The Dance of the Giant Planets


Orbits of the two planets circling Gliese 876
Orbits of the two planets circling Gliese 876. Note that the planets orbit their parent star closely, at a fraction of the distance between Earth and the Sun (Earth-Sun units, also called astronomical units). Graphic:


The Dance of the Giant Planets

A team of planet hunters January 9th announced a discovery that will help researchers better understand planet migration and how planets’ gravitational pulls influence each other. The discovery was announced at the American Astronomical Society meeting in San Diego.

The planet sleuths from the University of California at Berkeley, NASA and other institutions discovered the planetary pair locked in what appears to be "resonant" orbits, moving in synch around the star with orbital periods of 60 and 30 days. Because of the 2-to-1 ratio, the inner planet goes around the star twice for each orbit of the outer one. They gravitationally tug on each other to maintain this synchronicity.

Photo of Gliese 876
The red dwarf Gliese 876 lies just 15 light years from Earth.

"The resonance between the two orbiting planets is among the most exciting planet detection discoveries to date," said Dr. Jack Lissauer, a NASA Ames Research Center scientist and member of the NASA Astrobiology Institute. A "resonance" is similar to the harmonic vibration produced by plucking two notes on a stringed instrument. This gravitational pas de deux between the two planets is common among moons and asteroids, but not planets. The axes of the two newly detected planets’ elliptical orbits also appear to be nearly perfectly aligned.

Position of Gliese 876 in the sky
Gliese 876 is located in the constellation Aquarius. Though it is one of the Sun’s nearest neighbors, it is too dim to see with the unaided eye. Graphic: Extrasolar Visions.

Lissauer and State University of New York at Stony Brook graduate student Eugenio Rivera used a numerical model to demonstrate the stability of the nearly twin orbits around the star known as Gliese 876, a dim red dwarf 15 light years from Earth in the constellation Aquarius. "Questions about planetary migration and gravitational influence are still very much unsolved," Lissauer said.

"This discovery is significant for several reasons," said Lissauer. "This is the first extra-solar planetary system to show a strong resonance. It also is the smallest star known to have any orbiting planets, much less two," he said.

Photo of Gliese 876
Computer simulation of the two planets forming around Gliese 876. Graphic: Geoff Bryden.

The two gravitationally linked planets have masses of at least 0.5 and 1.8 times the mass of Jupiter, he said. The inner planetary companion was not recognized at first because the orbital resonance allowed the pair of planets to masquerade as a single planet with an elongated orbit.

The two orbiting planets are located relatively close to each other, within 0.08 Astronomical Units (the distance between the Earth and the sun) of each other, less than one-third the distance from the Earth to its nearest neighbor, Venus.

In our solar system, the only known resonances between a pair of planets is Pluto, which orbits the sun twice for every three times Neptune circles the sun.

Besides Lissauer, the planet-hunting team that discovered the system includes Drs. Geoff Marcy and Debra Fischer of the University of California at Berkeley; Dr. Paul Butler of the Carnegie Institution of Washington; and Dr. Steve Vogt of the University of California at Santa Cruz.

Though significant and unusual, the discovery will require more modeling before researchers can determine what the resonance they discovered actually means.

Photo of Gliese 876
The Lick Observatory overlooks Californina’s Silicon Valley. Photo: University of California Observatories.

The team based both sets of its conclusions on 6 years of precise Doppler measurements and observations made at the Keck I telescope atop Mauna Kea in Hawaii and the Lick Observatory telescope in California. The research is part of a multi-year project to look for planets among 1,100 stars within 300 light years of Earth. The project is supported by NASA, the National Science Foundation and Sun Microsystems.

Related links

More on This Story

Two Planets Orbiting Gliese 876
Announcement and data from the team of scientists who detected the planets.

The Formation of Short-Period Resonant Planets: How Did GL 876’s Planets Form?

Detection of Gliese 876b (Outer Planet) in 1998

Welcome to the Neighborhood

Astronomers have detected yet another new extrasolar planet, and this one’s not very far away. From Scientific American.

Two teams find planet orbiting nearby star
From Science News Online.

Gliese 876 : The Closest Extrasolar Planet

Gliese 876 and its Planets

Astronomy Picture of the Day: Gliese 876

Artist’s Conception of Gliese 876b: Lynette Cook
Painting by Lynette Cook depicting a gas giant and its red dwarf parent star as seen from a possible moon.

Artist’s Conception of Gliese 876b: David Egge
Painting by David Egge portraying a ringed Gliese 876b viewed from an icy moon.

Gliese 876
Diagram of the two-planet system, position of Gliese 876 in the sky, and other data on the star.

Gliese 876b (Outer Planet)
Diagram of orbit and other data.

Gliese 876c (Inner Planet)
Diagram of orbit and other data.

Notes for Gliese 876
Includes research bibliography on the detection of planets around Gliese 876.

Planet-Detection Methods

Capabilities of Various Planet Detection Methods
Explanation and comparison of planet-hunting techniques, such as Doppler spectroscopy (used in detecting the planets around Gliese 876).

Telescopes Used in Detecting Gliese 876b and c

Keck Observatory

Lick Observatory