The Habitability of Gliese 581d

by Jon Voisey on October 19, 2010

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The Gliese 581 system has been making headlines recently for the most newly announced planet that may lie in the habitable zone. Hopes were somewhat dashed when we were reminded that the certainty level of its discovery was only 3 sigma (95%, whereas most astronomical discoveries are at or above the 99% confidence level before major announcements), but the Gliese 581 system may yet have more surprises. When the second planet, Gliese 581d, was first discovered, it was placed outside of the expected habitable zone. But in 2009, reanalysis of the data refined the orbital parameters and moved the planet in, just to the edge of the habitable zone. Several authors have suggested that, with sufficient greenhouse gasses, this may push Gliese 581d into the habitable zone. A new paper to be published in an upcoming issue of Astronomy & Astrophysics simulates a wide range of conditions to explore just what characteristics would be required.

The team, led by Robin Wordsworth at the University of Paris, varied properties of the planet including surface gravity, albedo, and the composition of potential atmospheres. Additionally, the simulations were also run for a planet in a similar orbit around the sun (Gliese 581 is an M dwarf) to understand how the different distribution of energy could effect the atmosphere. The team discovered that, for atmospheres comprised primarily of CO2, the redder stars would warm the planet more than a solar type star due to the CO2 not being able to scatter the redder light as well, thus allowing more to reach the ground.

One of the potential roadblocks to warming the team considered was the formation of clouds. The team first considered CO2 clouds which would be likely towards the outer edges of the habitable zone and form on Mars. Since clouds tend to be reflective, they would counteract warming effects from incoming starlight and cool the planet. Again, due to the nature of the star, the redder light would mitigate this somewhat allowing more to penetrate a potential cloud deck.

Should some H2O be present its effects are mixed. While clouds and ice are both very reflective, which would decrease the amount of energy captured by a planet, water also absorbs well in the infrared region. As such, clouds of water vapor can trap heat radiating from the surface back into space, trapping it and resulting in an overall increase. The problem is getting clouds to form in the first place.

The inclusion of nitrogen gas (common in the atmospheres of planets in the solar system) had little effect on the simulations. The primary reason was the lack of absorption of redder light. In general, the inclusion only slightly changed the specific heat of the atmosphere and a broadening of the absorption lines of other gasses, allowing for a very minor ability to trap more heat. Given the team was looking for conservative estimates, they ultimately discounted nitrogen from their final considerations.

With the combination of all these considerations, the team found that even given the most unfavorable conditions of most variables, should the atmospheric pressure be sufficiently high, this would allow for the presence of liquid water on the surface of the planet, a key requirement for what scientists maintain is critical for abiogenesis. The favorable merging of characteristics other than pressure were also able to produce liquid water with pressures as low as 5 bars. The team also notes that other greenhouse gasses, such as methane, were excluded due to their rarity, but should the exist, the ability for liquid water would be improved further.

Ultimately, the simulation was only done as a one dimensional model which essentially considered a thin column of the atmosphere on the day side of the planet. The team suggests that, for a better understanding, three dimensional models would need to be created. In the future, they plan to use just such modeling which would allow for a better understanding of what was happening elsewhere on the planet. For example, should temperatures fall too quickly on the night side, this could lead to the condensation of the gasses necessary and put the atmosphere in an unstable state. Additionally, as we discover more transiting exoplanets and determine their atmospheric properties from transmission spectra, astronomers will better be able to constrain what typical atmospheres really look like.

About 

Jon is a science educator currently living in Missouri. He is a high school teacher and does outreach with the St. Louis Astronomical society as well as presenting talks on science and related topics at regional conventions. He graduated from the University of Kansas with his BS in Astronomy in 2008 and has maintained the Angry Astronomer blog since 2006.
For more of his work, you can find his website here.

Torbjorn Larsson OM October 19, 2010 at 11:35 AM

the certainty level of its discovery was only 3 sigma

Yeah… The area has its standards, but it isn’t like it is an isolated observation. Exoplanets are theory, Gliese is multicomponent and the data extraction is too. Especially the first lower the threshold to 3 sigma IMO – not so humble since I’m not astronomer and can deviate from standards willfully. :-)

Lawrence B. Crowell October 19, 2010 at 12:55 PM

This was the original planet cited as possibly habitable two or three years ago. For all we know there might be two biologically active planets here. There are lots of other circumstances though which could make these planets lifeless.

