TRUE clones of Earth may be rare indeed. It seems that many of the super-Earths spotted in our galaxy so far formed in a very different way to our own celestial home.
Super-Earths – rocky planets 1 to 10 times the size of ours – are just one type of weird world uncovered in recent years. Hot Jupiters – enormous balls of gas that sit closer to their stars than Mercury is to the sun – are another. Understanding how these types of worlds form should help our search for other inhabited worlds.
Earth formed in roughly the zone it still inhabits, but to grow so large, these planets had to start further out from their stars and migrate inwards, drawing in matter from discs of dust and gas around the star along the way. That, however, should have made them spiral into their stars within a few hundred thousand years.
Ralph Pudritz at McMaster University in Ontario, Canada, and colleagues simulated planets forming in systems with different discs. They found that rings of slow-moving gas slowed the planets, allowing them to live until the disc was gone. Jupiters piled up at about Earth's distance from the sun or closer in big, long-lived discs. Thin, short-lived discs tend to produce super-Earth-sized planets in similar orbits to the hot Jupiters (arxiv.org/abs/1310.2009).
That suggests super-Earths might have got bigger if the gas hadn't run out. "They get to 5 to 10 Earth masses, when gravity lets you pull in gas more quickly, but by the time they're ready to accrete the gas [and become gas giants], it is gone," says Pudritz. "We call them failed Jupiters."
Jonathan Fortney of the University of California, Santa Cruz, says what we know about the atmospheres of super-Earths supports the failed Jupiter theory.
Modelling their rocky cores should tell us how different super-Earths are from our planet and how suitable they are for life.
This article appeared in print under the headline "Earth's siblings grew up very differently"
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