The inner solar system may once have been a crowded and violent place 鈥� and Mercury its lone survivor. A new model suggests that most young planetary systems start with several close-in, rocky planets, which later destroy each other in a cascade of collisions.
鈥淚f forming tightly packed systems of inner planets is easy, there鈥檚 no reason it shouldn鈥檛 happen in our solar system,鈥� says at the University of British Columbia, Canada. 鈥淎nd if it happened here, it would solve several problems.鈥�
As astronomers discover more and more planets orbiting other stars, our solar system looks increasingly unusual. While we have four inner rocky planets and four outer gas giants, many other systems have 鈥渉ot Jupiters鈥� very close to their star. What鈥檚 more, observations by NASA鈥檚 Kepler space telescope suggest that between 5 and 10 per cent of planetary systems cram several planets closer to their host star than Mercury is to the sun.
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Within our solar system, Mercury itself is an oddball. Its dense iron core takes up 42 per cent of its volume, its orbit is less circular than that of the other planets, and current planetary formation models predict Mercury should be closer to the sun and bigger, so we know we鈥檙e missing something.
Cadre of rocky planets
In a talk at the American Astronomical Society meeting in Seattle on 6 January, Volk suggested a way to explain both mysteries at once: nearly all stars form a cadre of rocky planets close to them, and most destroy themselves within a billion years. Kepler just sees the ones that survive.
To test this idea, Volk selected 12 of Kepler鈥檚 systems with four or more planets within the orbital distance of Mercury, tweaked their orbits slightly, and simulated what happened next. Some planets collided almost immediately, while others remained stable for 100 million years.
The most violent collisions happen at such high speeds that the planets could simply vaporise each other. Because lighter material would be blown away more easily by the stellar wind, any surviving planets would have to be unusually dense 鈥� like Mercury.
鈥淲e鈥檙e imagining that Mercury would be the lone, highly dense, eccentric planet that鈥檚 the outcome of this process,鈥� Volk says.
It also means our solar system may have formed more like the rest of the planetary systems we see in the galaxy.
鈥淭his would make our solar system look a lot like the Kepler systems, at least in the beginning,鈥� Volk says. 鈥淚t makes us less of a weirdo.鈥�
鈥淚 think that鈥檚 perfectly viable,鈥� says of the University of Washington in Seattle. His one worry is whether the material closer to our sun would have been solid enough to clump into planets in the first place. If it were all vapour, that would also explain why there are no planets inside Mercury鈥檚 orbit. 鈥淪he鈥檚 kind of turning that on its head. I don鈥檛 think there鈥檚 anything intrinsically wrong with that, although it will be hard to prove that that鈥檚 what really happened.鈥�
