· What is our definition of
habitable, namely the possibility that some form of life could
exist there?
First, a caveat:
we often refer to "life-as-we-know-it",
involving chemical reactions in liquid water.
In principle,
there could be life in the liquid oceans, lakes and rivers of
methane and ethane on the surface of
Titan, Saturn's moon, where
the temperature is 94 degrees Kelvin, about a third of that on
Earth.
If alternative forms of life can form in other liquids,
the definition of habitable conditions must change.
In that
case,
habitable planets or moons could include alternative
liquids which can exist at other temperatures and atmospheric
pressures than liquid water.
Water molecules contain oxygen and hydrogen.
Methane and ethane
molecules contain carbon and hydrogen.
Whereas hydrogen was made
in the Big Bang, oxygen and carbon were produced in the
interiors of stars.
We know of some metal-poor stars which are
richer in carbon relative to iron by orders of magnitude
relative to the Solar abundances.
They are labeled
"Carbon-Enhanced Metal-Poor (CEMP)" stars, possibly representing
the earliest second-generation stars.
The planetary systems
around these stars might have hosted carbon-rich planets instead
of iron-rich planets like the Earth.
Having produced oxygen or carbon is a necessary but not a
sufficient condition for life. In particular, liquid water is
only possible under atmospheric pressure or under surface ice.
For the planet's gravity to retain an atmosphere, Solar system
planets must have had a mass larger than that of Earth.
Mars
lost its atmosphere.
· Just because the right ingredients for
habitable worlds might have been present very early in the
Universe, it doesn't mean the conditions existed...
There might
have been too much radiation and not enough time for life to
emerge?
The known factories for producing rocky planets like Earth, are
cold debris disks.
These contain the residual material from the
formation process of the star at their center. The temperature
of the cosmic microwave background was itself close to that of
Titan's surface, 94 degrees Kelvin, when the first stars formed.
This sets the temperature floor for debris disks at these early
cosmic times.
It is unclear whether rocky planets can form out
of warm debris disks so early in cosmic history.
· How long does it actually take for planetary
bombardment to slow down before conditions on a planet could be
hospitable to life?
In debris disks within the Milky-Way, heavy elements settle to
the midplane and form dust particles which coagulate to make
planetesimals.
These merge to make rocky cores of planets within
a few million years.
During the
Large Heavy Bombardment,
collisions of massive proto-planets with Earth or Mars melted
the surface rock of these planets.
It took the Earth and Mars
hundreds of millions of years to cool down enough for them to
become habitable. The Last Universal Common Ancestor (LUCA) of
life on Earth was recently dated to about
4.2 billion years ago.
This was
350 million years after the Solar system formed. The delay
required for cooling planets to habitable temperatures is longer
than the age of the Universe when the first stars formed.
Given
this delay, the first habitable planets should have appeared at
redshifts below 10 even if the first stars appeared at much
higher redshifts.
· Could we verify their existence today?
We can find solar-mass stars which are metal-poor in the Milky
Way galaxy and search for transiting habitable planets around
them.
The separation of the planets from their host star and
their chemical composition would allow us to infer whether the
planets might have been habitable hundreds of millions of years
after the Big Bang.
· Does the possible existence of habitable worlds
early in cosmic history deepen the mystery of the Fermi paradox?
Not really.
Most of the earlier civilizations that formed
billions of years ago are probably dead by now.
This is true on
Earth:
out of the
117 billion humans who ever
lived on Earth over the past few million years, only 8 billion
are alive today.
The rest died...
The best we can do is search for
relics they left behind.
Keep in mind that common archaeological
digs on Earth go back at most five thousand years, only a tenth
of a percent of human history and a millionth of Earth's
history.
Enrico Fermi was very presumptuous...
If I had the
opportunity to attend the lunch at Los Alamos where he raised
the question:
"Where is everybody?"
I would have replied:
"Most
of them are dead. You need to search for what they left behind".
· What do we know about the role of the earliest
stars in the chemical evolution of the universe?
The first generation of stars represent our cosmic roots.
From
our perspective, they were the first good news after the Big
Bang. For a hundred million years, the Universe did not have the
atomic building blocks of life, like oxygen or carbon.
Once
nuclear fusion started in stellar interiors, the Universe became
far more interesting.
I am an optimist. I believe that the best is yet to come in the
form of artificial intelligence and space exploration. We might
actually be late to the party if
other civilizations did both of
these activities billions of years ago.
My current research is
dedicated to checking if they actually did that before us.
If
so, we can learn from them...
