64 votes
Accepted

Serious alternate form of the Drake Equation, or graffiti?

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 ...
Rob's user avatar
  • 2,035
27 votes

Why there are no terrestrial planets with a subsurface ocean?

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 ...
David Hammen's user avatar
  • 33.7k
21 votes
Accepted

Why do aliens have to be carbon based lifeforms?

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 ...
zephyr's user avatar
  • 15k
16 votes

What is the probability of life having developed or going to develop in our solar system another time?

The answer is, it could be non-zero (some would argue it must be non-zero), but since we don't know what the probability of life emerging on Earth was, it is impossible to quantify. This is why this ...
ProfRob's user avatar
  • 149k
15 votes
Accepted

Why there are no terrestrial planets with a subsurface ocean?

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 ...
James K's user avatar
  • 119k
13 votes
Accepted

How is an exoplanet characterised as "Earth-Like"?

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 ...
planetmaker's user avatar
  • 19.1k
13 votes
Accepted

Are there plans to detect life on Earth from the outer solar system?

Somebody can fill in the details perhaps but both the Galileo and Osiris-Rex spacecraft analysed light received from the Earth and looked for the spectroscopic signatures of carbon dioxide, oxygen, ...
ProfRob's user avatar
  • 149k
8 votes

Why N$_2$ is a non-absorbing species in the spectrum of the Earth?

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 ...
SebaGM's user avatar
  • 81
7 votes
Accepted

How much competition there is for jobs in astronomy compared to other fields of science?

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 ...
J. O'Brien Antognini's user avatar
6 votes

Why there are no terrestrial planets with a subsurface ocean?

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 ...
Darwood Martin's user avatar
5 votes

Why there are no terrestrial planets with a subsurface ocean?

We may regard Earth's ocean as a subsurface ocean that has melted through. Temperatures on Earth's surface are too warm to maintain the global ice covering which, on the outer-planet moons, renders ...
Oscar Lanzi's user avatar
  • 1,113
5 votes
Accepted

Why search for water?

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 ...
SF.'s user avatar
  • 6,249
5 votes

What is the probability of life having developed or going to develop in our solar system another time?

There's nothing wrong with asking a question to which the answer is "nobody knows" -- so long as you are willing to accept that the answer is essentially "nobody knows". The only ...
David Hammen's user avatar
  • 33.7k
5 votes

Calculating "maximum" distance to an alien civilization

Suppose civilizations are randomly distributed in the galaxy (not necessarily evenly, but independent of each other). That means that there is a CDF $\Pr[d<x]=F(x)$, the probability for the ...
Anders Sandberg's user avatar
5 votes
Accepted

Have there been any searches for extraterrestrial life which doesn't require water, oxygen and carbon?

Some of the projects carried out by The Search for Extraterrestrial Intelligence (SETI) would appear to make no carbon/water-based assumptions - for example Laser Seti, which looks for optical beacons ...
ProfRob's user avatar
  • 149k
4 votes

Which elements are an indication of habitable exoplanets?

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 ...
userLTK's user avatar
  • 23.9k
4 votes
Accepted

Explaining the Drake Equation on a smaller scale

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 ...
Fattie's user avatar
  • 1,126
4 votes

Why do aliens have to be carbon based lifeforms?

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 ...
bandybabboon's user avatar
  • 4,242
4 votes

Why there are no terrestrial planets with a subsurface ocean?

It depends what you mean by ocean. Earth arguably has a subsurface ocean of liquid iron, usually called the "outer core".
Steve Linton's user avatar
  • 10.3k
4 votes

Why there are no terrestrial planets with a subsurface ocean?

As far as I know satellite data from Mars observers show significant amounts of ice blow the south pole and pointers to a similar though smaller amount at the northern pole of Mars. This is not ...
eagle275's user avatar
  • 311
4 votes
Accepted

Why are ice giants not considered suitable for life?

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 ...
userLTK's user avatar
  • 23.9k
4 votes

Did we adapt to Earth or Earth to us?

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 ...
James K's user avatar
  • 119k
4 votes

Could Enceladus be too salty to support life?

I actually found this very useful article, which seems to answer the question: https://phys.org/news/2021-05-salty-enceladus-ocean-ice.html It suggests that the liquid water in the Enceladus ocean has ...
4 votes

Could Enceladus be too salty to support life?

Limitations to life via osmosis is mostly a modern limitation. Modern in the 'geological era'-sense. Why osmosis can affect life negatively is, very roughly, when concentration gradients past a bi-...
AtmosphericPrisonEscape's user avatar
4 votes

Could life develop in a galaxy with a quasar at its centre?

There is some evidence that the Milky Way black hole has been active in the relatively recent past. Joss Bland-Hawthorn (et.al) describe large-scale ionization cones in the Galaxy that they say ...
James K's user avatar
  • 119k
4 votes
Accepted

Do M dwarf stars emit the wavelengths of the electromagnetic spectrum necessary for photosynthesis?

As ProfRob noted, M dwarfs emit mostly in the infra-red. But there is no reason to think that "photosynthesis" would be the same on an alien planet. Photosynthesis has evolved on Earth to ...
James K's user avatar
  • 119k
4 votes

Life around a former brown dwarf

The scenario is that the new life zone due to the star's increasing luminosity expands past the brown dwarf, making its moons lie in the life zone. The big issue here is the time span they are in the ...
Anders Sandberg's user avatar
3 votes
Accepted

Do we know the fluctuations and constant temperature depth of the moon?

Do we know the fluctuations and constant temperature depth of the moon? Yes, at some Apollo landing sites, it's been carefully measured! The astronauts make deep holes in the Moon's regolith and put ...
uhoh's user avatar
  • 31.3k
3 votes

Will biological process be disrupted under strong gravitational time dilation?

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 ...
James K's user avatar
  • 119k

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