64

Given the font, it's xkcd 384, The Drake Equation. The two $X$'s are the other $f$ terms from the original Drake equation, while the $B_S$ is "amount of bullshit you're willing to buy from Frank Drake"


21

Some hypothesize that the Earth did have a subsurface ocean during the Cryogenian period, which lasted from 720 to 635 million years ago. The Cryogenian saw the two greatest known ice ages in the Earth's history, the Sturtian and Marinoan glaciations. There is some evidence that the Earth was completely covered with ice and snow during those glaciations. (...


20

I want to start with a disclaimer that I'm not a chemist and hopefully I don't screw any of the chemistry up. If I do, please let me know. I think Bill Oertell hit the nail on the head. Just because you can imagine a species which can drink lead doesn't mean it's actually physically possible. Only specific atoms can form the basis of the (extremely) complex ...


13

There is no unique definition of Earth-like and it depends on what characteristics are important in the context of discussion. The most simple one is to just compare radii and masses. That is observationally sensible as radius is directly deducable from transits (assuming stellar size known) and mass is often an accessible properties for transiting planets ...


8

On the surface of Mars probably none, since it's too dry or too cold, or both, to stay active. Spores or other dormant forms probably could survive for centuries, until radiation will gradually destroy the organic molecules necessary to get back into an active state. But there are "Mars Special Regions", where either Earth microbes or potential Martian ...


8

The terrestrial planets are Mercury, Venus, Earth and Mars. Mercury and Venus are too hot for liquid water to exist at any level, Mars has lost nearly all its water and Earth has a surface ocean, not a subsurface one. The inner planets lost most of their volatiles (including water) as they formed, the water on Earth was provided by later icy asteroid ...


8

As your question is based on the plot you posted, I suggest you to look for a lower wavelength range of the atmospheric electromagnetic absorption. A quick search in google gave me this paper, which says: The importance of molecular nitrogen as the most abundant species in the Earth's atmosphere is evident. The strong absorption bands in the range 80–100 nm ...


7

From what I understand, James Webb, if used in conjunction with a successful starshade (being developed at MIT), should be able to detect close in planets orbiting nearby stars. However, getting good atmospheric spectra of these planets directly (from planet's blackbody IR emission) is unlikely. What we must hope for is that TESS, which should be going up in ...


7

The competition for permanent positions in astronomy is very tough. The field as a whole produces roughly ~200 Ph.Ds per year, but there are usually only a handful (say ~10) tenure-track positions that open up every year. So perhaps ~5% of Ph.Ds end up in tenure-track positions in astronomy. There are more permanent positions in astronomy that aren't ...


5

A lot of different "alternative biologies" have been considered, but if you analyze their required chemistry, this always falls apart at some point, some mechanisms necessary for life are simply impossible. Replace carbon with silicon, increase ambient temperature, and you're getting a lot of interesting reactions that could be conductive to life. But ...


4

No need to make it complicated: what about this... Just scribble a rectangle on a piece of paper, and say "there are 100 billion stars in our galaxy".... Then, color off (let's say) 1/3 of the rectangle, and say "only one third of those are the sort of star that could have life, so that's blah billion" Then, color off (say) 9/10ths of that box, and say "...


4

While not strictly relevant to your question, I'm very much looking forward to what the James Webb Telescope might tell us about exo-planet atmospheres. That's probably the thing I'm most looking forward to in astronomy discoveries over the next 5-10 years. The one thing I've heard that they are looking for specifically is a combination of Oxygen (which ...


4

To answer your question directly, it is quite unlikely that planets would be habitable without already having life. It would have to be in a near-perfect condition, which would be statistically unlikely. In case of planets already having life, the existence of life provides robustness to small changes and makes habitability possible in a larger range of ...


4

A basic approach would be to estimate the air-pressure at the top of the mantle. Uranus' surface gravity is a bit less than Earths, but, somewhat counter-intuitively, it's atmosphere is light enough that it's gravity increases as you move inside the planet towards the top of the mantle. (the same is true for Earth). For ballpark estimate, lets say the ...


4

Carbon is like lego-technic. It connects water into organic compounds which have every chemical and physical property from volatile solvent to syrup to oil to tar and rock. Silicon is like lego, try building a car out of it, it's difficult to find a good solvent for it. it can't even bond H. Silane which is the equivalent of methane doesn't occur naturally ...


