Astronomers have found an extrasolar planet with the largest solid core ever detected. The core is so immense 鈥� as massive as 70 Earths 鈥� that astronomers believe it might have formed through collisions with other planets.
The planet was discovered around a star called HD 149026, which is rich in elements heavier than hydrogen and helium. Such 鈥渕etal-rich鈥� stars are three times more likely than their Sun-like cousins to harbour detectable planets.
Astronomers using Japan鈥檚 powerful Subaru Telescope in Hawaii first detected the planet, called HD 149026b, in July 2004 by the gravitational tugs it exerted on its host star. Subsequent measurements taken by the Keck Observatory in Hawaii confirmed the star鈥檚 resultant wobble, revealing the planet orbits its star every 2.87 days.
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But in May 2005, astronomers using a relatively small robotic telescope in Arizona noticed a 0.3% dip in the star鈥檚 brightness every 2.87 days. That dimming showed the planet was among an elite group of eight which are known to pass directly between their host stars and Earth during their orbits. Such transits can reveal the planet鈥檚 exact physical size and mass, from which its density can be calculated.
鈥淲eird object鈥�
The results were surprising. The planet鈥檚 diameter was about three-quarters that of Jupiter. But its density suggested that half to two-thirds of its mass was locked in heavy elements 鈥� mostly in a solid core.
That differs strikingly from other transiting extrasolar planets which, like Jupiter, are mostly hydrogen and helium gas with no more than a quarter of their mass taken up by their cores.鈥漈his is a very weird object,鈥� says team member Greg Laughlin, an astronomer at the University of California in Santa Cruz, US.
Indeed, the discovery strains the two main planet-formation theories. In the 鈥渃ore accretion model鈥�, planets form in a snowball effect by gradually building up a rocky core as a result of random collisions. The increasing gravity of the growing core then begins to attract more material and the gases that make up an atmosphere.
In the alternative 鈥済ravitational instability model鈥�, gas giant planets form quickly by condensing directly from a cloud of gas and dust surrounding a young star.
Pushed to the limit
鈥淲e have no rigorous model for how this planet formed,鈥� Laughlin told New 杏吧原创. He says gas giants in the gravitational instability model could gain heavy elements through collisions with comets or asteroids, but would not be able to swallow enough material to explain this observation.
鈥淚鈥檓 a long-term cheerleader for the core accretion model,鈥� he says. 鈥淏ut this really pushes the model to its limits.鈥�
Laughlin says the new discovery suggests 鈥渟omething extra鈥�, such as a collision with one or more planets in the past, could explain the dense core. If so, the collision might have knocked the planet out of the star鈥檚 equatorial plane. Astronomers have re-observed the star to check its axis of rotation and are currently processing this data.
Inward migration
The planet is a scalding 1200掳C and lies about 25 times closer to its star than Earth does to the Sun. It probably migrated towards the star from a more distant orbit when friction with material in a disc around the star slowed it down, allowing the star鈥檚 gravity to pull it closer.
Laughlin hopes to find more transiting planets to see how unusual this object is. The best data so far is on three extrasolar planets that transit bright stars, but all three are quite different. One is about a third larger in diameter than expected by theory, one is the predicted size, and this object is smaller and denser than expected.
That suggests planets lying very close to their stars 鈥� which are easiest for astronomers to detect 鈥� 鈥渁re most likely to have suffered problems in the past鈥�, says Laughlin. 鈥淭he fact you see oddballs all close to the star is telling us we鈥檙e seeing the ruined fringes of the planetary distribution.鈥�
A paper on the newly discovered planet will be published in the Astrophysical Journal.
