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A Sub-Neptune Sized Planet Validated with the Habitable-Zone Planet Finder

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February 20, 2020

A Sub-Neptune Sized Planet Validated with the Habitable-Zone Planet Finder

Apache Point Observatory New Mexico

NORMAN, OKLA. – A multi-university team of researchers has validated that a candidate planet signal originally detected by the Kepler space telescope is an exoplanet—a planet orbiting a star outside of our solar system. The planet, called G 9-40b, is about twice the size of the Earth and orbits its low mass host star (an M dwarf star) only 100 light years away, making it the second-closest transiting planet discovered by the K2 mission to date.

The team corroborated their findings by observing G 9-40b through the 3.5m telescope at Apache Point Observatory in New Mexico, obtained through the University of Oklahoma’s institutional access to the telescope.

The details of the team’s discovery appear in the Astronomical Journal.

Using the Habitable-zone Planet Finder (HPF)—an astronomical spectrograph that provides the highest precision measurements to date of infrared signals from nearby stars that was built by a Penn State-led team and recently installed on the 10m Hobby-Eberly Telescope at McDonald Observatory in Texas—astronomers validated the planet by excluding all possibilities of contaminating signals to very high level of probability.

Kepler detected the planet by observing a dip in the host star’s light as the planet crossed—or transited—in front of the star during its orbit. G 9-40b completes a full orbit every six Earth days. This signal was then validated using precision spectroscopic observations from the HPF spectrograph—ruling out the possibility of a close stellar or substellar binary companion. Observations from other telescopes, including the 3.5m Telescope at Apache Point Observatory for precision photometry and the 3m Shane Telescope at Lick Observatory for high-contrast imaging, helped to confirm the validation.

“G 9-40b is amongst the top-20 closest transiting planets known, which makes this discovery really exciting,” said Gudmundur Stefansson, lead author of the paper and a former Ph.D. student at Penn State who is currently a postdoctoral fellow at Princeton University. “Further, due to its large transit depth, G 9-40b is an excellent candidate exoplanet to study its atmospheric composition with future space telescopes.”

The team used the University of Oklahoma’s institutional access to the 3.5m telescope at Apache Point Observatory in New Mexico to obtain another observation of the transiting planet, using a new photometric technique and the engineered diffuser on the ARCTIC imager, developed as part of Stefansson’s doctoral thesis. These follow-up transit observations helped further resolve the “transit shape”—the curve that represents how much of the host planet’s light is blocked—resulting in more precise planet parameters. 

“The ability of the APO/ARCTIC diffuser and its new Semrock filter to achieve the photometric precision needed for this work is impressive,” said John Wisniewski, OU Homer L. Dodge Department of Physics and Astronomy Presidential Professor and associate professor. “Our OU team is excited to continue to use APO to help validate additional candidate planets detected by NASA’s Kepler, K2 and TESS missions. In addition, high-contrast adaptive optics imaging observations using the 3m Shane Telescope at Lick Observatory showed that the host star was the true source of the transits.”

This research was supported by the National Science Foundation, Penn State, the University of Oklahoma, the Heising-Simons Foundation, the NASA Earth and Space Science Fellowship program, the Center for Exoplanets and Habitable Worlds at Penn State, the Mount Cuba Astronomical Foundation, and the Research Corp.

“The spectroscopic observations from HPF allowed us to place an upper-bound on the mass of the planet of 12 Earth masses,” said Caleb Cañas, a graduate student at Penn State and an author of the paper. “This demonstrates that a planet is causing the dips in light from the host star, rather than another astrophysical object such as a background star. We hope to obtain more observations of G 9-40b in the future with HPF to precisely measure its mass, which will allow us to constrain its bulk composition and differentiate between a predominantly rocky or gas-rich composition.”

HPF was first delivered to the 10m Hobby-Eberly Telescope at McDonald Observatory in late 2017, and started full science operations in late 2018. HPF is designed to detect and characterize planets in the habitable-zone—the region around the star where a planet could sustain liquid water on its surface—around nearby low-mass stars.

“Using HPF, we are currently surveying the nearest low-mass stars with the goal to discover exoplanets in our stellar neighborhood, around M dwarfs—the most common stars in the Galaxy,” said Suvrath Mahadevan, professor of astronomy and astrophysics at Penn State and principal investigator of the HPF spectrograph.

The Hobby-Eberly Telescope is a joint project of the University of Texas at Austin, Penn State, Ludwig-Maximilians-Universität München, and Georg-August Universität Gottingen. The HET is named in honor of its principal benefactors, William P. Hobby and Robert E. Eberly.

The full findings are available in the researchers’ article, published in the Astronomical Journal. A publicly accessible discussion of this result can be found on the HPF blog,