Two More Neptune-mass Exoplanets

55 Cancri and inner planet
An artist depicts the solar-type star 55 Cancri and its newly discovered inner planet. The planet has a minimum mass only 15 times that of Earth. Nobody knows whether it is a rocky body like Earth, a gaseous planet like Jupiter and Saturn, or a hybrid world like Uranus or Neptune.
Courtesy NASA/JPL.
The planet-hunting team led by Geoffrey W. Marcy (University of California, Berkeley) and R. Paul Butler (Carnegie Institution of Washington) continues to push the exoplanet envelope. As if discovering or codiscovering 98 of the 135 or so known planets around other stars weren't enough, the team has announced two new ones with minimum masses just 15 and 21 times that of Earth. Because we don't know the inclination of these planets' orbits, the most likely masses are roughly 18 and 25 Earths — slightly more massive than Neptune, which contains 17.2 Earth masses. These worlds, along with a third Neptune-mass body announced last week by the Swiss team led by Michel Mayor, are the lightest planets yet discovered around normal stars.

All three new planets orbit their stars extremely closely, which explains why such planetary lightweights could be found. One of the Marcy/Butler finds orbits 55 Cancri (also known as Rho1 Cancri), a G8 star a little smaller and dimmer than the Sun that was already known to possess three planets. This discovery thus makes 55 Cancri the first quadruple-planet system known outside our solar system. The 18-Earth-mass body races around the star every 2.81 days at just 0.038 astronomical unit (a tenth of Mercury's distance from the Sun). At that distance, its upper atmosphere (assuming it has an atmosphere) should be heated to a scalding 1,500° C.

55 Cancri system
The first known quadruple-planet system outside the system belongs to the solar-type star 55 Cancri. Three planets, including a newly found Neptune-mass object, huddle close to the star. The fourth planet orbits much farther out, at approximately Jupiter's distance from the Sun. Depending on your monitor, the planet would be roughly 2 meters (6.6 feet) from the star if you click on the image above to view a larger image. The white horizontal lines straddling the planets indicate their minimum and maximum distances from the star as they trace elliptical orbits.
S&T diagram by Gregg Dinderman.

The other new Marcy/Butler planet orbits the red dwarf Gliese 436, spectral type M2.5, 33 light-years away in Leo. This is just the second red dwarf known to possess planets, the other being Gliese 876, which has a two-planet system. The low rate of planets found around red dwarfs implies that these low-mass stars rarely have massive attendants in close orbits, probably because their protoplanetary disks had small masses too.

Gliese 436b whirls around its star every 2.64 days at an average distance of 0.028 a.u, which is just seven times the diameter of the star itself. Seen from the planet's surface, the star would appear about the size of a tennis ball held at arm's length. Given Gliese 436's low luminosity, the planet's upper atmosphere should be about 380° C. Although a circular orbit has not been ruled out, the orbit appears to be slightly eccentric. The data also show evidence for a more distant planet in this system.

Gliese 436 and its newfound planet
In this artist rendering, the newly discovered planet orbits the red dwarf Gliese 436. The planet has at least 21 times the mass of Earth. The nature of the planet itself remains a mystery.
Courtesy NASA / JPL.

"The composition of these new planets remains unknown," says Marcy. "They could be gas giants, like the Jupiters and Saturns we've found so far. But as Neptune-mass objects, they could have a rock-ice core and a thick envelope of hydrogen and helium gas. They could also be made of only rock and iron, like Mercury."

Marcy and his colleagues think 55 Cancri e (and possibly Gliese 436b) formed farther from its host star and migrated inward until it settled into its current tight orbit. Tidal friction and heating may have inflated the planet's atmosphere, causing much of it to escape and leaving behind a core that might be primarily iron and rock.

Gliese 436 radial velocity plots
Top: Spectra of the star Gliese 436 reveal subtle motions toward and away from Earth. Bottom: When these motions are fitted to a 2.644-day periodicity, the telltale signature of an orbiting planet emerges. Based on the planet's distance from the star (about 0.03 astronomical unit), the estimated mass of the star, and the amplitude of the wave, the planet must have a minimum mass 21 times that of Earth.
S&T diagram by Casey B. Reed; source: Geoffrey W. Marcy and R. Paul Butler.

Like the vast majority of known exoplanets, all three exo-Neptunes were discovered by tracking back-and-forth Doppler shifts in their stars' spectra induced by the planets' gravitational pull. These discoveries highlight the power of the Doppler method to tease out low-mass planets. "We should be able to easily detect planets only 10 times the Earth's mass," says Marcy. "I expect we'll find dozens of planets between 10 and 20 Earth masses in the next few years."

"We can't quite see the Earth mass planets, but we can see their big brothers," adds Butler.

The 55 Cancri planet was discovered in data taken by Debra A. Fischer (San Francisco State University) and her colleagues with the 3-meter Shane Telescope at Lick Observatory in California and by Barbara E. McArthur and her colleagues at the 9.2-meter Hobby-Eberly Telescope in Texas. The Gliese 436 planet was discovered in data taken by Butler, Steven S. Vogt (University of California, Santa Cruz), Marcy, and Fischer at the 10-meter Keck telescope in Hawaii.

In other news, Mayor's team has confirmed the sixth known exoplanet to transit the face of its host star. In 2002 the Polish OGLE III microlensing survey detected periodic dimmings of a 17th-magnitude star in the direction of the galactic bulge that would be consistent with a transiting planet. In March 2004, Mayor and his colleagues tracked the radial velocity of the star, known as OGLE-TR-111, with the 8.4-meter Very Large Telescope in Chile. These follow-up observations revealed that the dimmings were indeed caused by a transiting planet-mass object with an orbital period of 4.02 days. It has a mass of 0.53 Jupiter, and a diameter of 1.0 Jupiter, based on the amount of dimming. This means its density is roughly 0.65 gram per cubic centimeter, slightly less than that of Saturn. The OGLE survey had previously identified three other transiting planets, all of which have periods of less than two days.

The other two transiting planets are HD 209458b and TrES-1, which was announced last week.

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