ROCKY planets can form without heavy elements, suggests a survey of planets the size of Neptune and smaller. That means Earth-size planets could be found all over the galaxy instead of just round stars with plentiful supplies of 鈥渕etals鈥�, elements heavier than helium.
The first exoplanets discovered were mainly 鈥渉ot Jupiters鈥�, planets up to several times larger than Jupiter and orbiting closer to their sun than Mercury. These tug strongly on their host stars, making them easiest to detect. They tend to be found around high-metal stars, which makes sense because a proto-planetary disc full of metals would be denser and more likely to clump together into planets.
There was a problem, though. To get close to their sun, hot Jupiters must have hurtled inwards, tossing any smaller planets aside. That would make solar systems with small planets like ours very rare.
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But the survey team led by of the University of Copenhagen in Denmark found that the correlation didn鈥檛 hold. They used Earth-based telescopes to look at the spectra of 152 stars harbouring 226 possible planets, identified by the , to determine the stars鈥� composition. Stars with a metal content as low as a quarter of the sun鈥檚 can host planets between one and four times the size of Earth, the team found ().
That鈥檚 good news in the search for rocky, Earth-size worlds. 鈥淭hey could be widespread, since there鈥檚 not a particular kind of star they need to form around,鈥� Buchhave says.
Long-time planet hunter of Yale University isn鈥檛 surprised. 鈥淚 had this gut feeling that the planet-metallicity correlation was a good thing for gas giants, and a bad thing for rocky planets.鈥� She suggests that a low-metal content makes planets grow more slowly, favouring small, rocky planets.
聯I had this gut feeling that the high-metal correlation was good for gas giants and bad for rocky planets聰