We’d all like Gliese 581 c to be as Earth-like as possible, but not everyone puts high odds on the planet being even potentially habitable. In an e-mail discussion circulating among space professionals, Gerald Nordley took issue with the ‘terrestrial world’ concept and pointed out how the results of Stephane Udry and the Geneva exoplanet team shouldn’t be taken too far. Nordley, a retired Air Force astronautical engineer, is a familiar name to those who follow interstellar studies from his work in the Journal of the British Interplanetary Society as well as his essays in venues like Analog. He is also the author of numerous science fiction stories.
Here are Nordley’s comments, reprinted with permission:
Udry et al., make a good case for a planet being there, but the rest looks speculative at best. The planet has a minimum mass of 5 Earths, the “1.5 Earth radius” is based on a density assumption with no data behind it, and the planet’s insolation is about 2.44 times the Earth’s (L/a2 = 0.013/.0732). The effective temperatures calculated didn’t reference any atmosphere model. A similar calculation for Earth gets you about 256K (-17C), depending on albedo. They used a Venus-like albedo to get down to 273K — actually not bad for the Venusian upper atmosphere. Of course, we all know what the surface of Venus is like.
If an awful lot of things break the right way, well, maybe a terrestrial planet. But in my crystal ball G 581c is a rather hot mini-Uranus.
The next planet out has an insolation of 20% Earth’s. If it (big if!) were of similar density to the Earth, it would have a surface radius and gravity roughly twice as high as high as Earth’s. And even if the top of the atmosphere were much colder, if it were a few bars deep, the lapse rate would produce a liquid water surface.
Nordley’s thoughts come at a time when Greg Laughlin (UC-Santa Cruz) has pegged the odds on Gliese 581 c harboring “a clement surface or a temperate ocean-atmospheric interface” at a thousand to one against. Which is not to downplay the significance of the Gliese 581 c discovery, but only to point out that there is a wide gap between the actual facts we have on this planet and the speculation they have provoked. We can learn more through continuing observations — and we can’t rule out the possibility of a transit, which would help immensly.
So just what is the significance of Gliese 581 c? Whether or not it turns out to be habitable, this planet represents the first time we’ve ever looked at a world where all the pieces could fall into place. And the new planet should energize further investigation, quickening the pulse not only of those already involved in the search, but in a public that shows signs of becoming excited again about deep space exploration. That makes the outlook for the next few years in exoplanet studies more promising than ever, because Gliese 581 c is only the first of many terrestrial world candidates to come.
That fits quite well with what I had been saying previously. A world with a supercritical ocean could feasibly be called a mini-Uranus – it certainly isn’t anything like a world with liquid seas! On the other hand, Uranus has a hydrogen/helium envelope, which probably would be lost (if Gliese 581 c ever had one) on a hot inner system planet, especially given this planet appears to be less massive than Uranus. With these planets we’ve got a window into the transition between terrestrial planets, ocean worlds and gas giants, which makes this system important even if it isn’t habitable.
In fact, a recent revision of the paper seems to have dropped the word “habitable” from the title, and includes a qualification that while we might have a fairly decent handle on the effective temperature, the actual surface temperature is unknown and likely to be substantially higher.
Nice to see that my speculations with regard to any oceans on the outer planet seem to hold up too, though the higher mass might make it more likely to be a mini-Uranus rather than an ocean world.
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Planet-finder says search for alien life next
AP Apr. 25, 2007
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Swiss scientist Michel Mayor, who
heads the European team that
announced the discovery of the new
potentially habitable 581 c planet,
predicts that top researchers are
less than two decades away from
being able to detect real signs of
such life — if it…
http://www.kurzweilai.net/email/newsRedirect.html?newsID=6725&m=25748
Brings me to the next question: although M-stars are amazingly long-lived and stable (?), the HZ of sunlike stars tends to move outward in the course of its evolution, reducing the continuously HZ (CHZ). Is something like that also the case for M-stars? If so, it could make the CHZ of M-stars very narrow indeed.
