174 research outputs found
An extremely low-density and temperate giant exoplanet
Transiting extrasolar planets are key objects in the study of the formation, migration, and evolution of planetary systems. In particular, the exploration of the atmospheres of giant planets, through transmission spectroscopy or direct imaging, has revealed a large diversity in their chemical composition and physical properties. Studying these giant planets allows one to test the global climate models that are used for the Earth and other solar system planets. However, these studies are mostly limited either to highly-irradiated transiting giant planets or directly-imaged giant planets at large separations. Here we report the physical characterisation of the planets in a bright multi-planetary system (HIP41378) in which the outer planet, HIP41378 f is a Saturn-sized planet (9.2 ± 0.1 R⊕) with an anomalously low density of 0.09 ± 0.02 g cm⁻³ that is not yet understood. Its equilibrium temperature is about 300 K. Therefore, it represents a planet with a mild temperature, in between the hot Jupiters and the colder giant planets of the Solar System. It opens a new window for atmospheric characterisation of giant exoplanets with a moderate irradiation, with the next-generation space telescopes such as JWST and ARIEL as well as the extremely-large ground-based telescopes. HIP41378 f is thus an important laboratory to understand the effect of the irradiation on the physical properties and chemical composition of the atmosphere of planets
Characterizing K2 Candidate Planetary Systems Orbiting Low-mass Stars. I. Classifying Low-mass Host Stars Observed during Campaigns 1–7
We present near-infrared spectra for 144 candidate planetary systems identified during Campaigns 1–7 of the NASA K2 Mission. The goal of the survey was to characterize planets orbiting low-mass stars, but our Infrared Telescope Facility/SpeX and Palomar/TripleSpec spectroscopic observations revealed that 49% of our targets were actually giant stars or hotter dwarfs reddened by interstellar extinction. For the 72 stars with spectra consistent with classification as cool dwarfs (spectral types K3–M4), we refined their stellar properties by applying empirical relations based on stars with interferometric radius measurements. Although our revised temperatures are generally consistent with those reported in the Ecliptic Plane Input Catalog (EPIC), our revised stellar radii are typically 0.13 [subscript ⊙](39%) larger than the EPIC values, which were based on model isochrones that have been shown to underestimate the radii of cool dwarfs. Our improved stellar characterizations will enable more efficient prioritization of K2 targets for follow-up studies
Planet Candidates from K2 Campaigns 5-8 and Follow-Up Optical Spectroscopy
We present 151 planet candidates orbiting 141 stars from K2 campaigns 5-8
(C5-C8), identified through a systematic search of K2 photometry. In addition,
we identify 16 targets as likely eclipsing binaries, based on their light curve
morphology. We obtained follow-up optical spectra of 105/141 candidate host
stars and 8/16 eclipsing binaries to improve stellar properties and to identify
spectroscopic binaries. Importantly, spectroscopy enables measurements of host
star radii with 10% precision, compared to 40% precision when
only broadband photometry is available. The improved stellar radii enable
improved planet radii. Our curated catalog of planet candidates provides a
starting point for future efforts to confirm and characterize K2 discoveries.Comment: Accepted for publication in the Astronomical Journal; 17 pages, 8
figures, 2 tables, download source for full table
Two Transiting Earth-size Planets Near Resonance Orbiting a Nearby Cool Star
Discoveries from the prime Kepler mission demonstrated that small planets (<
3 Earth-radii) are common outcomes of planet formation. While Kepler detected
many such planets, all but a handful orbit faint, distant stars and are not
amenable to precise follow up measurements. Here, we report the discovery of
two small planets transiting K2-21, a bright (K = 9.4) M0 dwarf located
656 pc from Earth. We detected the transiting planets in photometry
collected during Campaign 3 of NASA's K2 mission. Analysis of transit light
curves reveals that the planets have small radii compared to their host star,
2.60 0.14% and 3.15 0.20%, respectively. We obtained follow up NIR
spectroscopy of K2-21 to constrain host star properties, which imply planet
sizes of 1.59 0.43 Earth-radii and 1.92 0.53 Earth-radii,
respectively, straddling the boundary between high-density, rocky planets and
low-density planets with thick gaseous envelopes. The planets have orbital
periods of 9.32414 days and 15.50120 days, respectively, and have a period
ratio of 1.6624, very near to the 5:3 mean motion resonance, which may be a
