Re: Habitable zones of stars

Dear The Insane One,

There are a few conditions to consider when determining
the habitable zone around a star.

1) Since the more massive a star the shorter its lifetime, the mass
of the central star should not be more than 1.5 times the mass of
the Sun. This is because we want the star to live at least 2 billion years
to allow life enough time to evolve.

2) On the other hand, the star needs to have a mass of at least 0.3 times
the that of the Sun since the less massive the star the cooler it is.
The star needs to be hot enough so that there is a region in which
the orbiting planet will be warm enough to have liquid water.

3) The the radiation from the central star should be stable in that there
should not be many X-ray or UV flares. This means that the star should
lie along the "main sequence" which represents the stable phase of a star's 
life when it is simply burning hydrogen in its core. The previous
temperature restrictions narrow down the choices of stellar spectral type
to F, G and K. The sun is a G type star with a photospheric temperature
of  around 5700 Kelvin. F stars are a bit hotter and K stars are a bit 
cooler.

4) The location of the habitable zone will then be determined by the 
distance in which liquid water will exist and not be permanently frozen or
evaporate.

5) The orbit of the planet should be close to circular so that
the temperatures on the planet surface are fairly stable throughout the 
year. This could rule out planets existing in binary or multiple star systems.

6) The planet itself must be massive enough so that it retains enough
of an atmosphere to support life.

This list of criteria for a habitable zone was adapted from "Astronomy: A
Cosmic Journey" by William Hartmann.

As for your specific questions:

Definition of habitable zones -
Despite the above criteria, our definition of a habitable zone
is indeed changing. For instance, new discoveries of
unique lifeforms around hydrothermal vents in our oceans have already
negated the requirement that all life depends on sunlight in some fashion.
Many universities are now developing Astrobiology
institutes which will combine biology, astronomy and planetary science
research in order to better define the necessary ingredients to
produce life on a planet or moon. So the above ideas about what defines a
habitable zone should be broadened to consider all possible forms of life.
There is agreement, however, that liquid water does play an important role 
in the development of life.

A white dwarf or red giant -
If there was a habitable zone around the star originally,
the red giant (or possible a supernova) stage which occurs before the star 
becomes a white dwarf would probably wipe out any life which had
originally formed on a planet or moon within the zone. Since our own Sun will
eventually become a redgiant, much thought has gone into the fate of the
Earth during this event.
Besides being fried since we will be inside the atmosphere of the Sun,
when the Sun's atmosphere reaches the Earth, our orbit will
be affected as well. The Earth may fall into the Sun and be destroyed or
it will be flung far from the Sun and we will freeze. Astronomers are still
debating over which scenario is most probable.

Brown dwarfs - Now that is a good question. Again, the important
factor is the presence of liquid water. We can consider this possibility
by looking within our own solar system since Jupiter is much like
a brown dwarf in temperature and composition. There has been recent
excitement about the possibility for life on Europa (one of Jupiter's
moons) which has a frozen outer crust. Scientists are planning a
mission to launch a space probe to Europa which will
try to land on the crust and somehow cut through the crust to sample the 
liquid beneath.

Read more about the Europa mission at:
 http://sse.jpl.nasa.gov/missions/jup_missns/europa.html

Planets close to stars -

The many recent discoveries of Jupiter-like planets around other stars
have made us rethink the formation of other solar systems. Many of these
planets are closer to their stars than the Earth is to the Sun. Having life
on a planet or moon close to the star will probably require a cooler star 
than our Sun.

For information on the search for exoplanets check out:
 http://cannon.sfsu.edu/~gmarcy/planetsearch/planetsearch.html

Thanks for your question. I enjoyed thinking about it.

                            Angelle Tanner
                            UCLA Astronomy