A Rush of New Planets

28 New Planets, 7 New Brown Dwarfs Reported by California, Carnegie Team

An artist’s concept of the Neptune-sized planet GJ436b (right) orbiting an M dwarf star, Gliese 436, at a distance of only 3 million miles.
Credit: Lynnette Cook

The world’s largest and most prolific team of planet hunters announced on Monday, May 28 the discovery of 28 new planets outside our solar system, increasing to 236 the total number of known exoplanets.


The planets are among 37 new objects – all of them orbiting a star, but smaller than a star -discovered by the teams within the past year. Seven of the 37 are confirmed brown dwarfs, which are failed stars that are nevertheless much more massive than the largest, Jupiter-sized planets. Two others are borderline and could be either large, gas giant planets or small brown dwarfs.

Wright said the research teams have become much more sophisticated in their analyses and are able to detect smaller planets as well as planets farther from their parent stars. In both cases, he said, these planets produce much smaller wobbles in the parent star, making them harder to detect.

"We’ve added 12 percent to the total in the last year, and we’re very proud of that," said Wright of the 28 new exoplanets. "This provides new planetary systems so that we can study their properties as an ensemble."


The ice giant planet GJ436b is like a hot-Neptune that orbits Gliese 436 every 2.6 days.
Credit: Lynnette Cook





Also among the 28 new exoplanets are at least four new multiple-planet systems, plus three stars that probably contain a brown dwarf as well as a planet. Wright said that at least 30 percent of all stars known to have planets have more than one. Because smaller planets and outer planets of a star are harder to detect, he predicts that the percentage will continue to rise as detection methods improve. Searching for smaller planets is a vital step in determining the potential for habitable, Earth-like worlds beyond our Solar System.

"We’re just now getting to the point where, if we were observing our own solar system from afar, we would be seeing Jupiter," he said, pointing out that the teams’ Doppler technique is now sensitive to stellar wobbles of a meter per second, much less than the 10-meter per second limit they started out with 15 years ago.

Wright keeps track of all known exoplanets for the California and Carnegie Planet Search team’s Web site, http://exoplanets.org, which hosts the only peer-reviewed catalog of exoplanets within 200 parsecs (652 light years) of Earth. This includes "everything that is close enough to study and possibly follow up with imaging," he said.

Drawing of the predicted interior of an ice-giant planet like GJ436b or Neptune.
Credit: Jason Wright/UC Berkeley

Three of the newly reported planets are around large stars between 1.6 and 1.9 times the mass of our sun. Johnson has focused on exoplanets around massive stars, known as A and F stars, with masses between 1.5 and 2.5 solar masses. Planets around these massive stars are normally very hard to detect because they typically rotate fast and have pulsating atmospheres, effects that can hide or mimic the signal from an orbiting planet. He discovered, however, that cooler "retired" A stars – "subgiant" stars that have nearly completed hydrogen burning and have stabilized for a short period of time – are more stable, making planet-caused wobbles detectable.


"Only one of the 9 planets is within 1 AU (astronomical unit, or 93 million miles), and none of them is within 0.8 AU, of their host stars, which is very different than the distribution around sun-like stars," he said, noting that many sun-like stars harbor hot gas giants that whip around their host stars in two to 100 days. Even though such planets are easier to detect, no such planets have been detected orbiting retired A stars, whose typical planets have an orbital distance about equal to Earth’s orbit or greater, with an orbital period of a few years.


Johnson will continue to focus on the retired A stars, 450 of which have been added to the teams’ target list. As more planets are discovered around subgiants, it should become clearer whether larger orbits are "a result of different formation and migration mechanisms in the disks of A-type stars, or simply a consequence of the small number of massive subgiants currently surveyed," he and colleagues wrote in a paper submitted in April to the Astrophysical Journal.

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