LC

William928 October 19, 2010 at 3:46 PM

The Gliese system certainly seems like the right candidate for a Cassini type craft. I realize this is many decades in the future from being feasible, at least in terms of receiving any sort of meaningful data in a reasonable period of time. Still, why not send a probe out utilizing current technology while continuing to work toward faster spacecraft? Those with the expertise(LC) please explain why sending a probe now is a poor idea(running out of power long before it reaches the Gliese is the first that comes to mind). I just wish I were going to be around 500 years from now.

Uncle Fred October 19, 2010 at 5:54 PM

William, we’ve been discussing this in detail over the last few weeks. There are many reasons why any probe launched today would be inadequate for an interstellar mission.

1. Propulsion dilemma. No probe has been launched that has or will obtain speeds necessary for realistic intersellar travel. If I’m not correct, New Horizons is the fastest probe we’ve sent so far. It was launched in 2006. It will reach Pluto in 2015. I’m no math expert but a reasonable Glieses Odessy would a need a craft many orders of magnitude faster.

2. Self sufficiency issues. LC mentioned that communicating with the craft could take years. Thus a sufficiently autonomous system that can manage the craft, solve technical problems and likely determine scientific priorities is needed. Moreover, this system needs to be able to fuction with near zero error for decades if not centuries. This may be very much akin to HAL 9000 from 2001 Space Odessy. Only, – this time – there is near zero tolerance for error. No such A.I. based system exists or is likely within the next 30 years.

3. Interstellar particle bombardment. Some scientists calculated we would need very hefty shielding system to protect the craft from near-relativistic bombardment. Scientists differ over the size of these particles but suffice it to say the shielding would need to be very thick and/or incorporate as yet conceived magnetic shielding. The problem is shielding is heavy and all this has to be brought to space.

4. Powering the craft. here’s something I don’t remember anyone mentioning but I think it is worth considering: despite the longevity of our Plutonium based crafts, these sources of power are grossly inadequate for a large-scale interstellar missions lasting decades/centuries. Lots of equipment to power and a sizeable ship would needed to be maintained during the mission. How do we realistically power this? Don’t tell me Fusion or anti-matter or some other pie-in-the sky tech; what can we actually power it with NOW?

5. A short-term goal orientated public and scientific community. Ever notice how ultra long-term projects don’t attract much funding or fresh blood? The SETI community is an example of this. Who wants to wait decades or centuries for an expenditure to pay off? How would something like this generate enough public and scholastic interest to get off the ground? Big challenges here and lots of money needed for this kind of mission. Perhaps Billions of dollars would be needed to develop systems and technology that don’t even exist. In a sadly ironic way, this isn’t the Cold War.

I’m sure others with much more specific knowledge can add to this. Needless to say, the challenges prevent any short-term realistic hopes for a mission now.

Besides, as fascinating as the Gliese system is, I’m betting one of those sun-like stars within 50-25 light years has an Earth-Analog planet. It’s just a matter of time before we can resolve one. Who knows? We might have Earth likes planets – or moons? lol – at 1 Au from Alpha Centauri A or B.

Kawarthajon October 19, 2010 at 8:22 PM

This is a great discovery and I’m enjoying the articles coming out of this story. Problem is, we’ll never (at least in my lifetime) figure out whether there is life there even if the planet does exist. If we can’t even figure out if there’s life on Mars or Titan, which are incredibly close by comparison, how can we figure this out for something that is 10′s of lightyears away? Especially since this planet doesn’t transit and we can’t even detect whether it has an atmosphere.

IVAN3MAN_AT_LARGE October 19, 2010 at 11:04 PM

*cough*

At the second paragraph, in the fourth line, it should be affect, not “effect”.

(Now where’s my coat…?)

