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Now that you know what kinds of stars would be good to explore further and
what criteria should be used for distinguishing lifeforms from other physical
processes, let us hone in on the right kind of planet to support life.
Unfortunately, our information about life is limited to one planet, the Earth,
so the Earth-bias is there. However, scientists do know of the basics of what life
needs and what sort of conditions would probably destroy life. With these
cautionary notes, let's move forward.
The habitable planet should have:
- a stable temperature regime and
- a liquid mileau. Liquid water is best for biochemical reactions
and could be very abundant but liquid methane and/or ethane, like what is found
on Saturn's moon Titan might work. Since liquid water dissolves other compounds better than liquid methane/ethane and biochemical
sort of reactions work better in liquid water than liquid methane/ethane, liquid
water will probably be a requirement for a habitable planet. Also, water in
some form (mostly either gas or solid) is actually quite abundant in the Galaxy
so we are not limiting ourselves too much with the water bias. The liquid mileau
is needed to mix...
- the essential building block elements together (carbon, hydrogen,
nitrogen, oxygen, phosphorus, sulfur, and transition metals like iron,
chromium, and nickel). Since the building block elements are only created in
the stars, the best places to look for life is around stars formed from
processed gas, ie., look at metal-rich stars.
- The planet should have a solid surface to concentrate the building block
elements together in the liquid on top. The more concentrated the
solution of water and molecules is, the more likely the molecules will react with
each other. If the
molecules were fixed in a solid, they would not be able to get close to each
other and react with each other. If the molecules were in a gaseous state, they
would be too far apart from each other to react efficiently. Though the reactions
could conceivably take place, they would be rare!
- The planet should also have
enough
gravity to keep an atmosphere. An atmosphere would shield lifeforms on the surface
from harmful radiation (charged particles and high-energy photons) and moderate
the changes in temperatures between night and day to maintain a stable temperature regime.
- A relatively large moon nearby may be needed to keep the planet's
rotation axis from tilting too much and too quickly. This prevents large
differences in temperatures over short timescales (life needs sufficient time to
adapt to temperature changes).
- Plate tectonics may be needed to: 1) regulate the surface temperature
of the planet via its crucial
role in the carbon cycle;
2) create a magnetic field to shield the planet from the deadly stellar winds;
3) create dry land on a water-covered world; and 4) promote a high level of
biodiversity across the planet by creating new environments that organisms
would have to adapt to.
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last updated:
June 6, 2007
Is this page a copy of Strobel's
Astronomy Notes?
Author of original content:
Nick Strobel