The team discovered the planets by detecting a wobble of the star HD 7924 as a result of the gravitational pull of the planets orbiting around it. All three planets orbit the star at a distance closer than Mercury orbits the Sun, completing their orbits in just 5, 15, and 24 days.
The APF facility at Lick Observatory offers a way for astronomers to speed up the exoplanet search. The fully robotic telescope searches for planets every clear night of the year, so planets and their orbits can be discovered and traced quickly.
“The APF is great for two reasons. One, it has the superb Levy spectrometer. Two, it is a modern computer-controlled telescope so we can automate it. This combination means that we can observe stars night and night out to look for the wobble,” said Bradford Holden from from UC Observatories (UCO), who helped to make the telescope robotic.
“The automation of the telescope means that astronomers can fully use all the time available, and make discoveries faster,” he said.
Training computers to run the observatory all night, without human oversight, took years of effort by the UCO 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 Benjamin “BJ” Fulton from the University of Hawaii.
The Keck Observatory found the first evidence of planets orbiting HD 7924, discovering the innermost planet in 2009 using the HIRES instrument installed on the 10-meter Keck I Telescope. This same combination was also used to find other super-Earths orbiting nearby stars in planet searches led by University of Hawaii astronomer Andrew Howard and UC Berkeley professor Geoffrey Marcy. It took five years of additional observations at Keck Observatory and the year-and-a-half campaign by the APF telescope to find the two additional planets orbiting HD 7924. The Automatic Photometric Telescope (APT) at Fairborn Observatory in Arizona also made crucial measurements of the brightness of HD 7924 to assure the validity of the planet discoveries.
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,” said Lauren Weiss from UC Berkeley.
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,” said Evan Sinukoff from UH.
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. “When the survey is complete, we will have a census of small planets orbiting Sun-like stars within approximately 100 light-years of Earth,” said Fulton.
