WE THINK of stars as having just one shot at forging planets 鈥� a narrow window when the infant stars are surrounded by a disc of dust and gas. Now it seems paired stars may regularly spawn two or even three generations of planets.
The mechanism for this, proposed by at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, is simple, if somewhat macabre. A first clutch of planets would form as normal from a disc around one or both of the young stars.
When one of the stars dies, it sheds material that then forms a disc around its surviving partner, providing the building blocks for a second generation of planets. Such discs have already been observed.
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A third generation may even rise from the ashes shed during the death of the second star. 鈥淢any people look for planets around young stars,鈥� says Perets (). Evolved, double-star systems offer 鈥渁 whole different regime for where to look for planets鈥�.
Finding such systems may not be too difficult. Because double stars drift apart or draw closer together as they age and lose mass, planets that are observed orbiting closer or farther away from a star than expected for first-generation planets may be second generation. Perets has identified several candidates.
Opposite directions
The planets might also form in environments 鈥� such as globular star clusters 鈥� low in the heavy elements needed to form planets, since dying stars shed material enriched in such elements.
Multiple litters may also be spotted orbiting in two different planes, or rotating in different directions within the same plane. Second-generation planets might also be identified if they are unusually massive: some second-generation planets form when material from the dying star flows onto existing planets, potentially causing them to become hefty objects called brown dwarfs.
The process could be bad news for existing planets. The addition of gas and dust could impart enough friction to knock them out of orbit, and perhaps even into the star, says Perets.
Mario Livio of the Space Telescope Science Institute in Baltimore, Maryland, agrees that planets likely form around evolved stars. 鈥淕iven that the process of planet formation is rather poorly understood, it would be very interesting to determine observationally whether some of the scenarios described here indeed materialise in nature,鈥� he told 快猫短视频. 鈥淭he detection or non-detection of such planets could serve to place constraints on theoretical models.鈥�
