15 new planets confirmed around cool dwarf stars

15 new planets confirmed around cool dwarf stars
Credit: Tokyo Institute of Technology

A research team led by Teruyuki Hirano of Tokyo Institute of
Technology’s Department of Earth and Planetary Sciences has
validated 15 exoplanets orbiting red dwarf systems. One of
the brightest red dwarfs, K2-155 that is around 200 light
years away from Earth, has three transiting super-Earths. Of
those three super-Earths, the outermost planet, K2-155d, with
a radius 1.6 times that of Earth, could be within the host
star’s habitable zone.

The findings, published in the form of two papers in The
Astronomical Journal
, are based on data from NASA Kepler
spacecraft’s second mission, K2, and follow-up observations
using ground-based telescopes, including the Subaru Telescope
in Hawaii and the Nordic Optical Telescope (NOT) in Spain.

The researchers found that K2-155d could potentially have
liquid water on its surface based on three-dimensional global
climate simulations. Hirano says, “In our simulations, the
atmosphere and the composition of the planet were assumed to be
Earth-like, and there’s no guarantee that this is the case.”

15 new planets confirmed around cool dwarf stars
Surface temperatures were plotted as a function of insolation
flux (the amount of incoming stellar radiation) estimated at
1.67±0.38. When the insolation exceeds 1.5, a so-called
runaway greenhouse effect occurs, signaling a cut-off point
for life-friendly temperatures. If the insolation is under
1.5, the surface temperature is more likely to be moderate.
Credit: Astronomical Journal

A more precise estimate of the radius and temperature of the
K2-155 star would be needed to conclude definitively whether
K2-155d is habitable. Achieving such precision would require
further studies, for example, using interferometric techniques.
A key outcome from the current studies was that orbiting may have remarkably similar
characteristics to planets orbiting solar-type stars.

“It’s important to note that the number of planets around red
dwarfs is much smaller than the number around solar-type
stars,” says Hirano. “Red dwarf systems, especially coolest red
dwarfs, are just beginning to be investigated, so they are very
exciting targets for future exoplanet research.”

For example, while the so-called radius gap of planets around
solar-type stars has been reported previously, this is the
first time that researchers have shown a similar gap in planets
around red dwarfs. “This is a unique finding, and many
theoretical astronomers are now investigating what causes this
gap,” says Hirano.

15 new planets confirmed around cool dwarf stars
Histogram of planet radius for the validated and
well-characterized transiting planets around red dwarfs: The
number counts for mid-to-late red dwarfs (those with a
surface temperature of under 3,500 K) are shown above those
for early red dwarfs (those with a surface temperature of
around 3,500-4,000 K). The results show a “radius gap”, or a
dip in the number of stars with a radius between 1.5-2.0
times that of Earth. Credit: Astronomical Journal

He adds that the most likely explanation for the lack of large
planets in the proximity of host stars is photoevaporation,
which can strip away the envelope of the planetary atmosphere.

The researchers also investigated the relationship between
planet radius and metallicity of the . “Large planets are only discovered
around metal-rich stars,” Hirano says, “and what we found was
consistent with our predictions. The few planets with a radius
about three times that of Earth were found orbiting the most
metal-rich red dwarfs.”

The studies were conducted as part of the KESPRINT
collaboration, a group formed by the merger of KEST (Kepler
Exoplanet Science Team) and ESPRINT (Equipo de Seguimiento de
Planetas Rocosos Intepretando sus Transitos) in 2016.

With the planned launch of NASA’s Transiting Exoplanet Survey
Satellite (TESS) in April 2018, Hirano is hopeful that even
more planets will be discovered. “TESS is expected to find many
candidate planets around bright closer to Earth,” he says. “This will
greatly facilitate follow-up observations, including
investigation of planetary atmospheres and determining the
precise orbit of the planets.”

Explore further:

Three ‘super-Earths’ orbiting a cool dwarf star discovered

More information: Teruyuki Hirano et al, Exoplanets
around Low-mass Stars Unveiled by K2, The Astronomical
(2018). DOI: 10.3847/1538-3881/aaa9c1

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