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The mass of the Mars-sized exoplanet, Kepler-138b

The mass of the Mars-sized exoplanet, Kepler-138b

A First: Exoplanet smaller than Earth gets its size and mass measured

A team of astronomers has measured the mass and size of the smallest exoplanet yet, a Mars-sized planet orbiting a red dwarf star about 200 light years from our solar system. The team, which includes astronomers at Penn State University, NASA Ames Research Center, the SETI Institute, and the University of Chicago, describes the achievement in a paper that will be published in the journal Nature on 18 June 2015.

The planet, named Kepler-138 b, is the first exoplanet smaller than the Earth to have both its mass and its size measured. It is one of three planets that orbit the star Kepler-138 and that pass in front of it on every orbit as viewed from Earth — a maneuver that astronomers call a transit. “Each time a planet transits the star, it blocks a small fraction of the star’s light, allowing us to measure the size of the planet,” said Dr. Daniel Jontof-Hutter, a research associate in astronomy at Penn State who led the study.

“We also measured the gravity of all three planets, using data from NASA’s Kepler mission, by precisely observing the times of each transit,” Jontof-Hutter said. The astronomers also were able to measure the masses of these planets. “Each planet periodically slows down and accelerates ever so slightly from the gravity of its neighboring planets. This slight change in time between transits allowed us to measure the masses of the planets,” Jontof-Hutter explained. After measuring both the mass and size of an exoplanet, astronomers then can calculate its density and its bulk composition.


“Current measurements are consistent with a variety of compositions, and favor compositions that are mostly rocky,” according to Dr. Jason Rowe of the SETI Institute, a coauthor of the study. All three planets orbiting Kepler-138 are close to their star and are most likely are too hot to be habitable.

“These results demonstrate the rapid progress of exoplanet science and the enduring value of data collected by the Kepler mission,” said Eric Ford, professor of astronomy and astrophysics at Penn State and a coauthor of the study. A previous study had measured the masses of the two outer planets, but this new study performed a more detailed analysis and incorporated additional Kepler data, which enabled measurement of the mass of the Mars-sized inner planet and also improved the mass measurements of the outer planets. “Kepler-138 b is roughly 3,000 times less massive than the first exoplanet whose density was measured 15 years ago,” Ford said. “We now are working to discover and characterize rocky planets in the habitable zones of nearby stars.”

The Kepler spacecraft continues to search for planets and to explore the cosmos as part of a new mission, known as K2. Both K2 and NASA’s upcoming Transiting Exoplanet Survey Satellite missions will hunt for planets transiting brighter stars that will be more easily studied by ground-based observatories such as the Hobby-Eberly Telescope (HET) and the Miniature Exoplanet Radial Velocity Array (MINERVA). Penn State is a partner in both of these planet-hunting observatories. Penn State also is leading in the development the new Habitable Zone Planet Finder instrument, which will be installed at the Hobby-Eberly Telescope in 2016.

More information about the Penn State Center for Exoplanets and Habitable Worlds is online at exoplanets.psu.edu.

Information for members of the public who would like to help in the search to find undiscovered planets by searching for clues in Kepler data about changes in the brightness of a star over time is at planethunters.org. So far, Kepler data has been used to discover more than a thousand planets around other stars. More information about the Kepler mission is at kepler.nasa.gov.

This research was supported by NASA, the Penn State Center for Exoplanets and Habitable Worlds, the Penn State Department of Astronomy and Astrophysics, the Penn State Eberly College of Science, the Pennsylvania Space Grant Consortium, and the Alfred P. Sloan Foundation.

Source:  The Pennsylvania State University Press Release

Artist Impression of Kepler-138

The planetary system harboring Kepler-138 b, the first exoplanet smaller than Earth with both it mass and size measured. All three planets known to orbit the star have been characterized by NASA’s Kepler mission observing their transits, i.e., when the planets pass in front of the star. In this artist’s conception, the sizes of the planets relative to the star have been exaggerated. Kepler-138 b, in the foreground, is likely an airless rocky world. Because of the gravitational forces between the planets, the planets are shifted slightly from the positions that would be expected without their mutual gravitational interactions. This enabled their masses to be measured. Artwork credit: Danielle Futselaar, SETI Institute.

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ANIMATION CAPTION

A team of astronomers has measured the mass and size of the smallest exoplanet yet, a Mars-sized planet named Kepler-138 b orbiting a red dwarf star about 200 light years from our solar system. Kepler-138b is the first exoplanet smaller than the Earth to have both its mass and its size measured.
Kepler-138 b is one of three planets that orbit the star Kepler-138 and that pass in front of it — or transit — on every orbit. Each time a planet transits the star, it blocks a small fraction of the star’s light, allowing astronomers to measure the size of the planet. All three Kepler planets were identified by NASA’s Kepler mission, which has discovered over a thousand planets around other stars.
This animation shows a mass-radius diagram based on measurements of 127 exoplanets. The animation begins by showing a range of planets with masses up to that of Jupiter’s, then gradually zooms toward the smaller masses and radii to display a comparison of the physical properties of the Kepler-138 planets relative to Earth, Venus, Mars and Mercury.
ANIMATION CREDIT: Jason Rowe, SETI Institute

Plot of masses and sizes of exoplanets.

This plot shows the masses and sizes of the smallest exoplanets for which both quantities have been measured. The solar system planets (shown in red) are for comparison. The three Kepler-138 planets (shown in orange) are among the four smallest exoplanets with both size and mass measurements, and their characteristics imply that their planets similar in size to Earth have a wide variety of compositions. Kepler-138b is the first exoplanet smaller than Earth to have both its mass and size measured. This significantly extends the range of planets with measured densities. Planets are primarily composed of three types of constituents: ‘rock’, which includes metals, is the densest of these components, whereas hydrogen and helium gases are the lightest. All planetary constituents get compressed when they are within massive planets, so for a given composition the density is larger for larger mass planets. Intermediate density planets can be composed of mixtures of rock plus gas and/or the third major planetary constituent, water and similar materials such as methane (in solid, liquid or gaseous form).
Credits: NASA Ames/W Stenzel

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