Spoodle58 October 20, 2010 at 12:51 AM

In a way we are lucky to dream about what these planets can be like, maybe get a few books and movies out of them like we did with Mars & Venus during the 20th century, before we found out the hard truth of reality.

It might even inspire a new generation of scientists. :) but for now we have to contend with the fact that we may truly never know what these worlds are like.
For me, knowing that they are simply there is an achievement within our lifetime.

Lawrence B. Crowell October 20, 2010 at 5:31 AM

I wrote a book on star probes with the message that it is not impossible, even if it is difficult. In fact I wrote it in part because it might be that before the end of this century our understanding of the universe might reach some limit. We might in 25-50 years arrive at some theoretical understanding of quantum gravity and cosmology, with maybe some observational and experimental support of that theory, but where the extremes of scales (cosmic and infinitesimal) are such that we are not able to press onwards. If so we might be faced with the dilemma:

http://abstrusegoose.com/308

Maybe interstellar space exploration will fill in some of that gap in the late 21st to 22nd century.

The only possible propulsion in the foreseeable future is collimated photon energy. If we get a spacecraft to reach v = .5c to .87c the gamma factors are 1.15 and 2. If you multiply that factor by the mass of the craft you get the total energy E = gamma*mc^2. For a gamma = 1.15 this means .15*mc^2 is the kinetic energy of the craft. If that craft has a mass of 100 tons, or 10^5kg that energy is about 10^{20}joules. This is an enormous value. Based on nuclear or even fusion energy it is unrealistic to propose carrying the fuel sufficient to carry this mass. There is also a relativistic version of the rocket equation, which makes this even more problematic.

While I think interstellar space science is not impossible, I do think that sending probes to other stars is probably unlikely to happen. It does require a long term focus, and the modern world tends to increase our attention to transient events. The fashionable “tech-toy” of late is twitter — a way of sending minimal transient messages with little content. So our modern world is time compressed, information compressed and focused on things which most often have the lasting quality of a tissue paper. So this does not bode well for space science programs which can take a century to complete. Further, this problem is most acute with the United States, which is a nation that has a comparatively short history, and where its citizens have an exceptionally narrow time frame of thought. Of course by the time humanity might send probes to other stars the US dominance in the world will have doubtless ended — in fact I think the US is on a precipice and about to fall into a pretty deep pit.

LC

Uncle Fred October 20, 2010 at 6:55 AM

I fear you might be right LC. I really don’t understand what’s going on politically and economically south of the border. For somewhat selfish reasons I would hope that the eventual American decline would be more analogous to the British style fade. I fear Canada would suffer greatly from the windfall of any major readjustment or hard soviet-style surprise collapse.

IVAN3MAN_AT_LARGE October 20, 2010 at 9:05 AM

I also think that L.B.C. is right; as Lenin said in 1917:

Germany will militarize herself out of existence, England will expand herself out of existence, and America will spend herself out of existence.

Lawrence B. Crowell October 20, 2010 at 9:52 AM

I don’t have time to go into it all. The US is in a crisis of various forms. A part of it is religious, where the intellectual decline of Christianity and the culture conflict this has generated is reaching a peak. The other is the whole cold war machine this nation developed, which is largely obsolete. The country has been thrashing around the world starting wars — a war machine giant with no purpose. The other issues range from economics to “KulturKampf” issues, where over the last 2 decades the rhetoric in this nation has become increasingly vapid and puerile, all the while richly laced with manic histrionics. It is a crisis, and the worry I have is that the sort of fascistic junta types of governments various agencies of the US have installed in other countries, particularly Latin America, will be internally installed here. Much of what I see has some appearances of being a populist facade meant for this ultimate goal.

LC

babybaby October 21, 2010 at 7:49 AM

I love that quote, Ivan3man. A veritable Twentieth-century Nostradamus. Ultimate irony: the countries he mentions have ed themselves out of dominance (or are in the process of doing it verily as we write), but not out of existence; the only modern major nation I can think of that ever ed itself actually out of existence is the one he founded.

Nevertheless, I thoroughly agree that the US is in crisis, although I doubt if it will cease to exist in my thankfully short remaining lifetime.

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