Japanese astronomers discover gas giant planets orbiting evolved stars

Japanese astronomers discover gas giant planets orbiting evolved stars
Top: The observed RVs and the best-fit single Keplerian
curve for 24 Boo. The horizontal axis is the orbital phase.
The derived stellar jitter is also included in the error
bar. Red circles and green triangles are data taken with
slit- and fiber-mode, respectively. Bottom: The RV
residuals to the orbital fitting. Credit: Takarada et al.

Using radial velocity method a group of Japanese astronomers
has found that two evolved stars, namely 24 Booties and Gamma
Librae, are orbited by gas giant planets. They discovered
that 24 Booties hosts one planet, while Gamma Librae is
circled by two alien worlds. The finding is detailed in a
paper published April 11 on arXiv.org.

Thanks to the radial velocity (RV) technique, over 600
exoplanets have been detected so far, but the number of known
RV planets orbiting evolved is still small. Filling this gap is needed
in order to provide more comprehensive image of the variety of
extrasolar .

One astronomical survey, which has the potential of improving
our knowledge about orbiting evolved hosts,
is the Okayama Planet Search Program. Recently, a team of
Japanese astronomers, led by Takuya Takarada of the Tokyo
Institute of Technology, has reported the finding of three such
exoplanets of this category, by conducting RV measurements at
the Okayama Astrophysical Observatory (OAO) in Japan.

“We here report two new planetary systems around evolved stars:
24 Boo and γ Lib. This result is based on RV measurements
performed at OAO with the 1.88-m reflector and HIDES,” the
researchers wrote in the paper.

24 Booties (24 Boo for short) is a star of spectral type G3IV,
similar in mass to our sun, but almost 11 times larger.
Takarada’s team found that this star is orbited by a planet, which received designation 24
Boo b, every 30.35 days at a distance of about 0.19 AU from the
host. The researchers estimate that the minimum mass of the
newly found planet is 0.91 Jupiter masses.

Gamma Librae, or γ Lib, is around 11 times larger than the sun,
has a mass of approximately 1.47 solar masses, and is of
spectral type K0III. The astronomers discovered that this star
hosts two —γ Lib b and γ
Lib c, with minimum masses of 1.02 and 4.58 Jupiter masses
respectively. Located about 1.24 AU from the parent star, γ Lib
b needs about 415 days to fully orbit its host, while γ Lib c
has an of nearly 965 days as
it is separated from the star by approximately 2.17 AU.

Notably, both 24 Boo and γ Lib have low metallicities (-0.77
and -0.30 respectively) among the giant stars hosting planetary

In concluding remarks, the astronomers emphasize what makes
their discovery unique. They note that the planet 24 Boo b has
the shortest orbital period ever found around evolved stars
with a stellar radius larger than 10 solar radii. Furthermore,
γ Lib turns out to have the second lowest metallicity among the
giant stars with multiple planetary systems. The researchers
also add that the orbital period of γ Lib c is about 2.33
longer than γ Lib b, noting that such ratio is rare among
planetary systems around giant and dwarf stars.

Explore further:
new ‘hot Jupiter’ exoplanets discovered

More information: Planets around the evolved stars 24
Booties and γ Libra: A 30d-period planet and a double
giant-planet system in possible 7:3 MMR, arxiv.org/abs/1804.04008

We report the detection of planets around two evolved giant
stars from radial velocity measurements at Okayama
Astrophysical observatory. 24 Boo (G3IV) has a mass of 0.99M⊙,
a radius of 10.64R⊙, and a metallicity of [Fe/H]=−0.77. The
star hosts one planet with a minimum mass of 0.91MJup and an
orbital period of 30.35d. The planet has one of the shortest
orbital periods among those ever found around evolved stars by
radial-veloocity methods. The stellar radial velocities show
additional periodicity with 150d, which are probably attributed
to stellar activity. The star is one of the lowest-metallicity
stars orbited by planets currently known. γ Lib (K0III) is also
a metal-poor giant with a mass of 1.47M⊙, a radius of 11.1R⊙,
and [Fe/H]=−0.30. The star hosts two planets with minimum
masses of 1.02MJup and 4.58MJup, and periods of 415d and 964d,
respectively. The star has the second lowest metallicity among
the giant stars hosting more than two planets. Dynamical
stability analysis for the γ Lib system sets a minimum orbital
inclination angle to be about 70∘ and suggests that the planets
are in 7:3 mean-motion resonance, though the current
best-fitted orbits to the radial-velocity data are not totally

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