Outlier Planet: Extrasolar places that are like home

After detecting more than half of the 90 or so extrasolar planets now known, a team of veteran hunters has scored a landmark finding. This week, they announced that they had found the first Jupiterlike planet orbiting a star at nearly the same distance that Jupiter orbits the sun.

JUST LIKE JUPITER. Drawing of the planetary system orbiting the star 55 Cancri, with the newly discovered Jupiterlike planet in the foreground. L. Cook

At least one other planet, discovered by the same research team in 1996 (SN: 4/27/96, p. 267), also orbits the star 55 Cancri. That planet lies much nearer to 55 Cancri than does Mercury, the solar system’s innermost planet, to the sun. Nonetheless, the planetary system bears the closest resemblance to our own of all systems detected so far, the researchers say. 55 Cancri has about the same mass as the sun and lies just 41 light-years from Earth.

A team led by Geoffrey W. Marcy of the University of California, Berkeley and

R. Paul Butler of the Carnegie Institution of Washington (D.C.) is announcing the

findings June 13 at NASA headquarters in Washington, D.C.

Like all the other extrasolar planets found to date, the newest discovery was detected indirectly. The planet’s tug pulls 55 Cancri ever so slightly to and fro. The resulting wobble in the star’s motion shows up as a periodic shift in the wavelength

of light it emits.

Massive planets that tightly orbit their parent stars, whipping around them in a matter of days, induce an extremely rapid, easily detectable wobble. But Jupiter, which lies 5.2 times as far from the sun as Earth does, takes a leisurely 12 years to complete one orbit. The newfound planet, 3.5 to 5 times the mass of Jupiter, takes 13 years to complete its elliptical orbit. Marcy and Butler reported early hints of a planet 6 years ago (SN: 7/6/96, p.11), but to confirm it, they and their collaborators had to rely on spectral data spanning most of the 15 years they’ve hunted planets.

“We haven’t yet found an exact solar system analog, which would have a circular orbit and a mass closer to that of Jupiter, but this shows we are getting close,” says Butler.

After finding many planets quite unlike those in our solar system, “it’s nice to know that solar system analogs are out there and that the [wobble-detection] technique is now probing planets on a solar system scale,” says Adam S. Burrows of the University of Arizona in Tucson.

55 Cancri’s wobble can’t be accounted for fully by the two planets now confirmed to be orbiting it. The data hint that a third planet, about as massive as Saturn, may orbit 55 Cancri at a distance about half that at which Mercury orbits the sun.

If a planet with the same mass as Earth were placed in the planetary system orbiting 55 Cancri, it could survive there for billions of years, according to calculations by Gregory P. Laughlin of the University of California, Santa Cruz. However, says Alan P. Boss of the Carnegie Institution, the chances are slim that an Earthlike planet resides among the known planets of the star.

Theorists believe that planets that tightly orbit their parent star formed farther out and then migrated inward. In the case of 55 Cancri, the one or two giant planets that now closely circle the star would have passed through the region where an Earth-mass planet would reside, “thereby ejecting or swallowing any proto-Earths that happened to get in their way,” says Boss. “I for one would be absolutely astounded if there was an Earth-mass planet in between the gas giants.”

At the NASA briefing, Marcy and Butler are also announcing the discovery of 13 other planets, including what may be the lowest-mass extrasolar planet ever detected.

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