If we put the Sun’s CHZ at 0.95 – 1.2 AU, the (C?)HZ of Gl 581c would be something like from 0.11 – 0.13 AU, all other things equal (I mean, purely based on luminosity). This is indeed not only well outside c’s orbit, but is it wide enough for a ‘living’ planet? Anyway, apart from all the other negative factors mentioned before (tidal locking, gravitational perturbation by subgiants, are flares still a problem for M-stars this age?), the statistics for finding an earthlike planet smack in the CHZ of an M-star (and long enough!) don’t seem to look very promising.
And I have a question about tidal locking in relation to planet mass and AU:
are the very slow rotations of Mercury and Venus also the result of the sun’s gravitational effect, i.e. a near-tidal locking? For Venus this seems unlikely, but Mercury?
Perhaps I should rephase the question to make it more general: is it established when tidal locking takes place i.e. in relation to stellar mass, plant mass, planet orbit (AU)? I would think this is governed by straight-forward physics, or is planetary history also a factor here?
Detailed Models of super-Earths: How well can we infer bulk properties?
Authors: Diana Valencia (1), Dimitar D. Sasselov (2), Richard J. O’Connell (1); ((1) Earth and Planetary Sciences Dept., Harvard University; (2) Harvard-Smithsonian Center for Astrophysics)
(Submitted on 25 Apr 2007)
Abstract: The field of extrasolar planets has rapidly expanded to include the detection of planets with masses smaller than that of Uranus. Many of these are expected to have little or no hydrogen and helium gas and we might find Earth analogs among them. In this paper we describe our detailed interior models for a rich variety of such massive terrestrial and ocean planets in the 1-to-10 earth-mass range (super-Earths). The grid presented here allows the characterization of the bulk composition of super-Earths detected in transit and with a measured mass. We show that, on average, planet radius measurements to better than 5%, combined with mass measurements to better than 10% would permit us to distinguish between an icy or rocky composition. This is due to the fact that there is a maximum radius a rocky terrestrial planet may achieve for a given mass. Any value of the radius above this maximum terrestrial radius implies that the planet contains a large (> 10%) amount of water (ocean planet).
Comments:
20 Pages, 5 figures. This article is under review in Astrophysical Journal
Subjects:
Astrophysics (astro-ph)
Cite as:
arXiv:0704.3454v1 [astro-ph]
Submission history
From: Diana Valencia [view email]
[v1] Wed, 25 Apr 2007 21:35:48 GMT (1977kb)
http://arxiv.org/abs/0704.3454
Ronald asks: ” although M-stars are amazingly long-lived and stable (?), the HZ of sunlike stars tends to move outward in the course of its evolution, reducing the continuously HZ (CHZ). Is something like that also the case for M-stars? If so, it could make the CHZ of M-stars very narrow indeed.”
My understanding is that M stars don’t get hotter with age the way F/G/K stars do. The core process that causes that heating in Sunlike stars doesn’t go on in M stars. So their HZs don’t migrate outward, or do so at an infinitesimal rate. That’s one of the reasons they’re regarded these days as such good candidates for long-term habitability.
“are the very slow rotations of Mercury and Venus also the result of the sun’s gravitational effect, i.e. a near-tidal locking? For Venus this seems unlikely, but Mercury?”
Mercury is in a 3:2 orbital coupling with the Sun, saved from a 1:1 tidal lock by the eccentricity of its orbit (if I recall correctly). So yes, that is influenced by Sol’s gravity. As for Venus, here’s a PDF of a paper talking about the effect of its atmosphere and tidal effects on its spin:
http://adsabs.harvard.edu/abs/2003Icar..163….1C
I think it’s saying that solar tides do contribute to slowing Venus down, but the inertia of its dense atmosphere prevents it from falling into a 1:1 tidal lock.
^^Uhh, sorry, that address didn’t work. Try this one:
http://www.imcce.fr/Equipes/ASD/preprints/prep.2002/venus1.2002.pdf
Hi All
The outer planet seems more likely to have a clement ocean than 581b, but red-dwarf spectra lack the UV that would photolyse an ocean in a short time like what happened to Venus, so 581b might have a Wet Greenhouse situation because of cloud cover instead of a super-critical ocean.
Still nasty, but not quite as nasty.
