BAD news for people hunting extraterrestrials: the cosy, rocky planets that are essential for supporting life might be rare, cosmological freaks. The only reason we are here is because a nearby star happened to explode next to our young Sun just as the Solar System was forming, claims an applied mathematician.
Thomas Clarke at the University of Central Florida in Orlando predicts that the vast majority of planets in the Milky Way are frigid gas giants like Jupiter, with hostile atmospheres and no solid surfaces to walk around on.
鈥淥n average, a solar system will consist of an extensive rocky asteroid belt and some gas giant planets and moons,鈥� says Clarke. 鈥淚t鈥檚 kind of a dismal conclusion.鈥�
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Astronomers agree that the planets and moons of our Solar System formed in a swirling disc of gas and dust around the Sun. In the outer regions, cold, slushy gases condensed into the giants Jupiter, Saturn, Uranus and Neptune. And in the inner regions, dusty particles melted and stuck together, forming hot blobs of rock that cooled and merged to make Mercury, Venus, Earth and Mars.
But it is not clear why the rock melted 鈥� the Sun then was not much hotter than it is now. Astronomers believe that the extra heat may have come from radioactive aluminium-26 that was sprayed out of a star that exploded up to 50 light years away when the planets were forming. Decay products of the isotope, which has a half-life of 720,000 years, have been found in meteorites.
At last week鈥檚 Lunar and Planetary Science conference near Houston, in Texas, Clarke suggested that without the heat from the aluminium, the Earth would not have formed. While asteroid-sized rocks would have aggregated in the inner Solar System, they would not have melted and clumped together to form planets.
According to Clarke鈥檚 calculations, the solid rocks would simply zoom past each other or collide and recoil like snooker balls. Only molten, squidgy rocks would deform and lose energy in a collision, he says, allowing them to stick together and grow.
But the chance of a star exploding at just the right time and place is very much against the odds. Stars only explode three or four times a century in our Galaxy. Clarke estimates that the probability of a supernova happening within 50 light years of any new solar system that is busy forming planets is only about 1 in 100. 鈥淪o only a small fraction of planetary systems would be expected to have terrestrial planets,鈥� says Clarke.
He concedes a small, close-in planet like Mercury could form without being warmed by radioactive aluminium, as it could be near enough to its star for the rock to melt. But such planets would probably remain far too hot for life to thrive. Farther out, it would only be possible to form asteroids and icy gas giants.
Astronomers have already discovered more than 100 planets in other solar systems. All of them are gas giants, which current astronomical techniques are best equipped to detect.
NASA and the European Space Agency have plans for ambitious missions to find Earth-sized planets, but if Clarke is right, they may not find what they are looking for. 鈥淚t鈥檚 a bit depressing to think that Earth-like planets are too special,鈥� says Clarke.
His idea is speculative and he hopes to test it further in computer simulations. But if it stands up, it could help explain why there are no signs 鈥� so far 鈥� of alien civilisations.