record of the system's formation history. Transit timing variations (TTVs) due
to gravitational interactions between the planets may be detectable using
ground-based telescopes. Finally, this system offers a convenient laboratory
for studying the bulk composition and atmospheric properties of small planets
with low equilibrium temperatures.Comment: Updated to ApJ accepted version; photometry available alongside LaTeX
source; 10 pages, 7 figure
60 Validated Planets from K2 Campaigns 5-8
We present a uniform analysis of 155 candidates from the second year of
NASA's mission (Campaigns 5-8), yielding 60 statistically validated
planets spanning a range of properties, with median values of = 2.5
, = 7.1 d, = 811 K, and = 11.3 mag. The
sample includes 24 planets in 11 multi-planetary systems, as well as 18 false
positives, and 77 remaining planet candidates. Of particular interest are 18
planets smaller than 2 , five orbiting stars brighter than = 10
mag, and a system of four small planets orbiting the solar-type star EPIC
212157262. We compute planetary transit parameters and false positive
probabilities using a robust statistical framework and present a complete
analysis incorporating the results of an intensive campaign of high resolution
imaging and spectroscopic observations. This work brings the yield to over
360 planets, and by extrapolation we expect that will have discovered
600 planets before the expected depletion of its on-board fuel in late
2018.Comment: 33 pages, 13 figures, 5 tables, accepted for publication in A
Dynamics and Formation of the Near-Resonant K2-24 System: Insights from Transit-Timing Variations and Radial Velocities
While planets between the size of Uranus and Saturn are absent within the
Solar System, the star K2-24 hosts two such planets, K2-24b and c, with radii
equal to and , respectively. The two planets have orbital
periods of 20.9 days and 42.4 days, residing only 1% outside the nominal 2:1
mean-motion resonance. In this work, we present results from a coordinated
observing campaign to measure planet masses and eccentricities that combines
radial velocity (RV) measurements from Keck/HIRES and transit-timing
measurements from K2 and Spitzer. K2-24b and c have low, but non-zero,
eccentricities of . The low observed eccentricities
provide clues regarding the formation and dynamical evolution of K2-24b and
K2-24c, suggesting that they could be the result of stochastic gravitational
interactions with a turbulent protoplanetary disk, among other mechanisms.
K2-24b and c are and , respectively; K2-24c is 20%
less massive than K2-24b, despite being 40% larger. Their large sizes and low
masses imply large envelope fractions, which we estimate at
and . In particular, K2-24c's large envelope presents an
intriguing challenge to the standard model of core nucleated accretion that
predicts the onset of runaway accretion when .Comment: 14 pages, 9 figures, 2 tables, accepted to A
K2 Discovers a Busy Bee: An Unusual Transiting Neptune Found in the Beehive Cluster
Open clusters have been the focus of several exoplanet surveys but only a few
planets have so far been discovered. The \emph{Kepler} spacecraft revealed an
abundance of small planets around small, cool stars, therefore, such cluster
members are prime targets for exoplanet transit searches. Kepler's new mission,
K2, is targeting several open clusters and star-forming regions around the
ecliptic to search for transiting planets around their low-mass constituents.
Here, we report the discovery of the first transiting planet in the
intermediate-age (800 Myr) Beehive cluster (Praesepe). K2-95 is a faint
() dwarf from K2's Campaign 5
with an effective temperature of , approximately
solar metallicity and a radius of . We
detected a transiting planet with a radius of and an orbital period of 10.134 days. We combined photometry,
medium/high-resolution spectroscopy, adaptive optics/speckle imaging and
archival survey images to rule out any false positive detection scenarios,
validate the planet, and further characterize the system. The planet's radius
is very unusual as M-dwarf field stars rarely have Neptune-sized transiting
planets. The comparatively large radius of K2-95b is consistent with the other
recently discovered cluster planets K2-25b (Hyades) and K2-33b (Upper
Scorpius), indicating systematic differences in their evolutionary states or
formation. These discoveries from K2 provide a snapshot of planet formation and
evolution in cluster environments and thus make excellent laboratories to test
differences between field-star and cluster planet populations.Comment: 14 pages, 8 figues. Accepted for publication in A
Do Close-in Giant Planets Orbiting Evolved Stars Prefer Eccentric Orbits?
Stars and planetary system
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