Discovery of Planets Around Cool Stars Enabled with Hobby-Eberly Telescope

A new astronomical spectrograph provides the highest precision measurements to date of infrared signals from nearby stars, allowing astronomers to detect planets capable of having liquid water on their surfaces that orbit cool stars outside our solar system. The Habitable Zone Planet Finder (HPF) allows precise measurement of a star’s radial velocity, measured by the subtle change in the color of the star’s spectra as it is tugged by an orbiting planet, which is critical information in the discovery and confirmation of new planets.

The HPF, located at The University of Texas at Austin’s McDonald Observatory, targets low-mass planets around cool nearby red dwarf stars in habitable zones, regions where liquid water might exist on a planet’s surface. Red dwarf stars are known to host rocky planets, but these stars are faint due to their size and their magnetic activity manifests as spots and flares, which pose problems for existing visible-light instruments. Coupled to the 10-meter Hobby-Eberly Telescope, HPF instead uses near-infrared light — a type of invisible infrared light closest in wavelength to the visible spectrum — to observe these stars at wavelengths where they are brighter and less active.

Led by Penn State astronomer Suvrath Mahadevan, the team that built HPF includes McDonald Observatory astronomers William Cochran and Michael Endl.

“The HPF was built to be incredibly stable, and we added a calibrator called a laser frequency comb to increase precision,” Mahadevan said. “The laser comb, which was custom-built by the National Institute of Standards and Technology, separates individual wavelengths of light into separate lines, like the teeth of a comb, and is used like a ruler to calibrate the near-infrared energy from the stars. This combination of technologies has allowed us to demonstrate unprecedented near-infrared radial velocity precision with observations of Barnard’s Star, one of the closest stars to the Sun.” These results appear in the February 20 issue of the journal Optica.

“We are especially interested in finding Earth-like planets that orbit in the habitable zone of the nearest stars,” McDonald Observatory’s Endl said. “These planets represent our best chance to characterize and study them in greater detail. We might even be able to search for signs of life in their atmospheres in the near future with telescopes like the Giant Magellan Telescope and the James Webb Space Telescope.

“Since most stars are cool, red dwarf stars, we need a very precise instrument that is optimized for the near infrared,” he continued. “The laser frequency comb at the HPF enables us to reach this high level of precision to detect these small planets. It opens a new window in planet search, that has predominantly focused on the visible bandpass to obtain highly precise Doppler measurements.”

The Hobby-Eberly Telescope recently underwent a $40-million upgrade, and was essentially rebuilt. As part of the upgrade, the telescope acquired a suite of four new instruments. HPF is the latest one to come online.

“The Habitable Zone Planet Finder is a powerful addition to the instrumentation at HET,” said McDonald Observatory director Taft Armandroff, “and it is particularly important to the scientific investigations of exoplanets that have been carried out by University of Texas faculty and researchers for many years.”

The observatory is a major partner in the HET collaboration, which is a partnership between The University of Texas at Austin, Penn State, and two German institutions, Georg-August-Universität Göttingen and Ludwig-Maximilians-Universität München.