Astronomers discover a solar system like ours

Posted: June 13, 2002

The search for Earth-like planets around other stars took a step forward Thursday when astronomers announced the discovery of a solar system that more closely resembles our own solar system than any other to date.

This artist's concept depicts a Jupiter-mass planet orbiting the star, 55 Cancri, about 41 light years from Earth. A possible moon around the planet is shown because such moons are thought to be common around this type of planet, but no moon has been detected. Credit: NASA/Lynette Cook
A team of astronomers led by veteran planet hunters Geoffrey Marcy and Paul Butler said that they had discovered a planet orbiting the star 55 Cancri, 41 light-years from the Earth, that is both similar in mass and orbit as Jupiter, the largest planet in our solar system. The discovery was one of 15 new extrasolar planets, or exoplanets, announced Thursday.

The new planet, known as 55 Cnc d, has an estimated mass four times that of Jupiter and orbits the star in a slightly elliptical orbit at an average distance of 5.5 astronomical units (825 million kilometers, 510 million miles). Jupiter, by comparison, is in a near-circular orbit around the Sun at an average distance of 5.2 AU (780 million km, 483 million mi.)

"This is the first near analog to our Jupiter," said Marcy, a professor of astronomy at the University of California Berkeley. "All other extrasolar planets discovered up to now orbit closer to the parent star, and most of them have had elongated, eccentric orbits."

The planet is also part of a solar system of at least three planets. One planet, 55 Cnc b, was discovered back in 1996. It has a mass slightly less than Jupiter but orbits the star at a distance of only 17 million km (10.7 million mi.) A third planet, 55 Cnc c, was discovered at the same time as the new Jupiter analog; it has a mass of only one-fifth that of Jupiter and goes around the star in an elliptical orbit at an average distance of 36 million km (22 million mi.)

"This is a full-fledged system of planets," Marcy said at the NASA press conference where the results were announced. "We've finally found a family of planets that have some similarities to our solar system."

This discovery is seen not as a quantum leap forward in exoplanet searches but as the result of continued gradual advancements in the field. Over the last few years Marcy and Butler, as well as other astronomers, have been discovering systems of two or more planets, as well as planets whose mass or orbit closely resembles Jupiter. This discovery, though, is the first planet that has both a similar mass and orbit as Jupiter.

This graphic depiction compares our solar system with a newfound planetary system, 55 Cancri. The new system has a Jupiter-mass planet in an orbit similar to the orbit of our Jupiter. In addition, two other planets are shown orbiting 55 Cancri at distances closer than the distance between Earth and our Sun. Credit: NASA
These planets, as well as most other exoplanets, were discovered using a technique known as radial velocity, or Doppler detection. Astronomers look for periodic shifts in wavelength of spectral lines in spectra of Sunlike stars. These shifts are caused by a slight wobble in the star created by the gravitational tug of one or more planets as they orbit the star. Measuring the amplitude and period of the variations allows astronomers to determine the orbit of the planet and estimate its minimum mass without directly observing the planet itself.

The nature of the radial velocity technique means that the easiest planets to detect are large planets that orbit very close to their parent stars, creating large variations with period of just days. So many of the first exoplanets discovered since 1995 are these "hot Jupiters" that some scientists wondered if solar systems like our own are the exception rather than the rule. However, as new instruments that are able to detect smaller Doppler shifts in stars entered use smaller planets have been found. Moreover, data sets spanning longer periods of time have allowed astronomers to detect more distant planets: the discovery of 55 Cnc d required 15 years' worth of observations by Marcy and Butler.

Besides the new Jupiter-like planet, astronomers also reported discovering the smallest exoplanet found to date. The planet, orbiting the star HD46974, a Sunlike star over 130 light-years away, has a mass just 12 percent that of Jupiter, or about 40 percent the mass of Saturn. The planet, however, orbits the star at a distance of just 9 million km (5.5 million mi.)

The radial velocity technique is not sensitive enough to detect Earth-like planets. However, astronomers said they could not rule out the presence of such planets around 55 Cancri. A theoretical analysis by Greg Laughlin of the University of California Santa Cruz found that terrestrial planets could exist in stable orbits between the two inner giant planets and the outer Jupiter-like planet. However, the question remains whether the processes that formed the giant planets and migrated them to their current orbits would have permitted any terrestrial planets there to survive.

"The astrobiology crowd is probably going to be pretty happy about this system," said Butler, Marcy's longtime colleague and a researcher at the Carnegie Institution of Washington. "We haven't yet found an exact solar system analog, which would have a circular orbit and a mass closer to that of Jupiter. But this shows we are getting close."

The 15 new planets brings the total number of exoplanets known around Sunlike stars to over 90. The large number of planet is now helping astronomers sort them into categories and draw conclusions about the development of planets and solar systems. "We feel lucky that we are discovering enough planets to study them in a statistical way," said David Spergel, a Princeton University astronomer not involved in the latest discovery.

The discovery also shows how dynamic the field of exoplanet searches can be. An early version of the press release announcing the discovery, written a few weeks ago, listed the number of new planets as 13, not 15. Marcy explained the discrepancy at the press conference. "Literally in the last two weeks we've discovered two more planets."

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