Distant Object May Be a Planet Similar to Ours
Scientists announced Monday the discovery of what may be a rocky, Earth-like planet orbiting a star 15 light-years away -- a milestone in the search for a world outside our solar system that could sustain life.
The new planet, dubbed a “super-Earth” by the team that found it, is about seven times as massive as Earth and about twice the diameter. It orbits the star Gliese 876 in the constellation Aquarius.
Until now, every planet found outside our solar system has been larger than Uranus, the giant ice planet at the edge of our system. Two other planets had previously been found orbiting Gliese 876, a small red star known as an M dwarf, the most common type of star in the galaxy, but those were Jupiter-size gas giants.
The new planet’s modest size and mass suggest that it may be the first rocky planet, like Earth, ever found orbiting a normal-size star. Researchers think it may resemble the inner planets of our solar system, which are made of nickel and iron.
“This is a big milestone, to get down to the region of rocky planets,” said team member Steven Vogt, a professor of astronomy and astrophysics at UC Santa Cruz.
“We keep pushing the limits of what we can detect, and we’re getting closer and closer to finding Earths,” he said.
Even though the new planet is the closest analog to Earth yet discovered, it could not support life as we know it. Whirling around its star in just two days, it is far too close for water to exist in liquid form. Surface temperatures range from 400 to 750 degrees Fahrenheit.
“It’s not the place to go for a vacation,” Vogt said.
That hardly dimmed the excitement of the team, composed of researchers from the University of California, the Carnegie Institution in Washington and NASA’s Ames Research Center near Mountain View, Calif.
“Today’s results are an important step toward answering one of the most profound questions that mankind can ask: Are we alone in the universe?” said Michael Turner of the National Science Foundation, which helped fund the research.
The detection of most planets found outside the solar system, including the new one, known as Gliese 876d, could not be done by direct observation because they don’t give off light. The presence of the planets is revealed by analyzing the subtle wobble in a star’s movement that is caused by the gravity of a nearby object in space. Like a child tugging on its mother’s hand, an orbiting planet tugs on its star.
Pinning down the size and location of the planets is done by using a spectrometer to break down the star’s light and measure tiny changes over time, indicating small movements. Until recently, the lower limit that could be measured in the movement of a star was about three meters per second, about the speed of a jogging athlete.
The breakthrough that led to the discovery of the planet was the installation of a new light sensor at the W.M. Keck Observatory in Hawaii. The instrument can detect changes in star movement of one meter per second, about the speed of a mother pushing a baby carriage.
“More precision is everything,” said Vogt, who designed the instrument over the last three years.
Although technology was not advanced enough to see the new planet, the research team told reporters at a news conference in Arlington, Va., that they were confident of their findings, which had been submitted to the Astrophysical Journal for publication.
There are three types of planets in our solar system: the rocky inner planets, consisting of Mercury, Venus, Earth and Mars; the gas giants Jupiter and Saturn; and the intermediate planets Uranus and Neptune, which are thought to be composed of rock, ice, water and other constituents.
The scientific team believes the makeup of the new planet lies somewhere between the rocky inner planets and the intermediate ones.
The researchers said the next step was to find a planet in what was known as the “habitable zone” around a star.
The right kind of planet is one with a solid surface and an orbit close enough to a star for heat to warm its surface, yet far enough away that water can exist in liquid form. None of the known extrasolar planets meets those conditions.
Theoretically, Vogt said, it should be possible with the new technology to find rocky planets with orbits of about 20 days, significantly farther from the star than the new planet. That rules out sun-like stars, which would bake the surface of a planet with a 20-day orbit. (Mercury, for instance, has an 88-day orbit; its surface temperature can reach 800 degrees.)
A prime candidate, it turns out, is once again the Gliese 876 system. Although the star is one-third the size of the sun, it puts out 1% of the luminosity and wattage.
“We’re already modeling this system to see if there’s anything else there,” Vogt said.
Seventeen other systems are also candidates for harboring an Earth-like planet. The best hope of finding one may lie with a 2.4-meter telescope being installed at the Lick Observatory east of San Jose. The sole job of the Automated Planet Finder will be to look for extrasolar planets. Researchers said it should be operating by next spring.
“We should find planets of a few Earth masses,” said team member Geoffrey Marcy, astronomy professor at UC Berkeley. “Some may be cool enough to have liquid water.”
