We’ve found a lot of planets far away from the Sun but know comparatively little about what may be circling nearby stars. The rationale is clear: The Kepler mission’s field of view was carefully chosen to provide a large sample (over 145,000 main sequence stars) that could be studied for transits by the spacecraft’s photometer. Looking out along the Cygnus arm of the Milky Way, far enough from the ecliptic plane to avoid the Sun, the Kepler stars have been providing statistical data to help us understand how common planets actually are in the galaxy.

But as we saw with the announcement of a candidate planet around Alpha Centauri B, the news of planets closer to home excites interest. These are places close enough to us that they could conceivably be the targets of future interstellar probes. As we continue to look at the Kepler inflow, we’re also anticipating missions like TESS (Transiting Exoplanet Survey Satellite), scheduled for a 2017 launch, and PLATO (PLAnetary Transits and Oscillations of stars), scheduled for 2024, both given the charter of finding habitable planets around nearby stars.

But we’re finding nearby worlds even now, as witness the discovery of two planets around the star HD 7924, a K-class main sequence star about 55 light years away in Cassiopeia. A single super-Earth was found here in 2009 using the 10-meter Keck I telescope working in conjunction with the HIRES spectrograph. Now further observations with Keck and a campaign using the Automated Planet Finder (APF) Telescope at Lick Observatory have uncovered two more planets in this system, making for a trio of ‘super-Earths.’

The HD 7924 Planetary System

Image: 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. Credit: Karen Teramura & BJ Fulton, UH IfA.

The robotic contribution to this work is a story in itself. We’re looking at a new degree of automation in the exoplanet hunt with Earth-based observatories, one that co-author Andrew Howard (University of Hawaii) likens to “owning a driverless car that goes planet shopping.” The Automated Planet Finder instrument at the Lick Observatory operates robotically every clear night of the year, the installation consisting of a 2.4-meter automated telescope and the high-resolution Levy spectrometer, searching a preprogrammed list of nearby stars using radial velocity methods in the hunt for low-mass rocky worlds.

Lead author Benjamin Fulton (a University of Hawaii graduate student) notes that robotic searches like these can run all night without human oversight:

“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.”

The Automatic Photometric Telescope (APT) at Fairborn Observatory in Arizona also factored in. Now in operation for fourteen years, APT is one of a cluster of automated instruments at the observatory. Its observations of the brightness of HD 7924 complemented the APF and Keck data to confirm the new planets, both of which are seven to eight times the mass of Earth and, in a configuration we’re now finding common, orbiting close to their host star, with periods of 15.3 and 24.5 days. They join the previously discovered super-Earth with a period of a scant 5 days.

What to make of close-in super-Earths like these? From the paper (internal references omitted for brevity):

The large population of super-Earths orbiting close to their host stars was a surprise. Population synthesis models of planet formation had predicted that such systems would be rare. Planet cores were expected to mostly form beyond the ice line and rarely migrate to close orbits unless they first grew to become gas giants. Nevertheless, close-in, low-mass planets are common and often appear in compact multi-planet systems. Theoretical models are catching up, with refinements to the disk migration and multiplanet dynamics in the population synthesis family of models. A new class of “in situ” formation models have also been proposed in which systems of super-Earths and Neptunes emerge naturally from massive disks.

The first HD 7924 planet discovered (2009) grew out of the Eta-Earth Survey at Keck, in which Howard and colleagues searched for planets from a field of 166 nearby G and K dwarf stars. The team is now observing a subset of the Eta-Earth Survey stars with the Automated Planet Finder, with the two new HD 7924 worlds emerging from this work. With incident irradiation values 114, 28 and 15 times that of the Earth for the three planets, they’re not habitable by our standards of liquid water on the surface. But transits cannot be ruled out here and the paper adds that HD 7924 will be an excellent candidate for the James Webb Space Telescope. The upcoming launch of TESS offers the chance to observe the star closely for transit signatures.

The paper speaks of building a census of small planets in the local stellar neighborhood (within 100 light years) as the Automated Planet Finder continues its operations. There are doubtless many planets to find, considering Kepler’s discovery of so many compact, multi-planet systems.

The paper is Fulton et al., “Three super-Earths orbiting HD 7924,” accepted for publication in the Astrophysical Journal (preprint). But see also Howard and team’s 2010 paper on the Eta-Earth findings. It’s “The Occurrence and Mass Distribution of Close-in Super-Earths, Neptunes, and Jupiters,” Science Vol. 330, No. 6004 (2010), pp. 653-655 (abstract). This University of California Observatories news release is also helpful.