When discussing habitable exoplanets, the one thing that always comes up is surface
liquid water.
There is a good reason for this, life on earth would not exist without it, and it's a good
bet that life in general can't exist without some kind of liquid solvent for the chemistry
to occur.
Water is the best, and most abundant, solvent in the universe, though there are other possibilities.
But even water has its limits and here are two scenarios, one real world, and the other
speculative that illustrate the idea that having surface liquid water can go horribly
wrong and result in bad consequences for life.
The first is the exoplanet HD 85512 b, which at first glance looks like it might be a good
candidate for liquid water and even life.
In fact, when it was first discovered it was considered among the best possible candidates
for this.
But subsequent research has cast this very much in doubt, and instead now paints a picture
of a world where having significant amounts of water might have been a liability rather
than an advantage.
This planet is a super earth, about 3.6 times the mass of our own planet.
It orbits a suitable star for life, a type K star, which are thought to possibly be even
better for the development of complex life than our own sun is.
And, it's close, at only about 36 light-years.
This is the kind of planet that we may some day visit; at currently possible speeds we
could get a probe to this world in a few hundred thousand years.
That may seem like a painfully long time, but so long as we don't destroy ourselves
and end up branching out into the universe with colonies, then we have nothing - but
- time on our hands.
Billions and billions of years worth of time.
A few hundred thousand years is a blink of an eye in that context, especially if your
species is sufficiently technologically advanced as to be immortal or nearly so.
But HD 85512 b may not be a world worth going to, despite being thought to be rocky.
And the reason is water.
This exoplanet lies just inside its star's habitable zone.
As it orbits, it sort of skirts in and out of the inner edge of the zone.
This means that this world is warm enough for liquid water, at least initially, and
may have had oceans early in its history.
The system is also old enough for life to have evolved at an estimated 5.61 billion
years, which if we're any indication is ample time to develop an intelligent species.
But the fact that it sits on the inner edge of the zone is where the problem is.
That makes it just warm enough on this world to have evaporated any water on the surface.
Water vapor is an efficient greenhouse gas, and as such would have made the problem worse.
Eventually, a very hot desert planet with a steamy high water vapor atmosphere would
have resulted.
But that period early in the history HD 85512 b illustrates the second problem with water.
It's thought that this world would have had a lot of water, enough to where it had
little or no land.
Too much water and no land would tend to lock life in, so even if it had been stable and
developed life, that life would be oceanic with no adaptation for land.
This is no problem for developing intelligence, our ocean is full of smart animals, but it
is a problem for developing a civilization.
But that's a topic for a future video.
Thanks for listening!
I am Eleanor Davenport in for John Michael Godier who is currently being attacked by
microbes and will be back just as soon as his voice is, and be sure to subscribe to
the channel for regular, in-depth explorations into the interesting, weird and unknown aspects
of this amazing universe in which we live.
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