Want to visit one of our nearest planetary systems? It’s only 54 light-years away.
Astronomers using ground-based telescopes in Hawaii, California and Arizona have discovered three "super-Earth" planets in the newly found system orbiting a star at a distance closer than Mercury orbits the sun. They complete their "years" in just five, 15 and 24 days — unlike the 365 days for Earth.
"It would be great to be a kid on these planets," said University of Hawaii astronomy professor Andrew Howard, one of the authors of a paper describing the new find. "You’d have a birthday every few days."
All joking aside, the planets — which orbit very close to their host star with masses seven to eight times that of Earth’s — are much too hot to support kids or any other life form as we know it, he said.
Howard was part of a team of astronomers from UH-Manoa, the University of California at Berkeley, the University of California Observatories and Tennessee State University that found the planets using measurements from the W.M. Keck Observatory on Mauna Kea, the Automated Planet Finder Telescope at Lick Observatory in California and the Automatic Photometric Telescope at Fairborn Observatory in Arizona.
The astronomers pointed their telescopes at a star known as HD 7924, which is one of the few hundred brightest stars in our galaxy, Howard said, but visible on Earth only from a darkened location under the best of conditions.
The team discovered the new planets by detecting the wobble of HD 7924 as the planets orbited and pulled on the star gravitationally. The Automated Planet Finder Telescope and Keck Observatory mapped out the planets’ orbits over more than a decade using the Doppler technique that has found hundreds of larger planets orbiting stars. The Arizona telescope took key measurements of the brightness of the star.
The paper presenting this work, "Three super-Earths orbiting HD 7924," has been accepted for publication in the Astrophysical Journal and is available at arxiv.org/ abs/1504.06629.
Lead author Benjamin J. Fulton, one of Howard’s graduate students, said the discovery was aided greatly with the help of the new Automated Planet Finder.
"It’s incredibly valuable," he said. "It can search for planets every clear night of the year." Fulton said he initially used the Automated Planet Finder like an ordinary telescope, staying up all night searching star to star while controlling the device remotely from Hawaii.
But the idea of letting a computer take the night shift was more appealing after months of little rest.
"We decided we needed more sleep," he said. "So we wrote software to replace ourselves with a robot."
The Keck Observatory in 2009 discovered the first evidence of planets orbiting HD 7924, finding the innermost planet using the HIRES (High Resolution Echelle Spectrometer) instrument installed on the 10-meter Keck I telescope.
This same combination was also used to find other nearby super-Earths — planets larger than Earth but smaller than an ice giant like Neptune — in searches led by Howard and UC Berkeley professor Geoffrey Marcy, who is also listed on the discovery team.
It took five years of additional observations at Keck Observatory and a year and a half of work by the Automated Planet Finder to find the two additional planets orbiting HD 7924.
Fulton said the 2009 Keck data hinted at the two other planets, but nobody realized it at the time.
Observations by the Hawaii, Arizona and California observatories helped verify the planets and rule out other explanations, such as starspots, which, like sunspots on the sun, "can momentarily mimic the signatures of small planets. Repeated observations over many years allowed us (scientists) to separate the starspot signals from the signatures of these new planets," said Evan Sinukoff, a UH graduate student who also contributed to the discovery.
The find launches a larger, two-year campaign to discover nearby planets. Fulton, who is originally from San Jose, Calif., will lead the effort as part of research for his doctoral dissertation.
When the survey of the 50 most promising nearby stars is complete, there will be a census of small planets orbiting sunlike stars within about 100 light-years of Earth, Fulton said.
"It will be exciting to chart out our nearest neighbors," Howard said.
The Kepler Space Telescope has discovered thousands of these extrasolar planets and demonstrated that they are common in our Milky Way galaxy. However, nearly all of these planets are far from our solar system. Most nearby stars have not been thoroughly searched for the small "super-Earth" planets that the Kepler found in great abundance.
Fulton said that if the Kepler data hold true, he would expect to discover about 10 more planets. He compared his upcoming planet-finding campaign to "prospecting" for enough data to allow a stronger telescope, like the planned Thirty Meter Telescope, to discover more details.
The HD 7924 system was named the Levy Planetary System to honor the donations of Gloria and Ken Levy that helped with the construction of the Levy spectrograph on Automated Planet Finder and supported one of the paper’s authors, Lauren Weiss of UC Berkeley.
Also supporting the effort was NASA, the U.S. Naval Observatory and the University of California for its support of Lick Observatory.
The other authors of the paper are Howard Isaacson (UC Berkeley), Gregory Henry (Tennessee State University) and Bradford Holden and Robert I. Kibrick (UC Observatories).