Adam
Adam – as far as I understand it, the lack of ultraviolet radiation from the red dwarf would mean water vapour stays around for longer in the atmosphere before being broken down into oxygen and hydrogen (which may or may not escape, depending on the escape velocity and exosphere temperature – this is presumably the way Venus lost its water, which seems to be backed up by its hydrogen/deuterium ratio), not that it wouldn’t kick off a horrendous greenhouse effect – in fact, having water staying around in the atmosphere is a good way of keeping the greenhouse effect going, and keeping the ocean supercritical.
Hi andy
I kind of garbled my argument. What I meant was that a “Wet Greenhouse” seems more likely than a super-critical ocean because of enhanced albedo from clouds – but 581b is hotter than Venus so maybe the planet is like Hal Clement’s Tenebrae, which has most of its atmosphere is near critical fluid (~ 650 K, 800 bar) – it briefly condenses as massive raindrops during the night. Perhaps 581b’s eccentricity allows sufficient cooling to create such a scenario in reality – is 0.16(+/-0.07) enough eccentricity to cause the planet to slowly rotate like Mercury? The effective temperature differs by roughly 18% so who knows if the atmosphere condenses every few days, to “boil” away a week later?
That’s be truly an alien world.
Adam – Venus has the enhanced albedo from a global cloud cover, and its surface temperature is above the critical point of water. I’m not going to disagree about the possibility of liquid water in the atmosphere, but like the rain on Venus, the raindrops probably never reach the surface.
Also, from a paper on ice planets in habitable zones, it seems an ocean world could quite easily go supercritical even at Earthlike levels of insolation.
Hi andy
That’s not the implication I got from that Kuchner paper. He implies that the ice planets would remain frozen below a certain depth of atmosphere due to the opacity. There’s a lot we don’t know about an atmosphere made of mostly water and how it would behave – which is what a super-critical ocean planet would be.
Which is a damned good reason to go and look. Inspite of our massive computing abilities our models of such worlds would still be full of hidden assumptions that only comparison against a real planet would reveal.
You’re right – on closer that paper suggests a boiling (but liquid) ocean. However, from“Could we identify hot Ocean-Planets with CoRoT, Kepler and Doppler velocimetry?”, it seems that for sunlike stars, an ocean planet has supercritical ocean if it is located closer than 0.84 AU (see page 9). Scaling this to the luminosity of Gliese 581 gives 0.10 AU, still outside the orbit of Gliese 581 c.
Definitely we should investigate these planets (if only the system were a transiting one!) – they represent a window into the transition between low-mass planets and gas giants that is unrepresented in our solar system, and could give good constraints on the behaviour of oceans and atmospheres in unfamiliar situations.
Hi andy
Have you checked out Greg Laughlin’s post on Gl581b at Systemic?
http://oklo.org/?p=207
His GCM work (borrowed fromn a similar super-Earth model) suggests a sub-solar temperature of about 330 K and a far-side temp of ~ 250 K – for a 0.4 bar atmosphere mind you. I think the point is that a tidally-locked planet would have a huge heat-sink on one hemisphere and lots of atmospheric heat-transport, so it may not be such an easy thing to determine if it will have a Wet Greenhouse or a Runaway Greenhouse steam-bath.
Like I said we need to go look!
Adam – yes I’ve seen that – as I understand it, it’s a simulation of the lower stratosphere, so this is some way above the actual surface. With an effective temperature for the planet of around 0 degrees (they’re using a reflective layer of water clouds), it would be surprising if the temperatures weren’t comfortable somewhere in the atmosphere – such a region exists on Venus for example. Of course, any oceans would be below the troposphere, which may exhibit rather higher temperatures. For comparison, the base of Earth’s stratosphere is around -50 degrees C, which is much colder than the surface temperature!
andy Says:
“Definitely we should investigate these planets (if only the system were a transiting one!)”
UBC team is doing that:
http://thetyee.ca/News/2007/05/11/NewEarth/
“Canada’s space telescope has spent the past two weeks straining for a glimpse of what an elite group of European astronomers claim is the first habitable planet discovered outside this solar system. The suitcase-sized Canadian satellite, called MOST, is the only instrument capable of quickly verifying the historic claim.”