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Robotically Discovering Earth’s Nearest Neighbors

HD 7924

Artists’ impression of a view from the HD 7924 planetary system looking back toward our Sun, which would be easily visible to the naked eye. Since HD 7924 is in our northern sky, an observer looking back at the Sun would see objects like the Southern Cross and the Magellanic Clouds close to our Sun in their sky. Art by Karen Teramura & BJ Fulton, UH IfA.

A team of astronomers, including IfA astronomer Andrew Howard and IfA graduate students Evan Sinukoff and BJ Fulton, discovered a planetary system orbiting a star that is only 54 light-years away. They found the planets using measurements from the Automated Planet Finder (APF) Telescope at the University of California’s Lick Observatory, the W. M. Keck Observatory on Maunakea, and the Automatic Photometric Telescope (APT) at Tennessee State University’s Fairborn Observatory in Arizona. All three planets orbit their star at a distance closer than Mercury orbits the Sun, completing their orbits in just 5, 15, and 24 days.

The team discovered the new planets by detecting the wobble of the star HD 7924 as the planets orbited and pulled on the star gravitationally. APF and Keck Observatory traced out the planets’ orbits over many years using the Doppler technique that has successfully found hundreds of mostly larger planets orbiting nearby stars. APT made crucial measurements of the brightness of HD 7924 to assure the validity of the discoveries.

The Keck Observatory found the first evidence of planets orbiting HD 7924, discovering the innermost planet in 2009. It took five years of additional observations at Keck and the year-and-a-half campaign by the APF Telescope to find the two additional planets. The new APF facility offers a way to speed up the planet search because APF is a dedicated facility that robotically searches for planets every clear night. Training computers to run the observatory all night, without human oversight, took years of effort by the University of California Observatories staff and graduate students on the discovery team. “We initially used APF like a regular telescope, staying up all night searching star to star. But the idea of letting a computer take the graveyard shift was more appealing after months of little sleep. So we wrote software to replace ourselves with a robot,” said Fulton. “This level of automation is a game-changer in astronomy,” said Howard. “It’s a bit like owning a driverless car that goes planet shopping.”

The Kepler Space Telescope has discovered thousands of 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 (larger than Earth but smaller than Neptune) that Kepler found in great abundance. This discovery shows the type of planetary system that astronomers expect to find around many nearby stars in the coming years. “The three planets are unlike anything in our solar system, with masses 7–8 times the mass of Earth and orbits that take them very close to their host star,” explained University of California, Berkeley graduate student Lauren Weiss, another member of the team.

Observations by APF, APT, and Keck Observatory helped verify the planets and rule out other explanations. “Starspots, like sunspots on the Sun, can momentarily mimic the signatures of small planets. Repeated observations over many years allowed us to separate the starspot signals from the signatures of these new planets,” Sinukoff explained.

The robotic observations of HD 7924 are the start of a systematic survey for super-Earth planets orbiting nearby stars. Fulton will lead this two-year search with the APF as part of the research for his doctoral dissertation. “When the survey is complete we will have a census of small planets orbiting Sun-like stars within approximately 100 light-years of Earth,” says Fulton.

In honor of the donations of Gloria and Ken Levy that helped facilitate the construction of the Levy spectrograph on APF and supported Weiss, the team has informally named the HD 7924 system the “Levy Planetary System.”