Extrasolar planet gets heavier

From College Park, Md., at the Annual October Astronomy Meeting

Among the more than 100 extrasolar planets discovered since 1995, nearly all have been detected by measuring the tiny wobble they induce in the motion of their parent stars. But a few have been found by recording the tiny dip in starlight that occurs each time the putative planet passes in front of the star it orbits. This so-called transit method, which requires a rare alignment among the star, the planet, and Earth, yields both the radius and mass of a planet. The wobble technique can reveal only a lower limit for the mass.

The astronomers who last January reported finding the first planet using the transit method (SN: 1/18/03, p. 38: Available to subscribers at Distant and Strange: Orb isn’t just another extrasolar planet) have now refined their estimate of the planet’s mass. Rather than weighing nearly as much Jupiter, the newfound planet, designated OGLE-TR-56b, is about 1.5 times as heavy as Jupiter, reports Guillermo Torres of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

The newly calculated mass comes from a combination of the transit data, as well as new readings of the parent star’s spectra, which indicate a star’s wobble. The planet appears to have a radius 1.23 times that of Jupiter. OGLE-TR-56b lies so close to its parent star that its surface must be hot enough to melt iron.

The revised mass puts the planet more in line with models of how such a sizzling planet could evolve, says Peter Bodenheimer of the University of California, Santa Cruz.

His team’s model, described in the July 20 Astrophysical Journal, assumes that OGLE-TR-56b was initially twice as large as Jupiter but that over 4 billion years, it contracted, despite being blasted by inflationary heat and radiation.


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