Planet hunters have found the first true twin to Jupiter around another star, a milestone discovery that confirms planets like those in our solar system do exist elsewhere in the galaxyA team of astronomers announced today the discovery of the first planet outside our solar system with an orbit similar to Jupiter's, a configuration that has the potential to support an Earth-like planet.
They also found the least massive world ever detected around another star, a planet just 40 times as heavy as Earth.
The primary discovery is a gas giant planet that circles a star called 55 Cancri every 13 years, comparable to Jupiter's 11.86-year orbit. The planet is between 3.5 and 5 times as heavy as Jupiter.
"It's the first extrasolar planet that reminds us of a planet in our solar system," lead researcher Geoffrey Marcy said in an interview with SPACE.com several days prior to the announcement.
Marcy, of the University of California, Berkeley, said he and colleague Paul Butler, of the Carnegie Institution of Washington, have dreamed of this discovery for 17 years as they compiled data using a technique that many scientists said would never work. The two astronomers, whose team has grown in recent years, also announced 11 other worlds today at a press conference at NASA headquarters, bringing the total of known extrasolar planets to 98.
Potential for Earth twin
The new planet orbits 55 Cancri at 5.5 astronomical units (AU). One AU is the distance from Earth to the Sun. Jupiter orbits at 5.2 AU. The same team had already spotted another planet around 55 Cancri, a place slightly less massive than Jupiter. It orbits so close to the star that it makes a complete orbit in just 14.6 days.
Marcy speculated that the two-planet system could harbor more intriguing worlds, possibly even rocky planets like Earth, known as terrestrials.
"A Jupiter at five Earth-Sun distance units might serve as the marquee of a planetary theater located within, where terrestrial bit players are racing around on smaller tracks," Marcy said. "We are left to imagine what geophysical and perhaps biological improvisation is taking place inside this planetary playhouse."
Armed with their new data, Marcy and Butler enlisted theoretician
Gregory Laughlin of the University of California, Santa Cruz, to look into whether the 55 Cancri system could also retain an Earth-sized planet in a life-sustaining orbit. Such a region, called a habitable zone, would maintain moderate temperatures suitable to the retention of surface water and the possibility of life. Laughlin ran the data through computer models of planet formation. The answer is "yes."
"We tried a hypothetical configuration of a terrestrial planet in the habitable zone around one AU from the central star and found it very stable," said Laughlin, who also is associated with Lick Observatory. "Just as the other planets in our solar system tug on the Earth and produce a chaotic but bounded orbit, so the planets around 55 Cancri would push and pull an Earth-like planet in a manner that would not cause any collisions or wild orbital variations."
Marcy and Butler caution, however, that there is no way to detect an Earth-sized planet with present technology. Meanwhile, their data does suggest a third planet in the system, a possible Saturn-sized object. Others could lurk there.
Laurance Doyle, a researcher at the SETI Institute who was not involved in the discovery, told SPACE.com the new finding "is a strong encouragement" that our solar system "may not, after all, be totally unusual."
The Jupiter-like planet has another potential benefit, Doyle points out: Its gravity would lure comets, shielding inner planets from life-threatening bombardment. Jupiter plays this protective role in our solar system.
Pushing the limit
Marcy, Butler and their colleagues also announced today the lightest extrasolar planet ever found, one 40 times as massive as Earth.
This discovery pushes the lower limits of their
wobble method, which spots movement in a star induced by the gravity of an orbiting planet. (No confirmed planet outside our solar system has ever actually been photographed.)This relatively small planet, whose possible presence was
first reported in May by SPACE.com, was detected around a star called HD 49674. It is just 15 percent the mass of Jupiter. Theory holds that it would be gaseous, not rocky. Previously, the lightest known extrasolar planet was more than 50 times heavier than Earth.For comparison, Neptune is about 17 times as massive as Earth and Saturn is about 95 times as heavy.
Marcy has said the wobble method will not be able to find planets weighing less than 10 Earth-masses.
