Extrasolar Planets & SETI, Page 16: [Previous Page] [Next Page] [Syllabus] Textbook pages 260 - 268

Other Stellar Limitations

We want to form planets that get the right amount of light, but also, planets that are the right kind of geology! Terrestrial planets (those with rocky surfaces and thin atmospheres) are only formed in the inner proto-planetary disk. For the Sun, terrestrial rocky planets only can form in the inner 2-3 AU, where ices can not form.

Very massive stars have several disadvantages.

1) Because they are so luminous, their HZ may be too far out, beyond the distance we believe terrestrial, rocky planets will even form.
2) Massive stars have very short lifetimes. We must allow at least 1-2 billion years for life to form before the star dies. This puts an upper limit on the mass of any planet hosting star at 1.5x the mass of the Sun (stars more than 1.5x the mass of the Sun die in less than 2 billion years).
3) Massive stars change in brightness quicker, meaning a CHZ of even a billion years might not be possible, even if the star can live 1-2 billion years.

Solar mass stars have several advantages

1) Unlike the high mass, solar mass stars have very, very long lifetimes.
2) Stars even smaller than the Sun, 0.5x Sun (an M0 star), may have a CHZ for tens of billions of years, or more. Their HZ lie just in the range we expect terrestrial planets to form.


What kind of stars are most common?
It just so happens, Solar mass and smaller stars are the most abundant stars in our galaxy. In fact, most stars in the galaxy are the Sun's mass or less.

There are probably a few hundred billion stars in the galaxy. This suggests there are more than a hundred billion viable star-planet systems from which a habitable planet may exist.

Complications
We saw that many local stars have large Jupiter like planets located in the `terrestrial' planet zone. Should this be common, such giant planets would disrupt and possibly destroy or eliminate any smaller rocky planets hoping to exist within the Stars HZ.