Neptune-Class Worlds Found
|Out of the Dust, A Planet is Born
Image Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)
Astronomers using telescopes in Hawaii, California and Texas have found the first Neptune-size planets outside our solar system, far smaller than any planets previously detected. One of them is perhaps only 14 times the mass of the Earth, which is small enough to have a solid surface and possibly temperatures conducive to life.
"It sounds a bit strange, but we're not thinking in terms of Jupiter masses or Saturn masses anymore, but Earth masses," said Geoff Marcy, a professor of astronomy at the University of California, Berkeley, and a member of one of two teams reporting the discoveries. Neptune is 17 times more massive than Earth, while Saturn and Jupiter are 95 and 318 times larger, respectively.
"All exoplanets found so far are almost certainly gas giants, but these new ones are a puzzle - they could be gaseous like Jupiter, but they also could have a rock-ice core and a thick envelope of hydrogen and helium gas, like Neptune, or they could be a combination of rock and ice, like Mercury."
Marcy and planet-hunting colleague Paul Butler of the Carnegie Institution of Washington, along with Barbara McArthur of the University of Texas, leader of the second team, announced their findings. Two papers detailing the new planet discoveries were submitted to The Astrophysical Journal in July and accepted earlier this month. The paper by Butler, Marcy and their colleagues will appear in the Dec. 10 issue, while that of McArthur and her colleagues has not yet been scheduled for publication.
|Neptune: Giant Ice Ball. Click here for 6-year animation
The detection of a third Neptune-sized extrasolar planet, not yet peer-reviewed, was announced by European astronomers on Aug. 25 just a few days after its discovery. All three of these planets were discovered by tracking the star's wobble caused by the planet's gravitational tug on the star as it orbits, which produces a Doppler shift in the light emitted by the star.
The new planets, though approaching the Earth in size, are still far from Earth-like. Both whip around their stars in a few days and are so close as to be roasting on the side facing the star. Because each planet is so close to its star, its rotation is no doubt locked into its orbital period, so that it always presents the same face to the star, Marcy said.
The smaller of the two planets, with a minimum mass of 14 Earth masses, was discovered by McArthur and her team around the star 55 Cancri (or rho-Cancri). It is the fourth planet found around 55 Cancri, a yellow G star not unlike the sun only 41 light years from Earth in the constellation Cancer. The larger planet, with a minimum size 21 times the mass of the Earth, orbits a red M star, Gliese 436, which is about 50 times dimmer than our sun and located 33 light years away in the constellation Leo. A light year is 6 trillion miles.
If the planet orbiting Gliese 436 has little atmosphere to spread the heat around, it is likely to have temperatures of 377 Celsius on the side facing the star, where it would be perpetual noon, and a frigid tens of degrees above absolute zero where it's perpetual midnight. However, temperatures on the twilit border could be more comfortable, at least from an Earthling's perspective, Marcy said.
|Geoff Marcy, a planet hunter from UC Berkeley. "We are witnessing the birth of a new observational science: the discovery and characterization of extrasolar planetary systems..."
Image Credit: astron.berkeley.edu
"The front is hot and the backside is probably cold, but the region in between could have moderate temperatures between 0 and 100 degrees Celsius," he said.
The near simultaneous discovery of these smallest-yet planets indicates they could be common, Marcy said.
"If you look at the 135 or so extrasolar planets found so far, it's clear that nature makes more of the smaller planets than the larger ones," he said. "We've found more Saturn-size planets than Jupiter-size planets, and now it appears there are more Neptune-size planets than Saturn-size. That means there's an even better chance of finding Earths, and maybe more of them than all the other planets we've found so far."
Butler, Marcy, and colleagues Debra Fischer of UC Berkeley and San Francisco State University, and Steve Vogt, professor of astronomy at UC Santa Cruz, discovered the planet around Gliese 436 last year after four years of observation on the Keck I telescope in Hawaii. They have been observing the Doppler shifts of 950 nearby stars, 150 of them very nearby, low-mass stars called M dwarfs, the most common type of star in the galaxy. All the M dwarfs are within 30 light years of Earth.
After following the star's wobbles for another year, the team concluded that Gliese 436 has a Neptune-sized planet of at least 21 Earth masses, though more likely 25 Earth masses, that zips around in a circular orbit once every 2.64 days. That corresponds to an orbital radius of roughly 2.8 million miles (4.5 million kilometers), or about 3 percent of Earth's distance from the sun. In our own solar system, Mercury, the closest planet to the sun, orbits in 116 days at a distance of 37 million miles (58 million kilometers).
|Solar System panorama, not to scale
McArthur and her colleagues in Austin discovered the smaller planet after analyzing data on the wobbling of 55 Cancri obtained with the 9.2-meter Hobby-Eberly telescope of the McDonald Observatory, in combination with data from the Marcy-Butler-Fischer-Vogt team, which had earlier discovered three planets around the star. They also used data from a rival planet hunting team directed by Swiss astronomers Michel Mayor and Diedre Queloz.
Once they had the data from previous years, most of it obtained by Fischer at Lick Observatory outside San Jose, Calif., McArthur, William Cochran, Michael Endl and Fritz Benedict organized a major campaign to study the 55 Cancri system and took another 100 Doppler measurements over the course of 190 days.
The new data revealed that the star has a fourth planet with a minimum mass of 14 Earth masses, an orbital period of 2.81 days, and an orbital radius of just 3.8 percent that of Earth. Because the orbit of the planet is probably tipped relative to our line of sight, the most likely mass for this planet is 18 times that of Earth.
Marcy predicts that more and more Neptune-size planets will be discovered in the near future, thanks to a new CCD digital camera being installed on the Keck telescopes' spectrometer in Hawaii. Built by Vogt and his UCSC team, it will allow detection of smaller stellar wobbles, around 1 meter per second, compared to today's best resolution of 3 meters per second. The planet around Gliese 436 caused it to wobble about 18 meters per second. Since most of these planets would be in close orbits around the star and with very short orbital periods, a few days of observation could yield several sub-Neptune planets, he predicted.
"This is a real breakthrough," Marcy said. "We should be able to easily detect planets only 10 times the Earth's mass. I expect we'll find dozens of planets between 10 and 20 Earth masses in the next few years."
|The Terestrial Planet Finder will search for Earth-like planets orbiting 250 of the closest stars.
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|SIM, scheduled for launch in 2009, will determine the positions and distances of stars several hundred times more accurately than any previous program.
Credit: NASA / JPL
- French COROT mission will look at 50,000 to 60,000 stars and should find a few dozen terrestrial planets and several hundred close-in gas-giant planets during a two- to three-year mission
- Kepler, Extrasolar Terrestrial Planet Detection Mission, designed to look for transiting or earth-size planets that eclipse their parent stars [survey 100,000 stars]. Scientists expect to find thousands of planets, and perhaps 50 Earth-like candidates.
- Likely de-orbit for Hubble Space Telescope [date announced is highly fluid but assumes no planned shuttle visits from NASA]
- Planned launch for NASA-ESA Next Generation Space Telescope, or NGST [James Webb Space Telescope], a near-infrared telescope that will succeed the Hubble Space Telescope.
- Planned launch for Space Interferometery Mission (SIM)
- Planned launch for TPF and Darwin missions
The members of the discovery team were supported jointly by the National Science Foundation and the National Aeronautics and Space Administration.
Related Web Pages
IAU Working Group on Extrasolar Planets
The University of California Planet Search Project
Astrobiology Magazine New Planets
Extrasolar Planets Encyclopedia
Planet Quest (JPL)
Space Interferometry Mission