Astronomers find a Neptune-sized exoplanet in a binary star system

K2 photometry of K2-nnnA: (a) immediately after
extraction from the pixel-level data; (b) after removal of
systematics, showing the stellar variability; (c) after
smoothing and detrending, with vertical ticks indicating the
locations of transits; and (d) the phase-folded photometry with
the best-fit transit model fit to the light curve. The feature
at time index 3033 is a residual systematic induced by K2’s
motion. Image credit: Ciardi et al., 2017.

(—Using NASA’s prolonged Kepler mission, known as
K2, astronomers have discovered a new Neptune-sized planet in
a binary star system in the Hyades open cluster. The newly
found exoworld, designated K2-nnnA b, is the first known
Neptune-sized planet in a binary system within an open
cluster. The finding was reported Sept. 29 in a paper
published on the arXiv pre-print server.

K2-nnnA b was initially spotted during K2’s Campaign 13,
conducted between March 8 and May 27, 2017. During this
campaign, a team of astronomers led by David R. Ciardi of the
California Institute of Technology (Caltech) observed a binary
system named EPIC 247589423 consisting of a K-dwarf star and a
late M-dwarf companion. As a result, the researchers identified
a transit signal in the light curve of the binary.

The planetary nature of this signal was confirmed by follow-up
observations using several observing facilities, including
Palomar Observatory in California, the NASA Infrared Telescope
Facility (IRTF) and the Keck Observatory, both located in
Hawaii. The new observations were also complemented by archival
imaging data from the Palomar Observatory Sky Survey (POSS).

“The detection was made with K2; we have performed a suite of
follow-up observations which include high-resolution imaging
and spectroscopy,” the paper reads.

According to the study, K2-nnnA b is three times larger than
Earth and orbits its parent star every 17.3 days. The planet’s
host is a bright star of spectral type K, designated K2-nnnA,
about 30 percent smaller and less massive than the sun. The
planetary system is located approximately 180 light years away
in the Hyades open – the nearest star
cluster to the sun.

Besides spotting the new planet, Ciardi’s team also found the
parent star’s companion named K2-nnnB. The observations show
that it is a star of M type separated from the primary star by
at least 40 AU.

The newly detected planet is the first Neptune-sized planet to
be found orbiting in a binary system within an . The researchers also noted that
K2-nnnA b is the fourth exoworld discovered in the Hyades
cluster, and only the second in this cluster. The first
transiting planet found in the Hyades cluster, known as K2-25b,
is also a Neptune-sized extrasolar world and orbits a
relatively low-mass star, just like K2-nnnA b.

In concluding remarks, the astronomers emphasized the
importance of their discovery, saying that finding new alien
worlds in star clusters could provide important hints about the
formation and evolution of .

“By finding and studying in clusters spanning a range of stellar
ages, we may begin to understand how and on what timescales
planetary systems form and evolve. (…) As we begin to
understand the planetary distribution in the nascent clusters
in which stars and their planetary systems are born, we can
begin to set constraints on and understand how planetary
systems form and evolve into the systems we see today in the
field of ,” the scientists wrote in the paper.

Explore further:

Newly discovered planet in the Hyades cluster could shed light
on planetary evolution

More information: K2-nnnA~b: A Binary System in the
Hyades Cluster Hosting a Neptune-Sized Planet, arXiv:1709.10398

We report the discovery of a Neptune-size planet (R_p = 3.0
R_Earth) in the Hyades Cluster. The host star is in a binary
system, comprising a K5V star and M7/8V star with a projected
separation of 40 AU. The planet orbits the primary star with an
orbital period of 17.3 days and a transit duration of 3 hours.
The host star is bright (V=11.2, J=9.1) and so may be a good
target for precise radial velocity measurements. K2-nnnA~b is
the first Neptune-sized planet to be found orbiting in a binary
system within an open cluster. The Hyades is the nearest star
cluster to the Sun, has an age of 625-750 Myr, and forms one of
the fundamental rungs in the distance ladder; understanding the
planet population in such a well-studied cluster can help us
understand and set constraints on the formation and evolution
of planetary systems.

© 2017