a greenhouse effect in its atmosphere. […]
thirst4knowledge on May 18, 2008 at 3:11 pm: 11
I believe 581d may be more habitable than 581c.For instance it is further away from its host star which gives a better chance of not being tidally locked as well as if 581c may be as close to 581d as is said to be then 581c can help to warm 581d if it’s said greenhouse effect doesn’t sufficiantly warm the planet.My question to you is when will they have enough spectrographical data to determine the composition of the atmosphere if there actually is one and are sure there is liquid water on either of these planets because for a planet to support liquid water then ther must be an atmosphere because the water most likely would have boiled off into space.