A young sub-Neptune-sized planet sheds light onto how planets form and evolve


Artist impression of planet K2-25b

Artist’s impression of the planet K2-25b. New detailed observations using the Habitable-zone Planet Finder reveal a young exoplanet that is slightly smaller than Neptune orbiting a young star in the Hyades cluster. K2-25b orbits an M-dwarf star – the most common type of star in the galaxy – every 3.5 days.

Image: NOIRLab/NSF/AURA/J. Pollard

The detailed characterization of a young planet slightly smaller than Neptune provides insight into how such planets form and evolve. Astronomers characterized the planet’s mass, radius, and the tilt of its orbit using the Habitable-zone Planet Finder (HPF), an astronomical spectrograph built by a Penn State team and installed on the 10m Hobby-Eberly Telescope at McDonald Observatory in Texas. The paper has been accepted for publication in the Astronomical Journal and is publicly accessible on arXiv.

“Planets at intermediate sizes between Earth and Neptune are some of the most frequent types of planets in the galaxy, but no such planets are known to exist in the Solar System,” said Gudmundur Stefansson, lead author of the paper, and a former doctoral student at Penn State who is currently a postdoctoral fellow at Princeton University. “Despite their sheer number in the galaxy, many aspects of their formation and evolution remain a puzzle. Our new observations help shed light on this process.”

The planet, referred to as K2-25b, is slightly smaller in size than Neptune and orbits an M-dwarf star – the most numerous type of star in the galaxy. It was originally detected using the Kepler spacecraft by observing a dip in the host star’s light caused by the planet crossing in front of – or transiting – the star and blocking some of the star’s light during its orbit, a trip completed every 3.5 days. The planetary system is a member of the Hyades cluster, a nearby cluster of young stars with similar chemical properties that formed about 600 million years ago about 150 light years away from Earth.


Hyades Cluster with inset showing planet K2-25b

A newly characterized sub-Neptune-sized planet, named K2-25b, orbits a low-mass star in the Hyades cluster, a cluster of young stars about 150 light years away from Earth in the constellation of Taurus. The detailed characterization sheds light on how such planets form and evolve.

IMAGE: Gudmundur Stefansson

“K2-25b is one of the very few young planets orbiting a low-mass star with a measured mass and orbital tilt,” said Gudmundur. “Although smaller in size than Neptune, the planet interestingly has a mass about 1.5 times larger than Neptune. The planet is dense for its size and age, in contrast to other young short-period sub-Neptune systems which are often observed to have low densities and extended evaporating atmospheres.”

The tilt of planetary orbits – the angle between the star’s equator and the planet’s orbit – encodes valuable information on how planetary systems form and evolve. One of the most effective ways to measure the tilt of planetary orbits is by studying the star’s spectra – the light it emits across many different wavelengths – taken during planetary transits. As the host star is rotating during a planetary transit, one half of the stellar disk is “blueshifted” – its light spectrum shifts toward shorter wavelengths-as seen from the observer, while the other half of the star is “redshifted” – a shift toward longer wavelengths. As the planet passes in front of different regions of the stellar disk, it blocks differently blue- and red-shifted light, causing anomalous variations in the velocity of the star. By carefully measuring these velocity changes, the tilt of the orbit can be inferred.

“K2-25b’s orbit is well aligned with the host star’s equator, giving insights into how planetary systems around low-mass stars form,” said Suvrath Mahadevan, professor of astronomy and astrophysics at Penn State and principal investigator of the HPF spectrograph. “Only three other planetary systems orbiting low-mass stars have had their orbital tilts measured. By leveraging the large 10m aperture of the Hobby-Eberly Telescope and HPF’s sensitivity at near-infrared wavelengths – where low-mass stars emit most of their light – we are excited to conduct similar observations of other M-dwarf planetary systems to further study how they form and evolve.”

The Habitable-zone Planet Finder was 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 M-dwarf stars, but is also capable of making sensitive measurements for close in planets, outside the habitable zone.

In addition to data from HPF, additional data were obtained with the 3.5m Telescope at Apache Point Observatory in New Mexico, and the 0.9m Telescope at the Kitt Peak National Observatory (KPNO) in Arizona, using a photometric technique and instrumentation called diffuser-assisted photometry developed as part of Stefansson’s doctoral thesis.

“The high-accuracy photometry from the 0.9m WIYN Telescope using the innovative diffuser technique is an important part of this study that allowed us to better define the shape of the transit and thereby further constrain the size, density and composition of the planet,” said Jayadev Rajagopal, astronomer at the National Science Foundation’s NOIRLab, which operates KPNO. “Smaller aperture telescopes, when equipped with state-of-the-art (but not expensive) equipment can be platforms for high impact science programs. Very accurate photometry will be in demand for exploring host stars and planets in tandem with space missions and larger apertures from the ground, and this is an illustration of the role that a 0.9m telescope can play in that effort.”

This research was supported by the U.S. 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-Universität München, and Georg-August Universität Gottingen. The HET is named in honor of its principal benefactors, William P. Hobby and Robert E. Eberly.

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