The artistic concept shows the planetary system
harboring Kepler-138b, the first exoplanet smaller than Earth with both a
mass and size measurement. The sizes of the planets relative to the
star have been exaggerated.Credits: SETI Institute/Danielle Futselaar. Hi-res Image
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. 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. Credits: NASA Ames/W Stenzel. Mars-size Kepler-138b gets a mass
Determining the size of an Earth-size exoplanet by the amount of
starlight it blocks hundreds of light-years away once was the realm of
science fiction. Measuring the mass of such a small planet based on its
gravity was another level entirely, but astronomers have done just that
for an exoplanet fifty percent the size of Earth.
Researchers using NASA's Kepler mission data have measured the mass
of a Mars-size exoplanet that is about one tenth the mass of Earth.
Called Kepler-138b, it 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.
To determine a planet's mass, astronomers typically measure the
minuscule movement of the star caused by the gravitational tug of an
orbiting planet. For planets the mass of Earth detecting such a tiny tug
is extraordinarily challenging with current technology. Fortunately,
when a star hosts multiple planets that orbit closely together,
scientists have developed another way to get at the planets’ masses.
Daniel Jontof-Hutter, a research associate at the Pennsylvania State
University’s Center for Exoplanets and Habitable Worlds, led a team of
astronomers in a study to measure the mass of all three planets by
precisely observing the times each planet passed in front of, or
transited, the star Kepler-138.
"Each planet periodically slows down and accelerates ever so slightly
from the gravity of its neighboring planets. The slight change in time
between transits allows us to measure the masses of the planets," said
Jontof-Hutter.
Each time a planet transits a star it blocks a small fraction of the
star's light, allowing astronomers to measure the size of the planet.
This is how the Kepler spacecraft has detected thousands of planets
around other stars.
By measuring both the mass and size of an exoplanet, scientists can
calculate the density and infer the bulk composition to determine if a
planet is predominantly made of rock, water or gas. Tiny Kepler-138b's
density is consistent with a rocky composition like Earth or Mars, but
further observations are needed before astronomers can definitively say
that it is a rocky world.
Kepler-138b is the innermost of three planets that orbit Kepler-138, a
star less than half the size of our sun and roughly 30 percent cooler.
The Kepler-138 system is located about 200 light-years from Earth in the
direction of the constellation Lyra.
The outer two planets, Kepler-138c and Kepler-138d, are approximately
the size of Earth. Kepler-138c is likely to be rocky, whereas
Kepler-138d is less dense and cannot be made of the same mix of material
as Earth. All three planets orbit too close to their star for liquid
water to exist on the surface and support life, as we know it.
"The substantial difference between the densities of the two larger
planets tells us that not all planets similar to Earth in size are
rocky," said Jack Lissauer, co-author and planetary scientist at NASA's
Ames Research Center in Moffett Field, Calif. "Further study of small
planets will help provide more understanding of the diversity that
exists in nature, and will help determine if rocky planets like Earth
are common or rare."
Much like astronomers in the early 20th century studied a wide variety of stars to characterize and classify different types, astronomers in the 21st century are doing the same to understand the diversity and demographics of planet populations in our Milky Way galaxy.
Scientists are working to use these new measurements of small planets
from Kepler and NASA's upcoming Transiting Exoplanet Survey Satellite
to identify patterns in the relationship between mass and size. These
insights will provide context for understanding the history of Earth and
other planets in our own solar system, and inform the next generation
planet hunters as they search for life beyond the solar system.
A previous study had measured the masses of the two outer planets.
This new study performed a more detailed analysis of the Kepler-138
system using additional Kepler data. This enabled the measurement of the
mass of the Mars-size inner planet and improved the accuracy of the
size and mass measurements for the outer planets. The results will
appear in this Thursday's issue of the journal Nature.
Ames Research Center in Moffett Field, California, manages the Kepler
and K2 missions for NASA’s Science Mission Directorate. NASA's Jet
Propulsion Laboratory in Pasadena, California, managed Kepler mission
development. Ball Aerospace & Technologies Corp. operates the flight
system with support from the Laboratory for Atmospheric and Space
Physics at the University of Colorado in Boulder.
For more information on the Kepler mission, visit: www.nasa.gov/kepler.
Source: NASA/Ames Research Center
