MANY giant planets come to a fiery end when their parent stars swallow them
whole, new observations suggest.
Guillermo Gonzalez, an astronomer at the University of Washington in Seattle,
was intrigued to discover three years ago that 51 Pegasi, the first star other
than the Sun that astronomers found to have a planet, had lots of heavy elements
in its atmosphere. He thought this might be because the star had swallowed one
or more giant planets. This made sense, according to calculations by Doug Lin of
the University of California at Santa Cruz. He had earlier predicted that forces
in the dusty discs where planets form would push them into their parent
stars.
Gonzalez and his colleague Chris Laws say that they have now analysed light
from eight stars with nearby giant planets or 鈥渉ot Jupiters鈥�. In a paper due to
be published in The Astronomical Journal, they report that all
the stars have higher than average levels of iron, carbon and other heavy
elements. 鈥淭hat鈥檚 what you鈥檇 expect if you throw planets into a star,鈥� Gonzalez
says.
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He admits that his idea is controversial. Planets may simply form more easily
in environments where there are lots of heavy materials: 鈥淭he whole debate is
now a chicken and egg question.鈥�
Alan Boss, an astrophysicist at the Carnegie Institution of Washington, finds
the question intriguing. 鈥淚t鈥檚 stunning for these stars with hot Jupiters to
have more heavy elements than average,鈥� he says. He thinks environments rich in
heavy materials help build planets, and suspects that planetary material falling
onto a star would become too diluted to be detected.
However, Lin thinks it is easier to explain Gonzalez鈥檚 findings if planets
have plunged into the stars. His computer simulations of a giant planet crashing
into a Sun-like star show that 90 per cent of the planet鈥檚 remains stay in the
star鈥檚 outer envelope. 鈥淚f you contaminate the outer layers of these stars with
10 to 100 Earth masses of heavy material, that would be enough to be seen.鈥�
All the astronomers agree that more observations should resolve the
controversy. Gonzalez hopes to study pairs of stars that formed in the same
environment, one with planets and one without. 鈥淭hat would convince me,鈥� he
says.