Where should we look?

Last updated: 30/1/01 - entire text, additional external link

The question being asked in this section is not, 'Where can we find planets' but 'Where can we find planets that might have life on them'?

First, we have to establish exactly what a habitable planet needs. The general scientific consensus is that liquid surface water and 'healthy' biogeochemical cycles are both required; water is an important solvent and is involved in many biological reactions, and as far as we know, all life is carbon-based (hence the requirement for a stable carbon cycle).

Now we've done that, we have to find out what type of stars habitable planets will orbit. If we assume that for life to evolve, a planet needs to have liquid surface water for a long period, then we can say that most stars around the size of our sun (G-type stars) will be able to sustain stable habitable zones for billions of years. The habitable zone is the region around a star in which planets can sustain life.

Why only G-type stars? Stars that are larger than our sun (A and F-type stars) are much hotter than smaller stars and 'burn out' more quickly. This means that even if Earth-like planets are present in the habitable zones of large stars, there may not be enough time for life to evolve on them before the star either explodes or shrinks. However, since large stars emit more ultraviolet radiation (UV light causes mutations in organisms) they might be able to speed up evolution - but this is only a theory.

With stars that are smaller than our sun (K and M-type stars), their habitable zones will be much closer to the star - which means that any habitable planets will be orbiting very close to the star. So far so good, but if a planet is orbiting so close to a star, it becomes 'tidally-locked' to the star so that the same face of the planet is always facing the star (like the way the Moon is tidally locked to the Earth). This means that one side of the planet would be in perpetual daylight and the other side in perpetual night. Even so, it is conceivable that life might evolve under such circumstances.

Yet such planets have even more problems. Small stars can vary their luminosity (their brightness) by 0.1% due to flares and 'starspots'. 0.1% might not sound like a lot, but it can affect a planet's ecosystem extremely adversely.

So essentially, if you're looking for habitable planets, the best place to start is by searching for G-type stars.

Once you've found your G-type star, that doesn't automatically mean it'll have any habitable planets around it. What you need to do now is to find out if there are any terrestrial (rock-based) planets within the star's habitable zone. At the moment, we don't know how likely this might be, but we can give reasons for why you might, or might not, find such planets.

	51 Pegasi B
	The planet orbiting 51 Pegasi (named 51 Pegasi B) may be the first ever planet to be discovered orbiting a normal star roughly the size of the sun. 51 Pegasi is a G-type star (G2-3 main sequence, to be exact) 42 light years from Earth. 51 Pegasi B was detected using the 'wobble' method where anomalies in the star's radial velocity indicated that it was under the influence of a large planet nearby. The planet is estimated to be a mere 7 million km from the sun (much closer than Mercury is to our sun) and has a mass of at least half that of Jupiter. This would result in a red-hot molten ball of iron and rock with 7 times Earth's mass and gravity, tidally locked towards its star. Above: an artist's impression of 51 Pegasi B. Used with permission, © 2000 John Whatmough.

Solar Systems are very delicate systems with each planet depending on the gravity of the sun to keep it in orbit. However, planets can influence each others' orbits with their own gravity. Of course, if you have a star with only one terrestrial planet orbiting it right in the middle of its habitable zone, you have no problem with gravitational influences, or 'perturbations' (but you will have many other problems, as will be outlined later).

Most solar systems have more than one planet. It's difficult to say what the stability of a solar system will be with x numbers of planets, but generally if you want a stable solar system with no planets flying out of their orbits due to perturbations (and causing a chain reaction of more perturbations) there needs to be a large spacing between the orbits of planets, and all the orbits need to be in the same plane.

Interestingly, this 'large spacing' between the orbits of terrestrial planets (like Earth) is roughly the width of a star's habitable zone. This means that the largest number of planets you could have in a star's habitable zone is two.

(You have to take all of this with a pinch of salt - it is possible that by varying the amount of CO₂ in the atmosphere of planet just a little outside the habitable zone ('fiddling with the Greenhouse Effect') you could increase or decrease the temperature so that liquid water can exist on the surface.)

Finally, now that you've found your terrestrial planet residing in the habitable zone of a G-type star with a good distance separating its orbit from the orbits of other planets, you might think you've finished. You haven't. There's no doubt that this planet is habitable - but does that mean life will definitely evolve on it?

As it happens, no. Possibly the largest threat to life developing on a habitable planet is the possibility of an asteroid impact (like those seen on Deep Impact and Armageddon but with a little more science and a little less gung-ho). If you have a large number of asteroid impacts on your planet, life will be wiped out every time it tries to evolve.

This doesn't mean that you can't have any asteroid impacts - after all, life on Earth survived the impact of the K/T meteorite that measured 10km in radius (the dinosaurs, unfortunately, were not so lucky). But imagine a K/T event or an even bigger impact occuring every million years! Life wouldn't get the chance to recover (there are also other threats to life to civilizations than meteorites, as explained in Life Islands).

We can estimate the number of impacts on planets by looking at the other planets in the solar system. If you have a solar system like ours, then the large gas giants such as Jupiter and Saturn will tend to snag or deflect any comets heading on a collision course towards us due to their large gravitational influence (these comets tend to originate from the Oort cloud, a vast collection of comets that encircles the solar system about 5000 to 50,000 times the distance from the sun that we are from the sun).

However, if the gas giants were replaced with smaller planets, they wouldn't be able to deflect these comets and we'd be hit by many more impacts - not a good thing for life. Which explains why a single terrestrial planet orbiting a sun (as described earlier) is not a good thing at all since there wouldn't be anything stopping all the comets hitting it.

So, in conclusion, what you're looking for is a G-type star with a terrestrial planet in its habitable zone that is a safe distance away from any other terrestrial planets, but also has a few large gas giants in the outer solar system. Easy, right? Now all you have to do is to find it.