The SETI Institute's Doyle uses a different method for planet hunting, however. He looks for slight dips in a star's light that indicate the passage of a planet. The method has yet to discover a planet, but it has been used to
detect the atmosphere of a known extrasolar planet.This so-called
transit method could spot a planet twice as big as Earth, Doyle says, if the planet's path is properly aligned so that it passes in front of the star as seen from Earth.Such a planet would have roughly eight times the mass of our own. It would still be rocky and could, theoretically, harbor life.
Doyle said the existence of two planets bracketing the habitable zone around 55 Cancri "indicates that planet production may have taken place within the habitable zone of that system."
Next Page: A dream come true, plus what's next
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Dream come true
The discovery of the Jovian twin caps 17 years of planet hunting by Marcy and Butler, who were not deterred by early skepticism in their technique.
"Way back in 1985, Paul Butler and I began sketching the idea for a new instrument, attached to a telescope, that might someday detect planets around other stars," Marcy told SPACE.com. "Some very smart people told us that we wouldn't succeed, that we would never detect the wobble of a star caused by its attendant planets."
They did, beginning in 1995 just months after a European team found the first planet around a star besides our Sun. Marcy and Butler confirmed that finding and went on to become the world's most prolific planet-hunting team.
"We always dreamed that maybe, with a wisp of phenomenal luck and dogged perseverance, we might capture evidence of a Jupiter-like planet," Marcy said.
Prior to today's announcement, all known extrasolar planets orbited more closely to their host stars, some as close as Mercury is to our Sun.
Because the planet around 55 Cancri takes 13 years to make a complete orbit, it took equally long for enough data to accumulate to definitively identify the object. Its orbit is elongated instead of being nearly circular like Jupiter's. "We haven't yet found an exact solar system analog," Butler said. "But this shows we are getting close."
Other recent discoveries have shown that
circular orbits do exist around other stars.Butler said more Jupiter-like planets will likely flow from the data they are collecting on 1,200 Sun-like stars.
What's next
While Doyle or someone else might find a planet twice the size of Earth, the discovery of a true Earth-sized planet won't come for at least a few years, most researchers agree.
But now there is a perfect place to look.
The 55 Cancri system "will be the best candidate for direct pictures" by a next-generation space-based observatory, said Debra Fischer, a UC Berkeley astronomer who is part of the Marcy-Butler team.
Two such missions are planned by NASA, first the Space Interferometry Mission and then the
Terrestrial Planet Finder. The discovery of a solar system with elements similar to our own "adds urgency to missions capable of detecting Earth-sized planets," said Charles Beichman, NASA's Origins Program chief scientist at the Jet Propulsion Laboratory in Pasadena.No firm launch dates are set for either of these satellites, however. Both would follow the less ambitious
Kepler mission, set to launch in 2007. Kepler will use the transit method to detect and generate a census of Earth-like planets around other stars, assuming such planets exist, but it won't photograph any. Details of the research
The star 55 Cancri is in the constellation Cancer. It is roughly 41 light-years from Earth and about 4.7 billion years old, comparable to our Sun.
The new discoveries were funded by NASA and the National Science Foundation. Observations of 55 Cancri were made at the Lick Observatory. The Anglo-Australian telescope was used to find two of the other planets announced today.
Other scientists who collaborated in the new findings: Steve Vogt, UC Santa Cruz; Greg Henry, Tennessee State University; Dimitri Pourbaix, Universite' Libre de Bruxelles; Hugh Jones, Liverpool John Moores University in the United Kingdom; Chris Tinney, Anglo-Australian Telescope; Chris McCarthy, Carnegie Institution of Washington; Brad Carter, University of Southern Queensland, Australia; and Alan Penny of the Rutherford Appleton Laboratory in the United Kingdom.
The wobble method, which is so far responsible for all extrasolar planet discoveries, is also known as the Doppler technique. The researchers employ special filters in a telescope to measure a change in the wavelength in light coming from a star. The change results from the star moving toward the telescope and compressing the waves, and then moving away from the telescope and lengthening the waves.
The effect is similar to the change in sound of a siren from an ambulance rushing toward you and then heading away.
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