Jupiter's Brother Joins the Family

Dick writes about Earth and planetary science for Science magazine.

Astronomers at a meeting in Washington, D.C., will announce on 18 June the discovery of a near twin of Jupiter orbiting another star. Last week, a "first cousin" of our solar system made front-page news, but the new find looks to be a much closer relation: An exoplanet resembling Jupiter in a planetary system like our own. The find is the most promising discovery of a planetary system where Earth-like planets may be hiding.

THE JUPITERS
55 Cancri's HD190360's Our Jupiter
Minimum mass 4 x Jupiter's 1.1 x 1.0
Mean orbital distance 5.9 AU 3.7 AU 5.2 AU
Eccentricity 0.16 Less than 0.1 0.05
Orbital period 13 years 7.1 years 11.9 years
Who's most brotherly of them all? A newly discovered planet, orbiting the star HD190360, bears a startling resemblance to Jupiter.
CREDIT: G. MARCY ET AL./M. MAYOR ET AL.

Before last week's announcement, 76 exoplanets had been discovered. Most were "hot Jupiters" orbiting closer to their stars than Mercury does to the sun. All the latest discoveries came as astronomers searched for telltale stellar wobbling, a sign that the gravity of a massive unseen planet tugs the parent star back and forth.

Last week researchers led by Geoffrey Marcy of the University of California, Berkeley, described a "near analog" of Jupiter (Science,NOW, 13 June). Now astronomer Michel Mayor of the Geneva Observatory in Sauverny, Switzerland, and his colleagues report what they feel is a true Jupiter. Orbiting the star HD190360, this new exoplanet has a minimum mass just 1.1 times that of Jupiter and orbits at 3.7 times Earth's distance from the sun (astronomical units, or AU). In the solar system, that would put it nearer Jupiter than Mars.

The new planet's strongest claim to Jupiter-likeness lies in its familiar surroundings: In Doppler-shift observations, its planetary system looks nearly identical to ours. In contrast, the Berkeley team had already found one hot Jupiter orbiting their star, 55 Cancri, before they announced a second last week. Like all hot Jupiters, these must have formed farther out and drifted inward, driving everything before them into the star and vaporizing any inner, Earth-like planets.

Astronomers are welcoming both discoveries as the vanguard of a coming Jupiter bonanza. "I think it's great," says astronomer David Trilling of the University of Pennsylvania in Philadelphia. "In the next few years, there will be dozens and dozens more."

Related sites
California & Carnegie Planet Search
Geneva Extrasolar Planet Search Program

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