Sub-Neptune sized planet validated using Habitable-zone Planet Finder

Washington D.C.: The Habitable-zone Planet Finder (HPF), an astronomical instrument has confirmed an object initially observed by the Kepler spacecraft is an exoplanet.

An exoplanet or extrasolar planet is a planet outside the Solar System.

The instrument designed by a Penn State team and recently installed on the 10 m Hobby-Eberly Telescope at McDonald Observatory in Texas provides the highest precision measurements to date of infrared signals from nearby low-mass stars, and astronomers used it to validate the candidate planet by excluding all possibilities of contaminating signals to a very high level of probability.

The details of the findings appear in the Astronomical Journal.

The planet, called G 9-40b, is about twice the size of the Earth, but likely closer in size to Neptune, and orbits its low mass host star, an M dwarf star, only 100 light-years from Earth.

Kepler detected the planet by observing a dip in the host star's light as the planet crossed in front of--or transited--the star during its orbit, a trip completed every six Earth days. This signal was then validated using precision spectroscopic observations from the HPF, 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 and the 3m Shane Telescope at Lick Observatory, helped to confirm the identification.

A lead author of the paper, Gudmundur Stefansson said: "G 9-40b is amongst the top twenty closest transiting planets known, which makes this discovery really exciting. 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 spectroscopic observations from HPF allowed us to place an upper bound of 12 Earth masses on the mass of the planet. 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, " said Caleb Canas, a graduate student at Penn State and an author of the paper.

"We hope to obtain more observations 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," said Canas.

HPF was delivered to the 10m Hobby-Eberly Telescope at McDonald Observatory in late 2017 and started full science operations in late 2018.

The instrument 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.

A unique feature of HPF is its precise spectral calibration with a laser frequency comb built by collaborators at the National Institute of Standards and Technology and the University of Colorado Boulder.

"Using HPF, we are currently surveying the nearest low-mass stars--also called M-dwarfs, which are the most common stars in the galaxy--with the goal of discovering exoplanets in our stellar neighbourhood," said Suvrath Mahadevan, professor of astronomy and astrophysics at Penn State and principal investigator of the HPF spectrograph.

In addition to the data obtained with HPF, the scientists obtained another observation of the transiting planet using the 3.5m telescope at Apache Point Observatory in New Mexico using a photometric technique and instrumentation developed as part of Stefansson's doctoral thesis.

These 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 planetary parameters. In addition, high-contrast imaging observations using the 3m Shane Telescope at Lick Observatory showed that the host star was the true source of the transits.

"It is exciting to see this first result of the HPF survey coming out. HPF was built from the ground up to enable precision measurements to discover and confirm planets," said Larry Ramsey, emeritus professor of astronomy and astrophysics at Penn State.

This research was supported by the National Science Foundation (NSF), Penn State, the Heising-Simons Foundation, the NASA Earth and Space Science Fellowship Program, the Center for Exoplanets and Habitable Worlds at Penn State, and the Research Corporation.

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