North Hawaii News Articles from CFHT

Life in the Universe III: ETs around Extra-Solar Planets?

Life exists in our Universe. It has conquered the Earth where it flourishes in innumerable forms, and it may also exist in a few unique places in our Solar System. How about elsewhere, outside our Solar System? What is needed to produce Life? What is the secret recipe?

Having only one single example of a suitable set of conditions that can promote Life (the Earth), it is difficult to project and predict where else in our Universe Life may exist. Nonetheless, based on what we know (or what we think we know) scientists have some theories (that may be proven wrong, or right, in a few years).

First, we need a star, but not any will do! A star which is too massive will burn very fast and have a very short life, too short too allow life to appear and develop. For example, a star which is born with about 10 times the mass of our Sun will live only 10 million years. Compare with the Sun which is already 5 billion years old, and saw its first life-forms only after 1.5 billion years!

On the other hand, if a star is too small, it will have a very long life (tens of billions of years), but will shine very faintly, putting out very little energy (in the form of starlight) for Life to use.

So a star about the Sun's mass, or at least not much smaller or bigger, is the first ingredient. We know that our Galaxy has of the order of 200 billions stars, but only a modest fraction, maybe around 15%, is comparable to the Sun.

Then, a single star would probably be preferable to a double one, or to a member of a more complex system (3 stars or more). The gravitational influence of many stars on a planet can modify its orbit (sometimes even eject the planet from the system!), bringing it close to the scorching hot surface of one star, then flinging it in the cold space away from any source of heat, not giving life a chance for a stable environment. Some planets may exist on a stable orbit around many stars, or in a system where the second star is far away, but we think it is more difficult to have stable conditions in those cases. We know that most stars (85%) are found in double or multiple stellar systems, so a single lonely star is hard to find.

Once we have a suitable star, we need a suitable planet. But are planets common in other stellar systems? Until about a decade ago, astronomers were not even sure planets could exist elsewhere, because they did not have solid proof. But the good news is that astronomers have started to detect planets around other stars! They are mostly giant planets circling very close to their parent star, but smaller, rocky-type planets will soon follow.

So far, Life in our Solar System has appeared or could appear on rocky planets or satellites, but not on gaseous planets like Jupiter. The size of this rocky body might not be very relevant, but the distance to its star is. Too close, and the high temperature will boil off anything from the surface. Too far away, and not enough energy will reach it, forbidding liquid water.

And even then, a rocky planet at a suitable distance from a single Sun-like star might not be enough. Earth rotates about an axis whose orientation has been relatively stable over its life, and our Moon has played a crucial role in that stability. If that stability had not been there, the Sun would have lighted the Equator, then the Poles, and all intermediate latitude of our globe, thawing then freezing every part of the surface, perturbing the delicate equilibrium in temperature and illumination, disrupting any ecosystem that might have appeared, thus rendering the emergence and survival of life more difficult. It seems that a big satellite can control the orientation of the rotation axis and keep it from wandering chaotically.

But one planet might not be enough to ensure Life! In recent years, it has become clear that, at least in our Solar System, Jupiter, 320 times more massive than the Earth, and the other giants have played an important role for Life on Earth! Those big planets have attracted to themselves numerous asteroids and comets, thus diverting them and preventing them from hitting Earth. Without giant planets and their shielding effect, Earth might have been bombarded so many times that Life would not have had the time to flourish once started.

So, how many stellar systems with a not-too-massive single star and one or more rocky planets at the right distance with maybe a satellite and giants planets to help it harbor life might we find in the Universe? There are so many possibilities, so many unknowns, that we are really not sure yet. The answer could be anywhere between "zero" and "millions", but something in the middle would seem more reasonable.

Let's push it a little bit more... If there are stars around which planets suitable for life exist, what is the probability that life has really appeared and developed? What is the probability that that life is intelligent and trying to communicate with other life forms (us!) in our Universe?

That question is addressed by a famous equation, the Drake Equation. It just takes the number of stars in our Galaxy (we're pretty sure it's around 200 billion), multiplies that by the fraction of stars which have planets (still a hard number to estimate), then by the fraction of planets which can sustain life (a big unknown), the fraction of planets where life has indeed developed, the fraction where that life is intelligent, and wants to communicate, and survives long enough to do so!

If you want to play with the numbers, go check the web site I have tried, and have come up with less than 10 planets in our Galaxy, or as many as 100,000!

Nadine Manset
Resident Astronomer
Canada-France-Hawaii Telescope Corporation
Jan. 18, 2001