4

Your reasoning is at least true in part. First off. We have only one example of life. It is based on carbon chemistry and water solutions. As we only have one example, we don't know if fundamentally different chemistries are possible. We just don't know if the Earth is unique or special, or whether life is commonplace throughout the universe. If you head ...


3

Remember this is a theory of "relativity". Now, time dilation due to gravitation effects is rather outside our normal experience. But there are relativistic effects that that you experience all the time. I'm sitting on a train. Relative to my computer I'm not moving. Relative to the track I'm travelling at 100km/h. Relative to the sun I'm moving about 30 km/...


3

As far as I know, and aside from direct communication, there's only one way to "find life" on another planet. That method is to look for molecular oxygen in the atmosphere. Molecular oxygen hates existing by itself and loves to combine with other atoms and molecules through oxidation. The Earth's atmosphere is currently ~30% $\mathrm{O_2}$, but that's only ...


3

To copy my answer from Space.SE: Virtually all chemical reactions take place in a liquid or gaseous phase. Uncontained gasses aren't a likely candidate for life, so we're looking for solvents for life-related reactions to take place in. A good solvent is (a) common, (b) dissolves a wide variety of substances, and (c) is liquid at a wide range of ...


3

I don't think there is necessarily a problem with it being dry or cold. An excellent similacrum of the Martian surface conditions is found in Antarctica. There are plenty of anaerobic microbes that exist here. A brief search reveals Shivaji (1988); Franzmann & Rhode (1992); Dube et al. (2001). To quote the wikipedia entry on the "McMurdo dry valleys" - "...


3

We have not found life, bacterial or otherwise, anywhere other than Earth ... yet. I think you mean the difference between liquid water and water ice. This difference relates to the hospitality to life. Life of the type we are used to needs liquid water, even if this is just trapped moisture, unless the life is temporarily in suspended animation. If there ...


3

Anaerobic life has much less impact on the atmosphere than aerobic life. Anaerobic life could exist in the deep sea and have no gas exchange with the atmosphere at all. Methane would get everyone excited. Biotic methane might be expected to have a different isotope mix compared with abiotic methane. But it is hard enough to distinguish isotope mixes when ...


3

I agree with David Hammen. Hyperphysics is mostly a very good site but they dropped the ball on that page IMHO. Hope you don't mind a partially speculative answer, but here goes: Why does it matter if there are some areas of a planet with extreme temperatures, as long as there are other spots on the planet that are not extreme? It shouldn't ...


3

There are no terrestrial planets with subsurface oceans because of differentiation. Denser materials move toward the center of the body. Iron is denser than rock which is denser than water which is denser than ice. The icy surface of these moons and dwarf planets is essentially floating on water which is floating on rock. You can actually see this on ...


3

This figure is from the paper "Phosphine as a bio signature gas in exoplanet atmospheres". It shows the absorption cross section of Phosphine compared to other molecules. We can see that Phosphine has a distinct enough profile from the others molecules in the 7.8-11.5 microns range, with the exception of NH3. Probing from 2-11.5 microns should ...


3

The masses of the two objects are completely determined by a careful measurement of their relative positions on the sky over decades, combined with an accurately known distance. There is no degeneracy in this solution. For a given orbital period and separation of the two objects, the masses are entirely determined. If Sirius B were not a massive white dwarf ...


2

This is a case where the Southern US phrase might could is appropriate. Intelligent life1 might could arise only on terrestrial planets with just the right axial tilt. Then again, intelligent life might could arise on all kinds of other planets as well. Extrapolating from a sample size of one is always a risky endeavor. The hyperphysics page referenced in ...


2

The Exomars rover of ESA and Roscosmos would be the obvious answer. To be launched in 2018. It will drill 2 meters deep and as far I know is the only mission since the Vikings in the 1970's, to explicitly be equipped to find biosignatures, signs of life. But the Russians, who will land the rover, have had a very poor Mars mission success rate, ESA cooperates ...


2

If you are asking about second earth for us Carbon based life forms then atmospheric composition of "second earth" should be similar to Earth. Primarily Nitrogen and Oxygen. Some green house gases like CO2, CH4, water vapors for temperature regulation. Water Vapors can also be sign of active water cycle. Some O3 will be nice for UV protection.


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