Can life emerge on planets around cooling stars?

Nov 20, 2012 by Peter Kelley

(Phys.org)—Astronomers find planets in strange places and wonder if they might support life. One such place would be in orbit around a white or brown dwarf. While neither is a star like the sun, both glow and so could be orbited by planets with the right ingredients for life.

No terrestrial, or Earth-like planets have yet been confirmed orbiting white or , but there is no reason to assume they don't exist. However, new research by Rory Barnes of the University of Washington and René Heller of Germany's Leibniz Institute for Potsdam hints that planets orbiting white or brown dwarfs will prove poor candidates for life.

are the hot cores of and brown dwarfs are failed stars, objects not massive enough to start nuclear burning as the sun does. In theory, both can be bright enough to theoretically support a —that swath of space just right for an orbiting planet's surface water to be in liquid form, thus giving life a chance.

The inner edge of that just-right zone is where a planet starts to become a runaway greenhouse, such as Venus. That heating phenomenon removes the planet's surface water and all chance of life  — of habitability—is forever lost.

White and brown dwarfs share a common characteristic that sets them apart from normal stars like the sun: They slowly cool and become less luminous over time. And as they cool, their habitable zones gradually shrink inward. Thus, a planet that is found in the center of the habitable zone today must previously have spent time near the zone's deadly inner edge.

Because of their past, such planet would "face a difficult path to ," Barnes said, even if they're discovered right in that habitable zone.  Call it a sort of check, revealing that the worlds probably lost the means to host life long before they became habitable zone residents.

"These planets, if we find them today in a current habitable zone, previously had to have gone through a phase which sterilized them forever," Barnes said. Heller added, "So, even if they are located in the habitable zone today, they are dead."

Barnes is the lead author of a paper published in November in the journal Astrobiology; and Heller is co-author.

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Digi
5 / 5 (5) Nov 20, 2012
There are many paths to habitability, in particular tidal flexing of moons orbiting giant planets like Jupiter. These moons can sustain life under land and ice where the water is liquid. I think the conventional goldilocks zone idea needs to be updated - the bear with the cold porridge is deep underground enjoying his meal.
GSwift7
3.4 / 5 (5) Nov 20, 2012
That seems to make sense, based on what we have seen so far.

So, a brown dwarf is not good for habitability, but if you add just a bit more mass you get a red dwarf. It still looks like red dwarfs are the ultimate abode for life. Red dwarfs can remain stable for tens of billions of years because they take so long to burn up their fuel. Some of the earliest red dwarf stars, formed shortly after the big bang, should still be burning today. Imagine a planet that has been habitable for 13 billion years. If life is likely to start on its own when the conditions are right, that would have to be the ultimate place for it to happen. Any space-faring race seeking a long term home would surely investigate red dwarfs. That might be a good place for something like SETI to listen.
GSwift7
3 / 5 (4) Nov 20, 2012
in particular tidal flexing of moons orbiting giant planets like Jupiter. These moons can sustain life under land and ice where the water is liquid. I think the conventional goldilocks zone idea needs to be updated


Perhaps. I think that depends on what we find when we eventually get a better look at the moons here in our solar system. If we don't see any sing of life there, then should the 'idea need to be updated'?
Digi
3 / 5 (1) Nov 20, 2012
Perhaps. I think that depends on what we find when we eventually get a better look at the moons here in our solar system. If we don't see any sing of life there, then should the 'idea need to be updated'?


There are a host of conditions which have to be right for life to begin, which may of course not be present in our solar system. I do feel life will find a way if conditions are right whether on the surface of a planet in the habitable zone or deep under the ice of a moon orbiting a starless wandering planet.
GSwift7
2.3 / 5 (3) Nov 20, 2012
There are a host of conditions which have to be right for life to begin, which may of course not be present in our solar system. I do feel life will find a way


Yeah, that's the big question, isn't it?

Based on latest observations, it seems like there are so many planets that no matter what condition you are looking for, it probably exists in plenty of places at different times.

So the remaining question is whether the second part of your statement is true or not. If the conditions are right, how likely is it that life will form and survive?
Torbjorn_Larsson_OM
5 / 5 (5) Nov 20, 2012
Not to mention that white dwarfs stars have been read giants.

But in such a case, planets likely migrate anyway. And in both cases we know rogues, previously ejected planets, can be captured at any time. So we should still look a these planets for life.

@ Digi: The habitable zone concept is elaborated all the time. It is a useful quick estimate for surface habitability.

But as we see from our own crust and Mars we need to look for crustal biospheres, and from Europe and Enceladus we see that we need to look at ice covered ocean biospheres, as well.

@ GSwift7: As we see from the short time life evolved on Earth, the generic process from chemical to biological evolution isn't difficult and/or seldom proceeding. In either case it will happen a lot.

And as we see from our long history, despite mass extinctions to boot, life is robust. In models, unicellular life survives heavy bombardments as well, because it repopulates faster than sterilizing impactors. Life is a plague on a planet.
Digi
5 / 5 (2) Nov 21, 2012
Life is a plague on a planet
Nice analogy and quite apt - once life begins it will be very difficult to extinguish. I think life will be found on multiple moons throughout our solar system and probably even Mars. I am awaiting the latest news from Curiosity with bated breath hoping for a sign of organics!
GSwift7
1 / 5 (2) Nov 21, 2012
As we see from the short time life evolved on Earth, the generic process from chemical to biological evolution isn't difficult and/or seldom proceeding. In either case it will happen a lot


That depends on the mechanism by which life started. There are many competing theories, and it is possible that none of them are correct. Some of the theoretical pathways are require very precarious sequences of circumstances. If such elaborate combinations of specific conditions are required, then it takes the probability of finding life at any given place and time in the Universe down to nearly zero. On the other hand, if life here originated from material formed on asteroids and comets, then the probability of life forming elsewhere would be much higher.

I 'believe' that life is probably abundant, but 'belief' isn't science. It seems unlikely that we are 'special' but perhaps we are. If/when we do find life that FORMED elsewhere, it will be like opening a flood gate.
GSwift7
1 / 5 (2) Nov 21, 2012
Nice analogy and quite apt - once life begins it will be very difficult to extinguish.


That may not be true. How do you know that life did not 'begin' many times here on Earth before a type formed which could persist?

Furthermore, in human terms, it would be nearly impossible to wipe out all the life on a planet, but sterilization by stellar, or cosmic forces is quite easy and common, especially in dense parts of space.
Digi
not rated yet Nov 21, 2012
That may not be true. How do you know that life did not 'begin' many times here on Earth before a type formed which could persist?

Furthermore, in human terms, it would be nearly impossible to wipe out all the life on a planet, but sterilization by stellar, or cosmic forces is quite easy and common, especially in dense parts of space.


If life began many times on Earth then it is even more common and will arise more often. As for sterilization, there are countless stable systems. The Earth will be sterilized one day, but there still will have been life on this planet for over a third of the age of the universe.