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A new class of exoplanet

Two groups discover planets with masses similar to Neptune's, unexpectedly bringing astronomers one step closer to finding Earth-size worlds around other stars.
Artist's rendition of newfound Neptune-sized planet
An artist imagines this view of the newfound Neptune-mass planet circling 55 Cancri, a Sun-like star 41 light-years away.
NASA
August 31, 2004
Astronomers announced today the first discovery of a new class of extrasolar planets far smaller than any previously detected around a Sun-like star. Both planets have masses and sizes comparable to the planet Neptune in our own solar system. One of the new planets was discovered around a star already known to have three others, making this the first four-planet system identified so far.

Some 135 exoplanets have been securely identified, but most of these are "hot Jupiters" — planets of Jupiter's mass or greater that orbit very close to their stars. The discovery of Neptune-mass worlds "puts us in that transition region where we can't quite see the Earth-like planets yet, but we're seeing their big brothers," said Paul Butler of the Carnegie Institution of Washington, speaking during today's press conference at NASA headquarters.

The composition of these "exo-Neptunes" remains unknown, say scientists. The planets could be true gas giants, similar to our Jupiter and Saturn — and most of the exoplanets found so far. But with a mass near that of Neptune, there are new possibilities. The planets could have rock-ice cores covered by a thick envelope of hydrogen and helium gas, as do our Uranus and Neptune, and they could even be solid chunks of rock and iron, like Mercury.
Red dwarf
The first hint of these new worlds came in July 2003. Butler and collaborators have been measuring Doppler shifts in the spectra of 950 nearby stars, about 150 of which are cool, low mass red dwarfs. The spectrum of one dwarf, called Gliese 436, showed a regular Doppler shift, indicating that the gravity of some unseen object was causing the star to wobble slightly. The star, which lies 33 light-years away in the constellation Leo, has less than half the mass of our Sun and is probably more than 3 billion years old.

"About 70 percent of all stars are red dwarfs . . . but they only put out about 2 or 3 percent the amount of light of the Sun, so they're very, very faint," Butler said. While red dwarfs are the most numerous type of star in the galaxy, astronomers know the least about them because they're so faint.

Based on the regularity of the wobble, Butler's team determined that the object followed a nearly circular orbit, while the amount of shift implied a body with a mass no less than 21 times that of Earth. Team member Geoff Marcy of the University of California at Berkeley told Astronomy that, after taking into account the way the system's orbital plane tilts toward us, the planet likely weighs in at roughly 25 Earth masses.

The new world whips around Gliese 436 in just 2.64 Earth days at a distance of 0.028 AU, or about 3 percent of the distance between Earth and our Sun. At such close proximity, tides likely have stopped the planet's rotation. Astronomers estimate the temperature at high noon on the planet's sunward side to be around 700° Fahrenheit (650 K).

Only one other planet has been found around a red dwarf (Gliese 876), but the enormous number of these stars suggests exoplanets aplenty. "We estimate that something like 20 billion planetary systems exist within our Milky Way Galaxy alone," Marcy said.
Quadruple planets
The other Neptune-mass planet orbits the star 55 Cancri. It has about 18 times the mass of Earth and, added to the three other planets astronomers had already discovered around this star, makes it the first quadruple-planet system known. "It's quite a remarkable discovery," said Marcy. "Obviously, we're edging closer and closer to planetary systems . . . like the nine planets that go around our Sun."

In terms of mass and age, 55 Cancri is a star very much like our Sun. "This is a well studied system and is the closest analog to our own solar system," said Barbara McArthur of the McDonald Observatory in Texas. The star is located 41 light-years away in the constellation Cancer.

The three planets already known revolve around 55 Cancri every 15 days, 44 days, and 12.3 years, respectively. Marcy and Butler discovered the outermost planet in 2002. It remains the only known Jupiter-like gas giant to reside as far away from its star as our own Jupiter.

Debra Fischer, an astronomer at the University of California and a member of Butler's team, had been studying the spectrum of 55 Cancri for years. She found a residual wobble that could not be accounted for by the three planets already known and sent Doppler measurements — going back to 1987 — to McArthur, who had been organizing a campaign to study the system.

Using the 9.2-meter Hobby-Eberly Telescope, McArthur obtained more than 100 Doppler measurements in a span of 6 months. Such a "snapshot" of 55 Cancri, in which time-varying changes were greatly reduced, promised to be the best way to define the orbits of the system's two known inner planets.

To better characterize the orbit of the distant planet, McArthur combined Fischer's measurements with her own and employed statistical techniques to model the three orbits. "I must have run, I would say, 10,000 simulations of this system to find the orbits," she said. Out of this difficult analysis came what Marcy described as "incontrovertible evidence" of the fourth planet. McArthur's team also used archived data from the Hubble Space Telescope's Fine Guidance Sensor to estimate the tilt of the 55 Cancri system, which helped refine the planet's mass.

The new world at 55 Cancri whirls around its sun on a nearly circular orbit every 2.81 days. It lies 0.038 AU from its star, or just 4 percent of Earth's distance from the Sun, so tides have probably stopped its rotation, too. However, the star is brighter than Gliese 436, and temperatures on the planet's sunlit side likely hover around 2,200° F (1,500 K).
Waiting in the wings
On August 25, a third group announced the discovery of a Neptune-like world. A team led by Michel Mayor of Switzerland's Geneva Observatory found Doppler shifts in the spectrum of Mu (μ) Arae, a Sun-like star about 50 light-years from Earth. Although the study has just entered the peer review process, Mayor's team may have found the least massive planet so far — a 14-Earth-mass "exo-Uranus." It orbits 0.09 AU from its star, completes a circuit every 9.6 days, and is probably "blowtorched" to a temperature of 1,100° F (900 K) on its sunward side.

"The very fact that these small-mass planets exist — and there's a hint that there's a lot more small-mass planets than bigger planets — is causing us . . . to completely revamp our own search strategy," Butler said. He hopes to refine current techniques to "push down into the super-Earth regime" — that is, planets with 10 Earth masses or less — before the end of the decade.

By then, planet-hunting space missions such as Kepler and Terrestrial Planet Finder should be ready to seek true Earth-like planets.
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