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Artist’s 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
A team of astronomers using ground-based telescopes in Hawaii,
California, and Arizona recently discovered a planetary system orbiting a
nearby star that is only 54 light-years away. 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.
Astronomers from the University of Hawaii at
Manoa, the University of California, Berkeley, the University of
California Observatories, and Tennessee State University found the
planets using measurements from the Automated Planet Finder (APF)
Telescope at Lick Observatory in California, the W. M. Keck Observatory
on Maunakea, Hawaii, and the Automatic Photometric Telescope (APT) at
Fairborn Observatory in Arizona.
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 planet discoveries.
The new APF facility offers a way to speed up the planet search.
Planets can be discovered and their orbits traced much more quickly
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
“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 University of Hawaii graduate student BJ Fulton.
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 UH 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 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,” explains UC Berkeley graduate
student Lauren Weiss.
“This level of automation is a game-changer
in astronomy,” says Howard. “It’s a bit like owning a driverless car
that goes planet shopping.”
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,” explains Evan Sinukoff, a UH graduate student who
contributed to the discovery.
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 his
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.
Telescope automation is relatively new to
astronomy, and UH astronomers are building two forefront facilities.
Christoph Baranec built the Robo-AO observatory to takes high-resolution
images using a laser to remove the blur of Earth’s atmosphere, and John
Tonry is developing ATLAS, a robotic observatory that will hunt for
The paper presenting this work, “Three
super-Earths orbiting HD 7924,” has been accepted for publication in the
Astrophysical Journal and is available at no cost at http://arxiv.org/abs/1504.06629.
The other authors of the paper are Howard Isaacson (UC Berkeley),
Gregory Henry (TSU), and Bradford Holden and Robert I. Kibrick (UCO).
In honor of the donations of Gloria and Ken
Levy that helped facilitate the construction of the Levy spectrograph on
APF and supported Lauren Weiss, the team has informally named the HD
7924 system the “Levy Planetary System.” The team also acknowledges the
support of NASA, the U.S. Naval Observatory, and the University of
California for its support of Lick Observatory.
Founded in 1967, the Institute for Astronomy at the University of
Hawaii at Manoa conducts
research into galaxies, cosmology, stars, planets, and the
sun. Its faculty and staff are also involved in astronomy
deep space missions, and in the development and management
of the observatories on Haleakala and Maunakea. The
Institute operates facilities on the islands of Oahu, Maui, and Hawaii.