Abstract
Recent astronomical observations obtained with the Kepler and TESS missions and their related ground-based follow-ups revealed an abundance of exoplanets with a size intermediate between ...Earth and Neptune (1
R
⊕
≤
R
≤ 4
R
⊕
). A low occurrence rate of planets has been identified at around twice the size of Earth (2 ×
R
⊕
), known as the exoplanet radius gap or radius valley. We explore the geometry of this gap in the mass–radius diagram, with the help of a
Mathematica
plotting tool developed with the capability of manipulating exoplanet data in multidimensional parameter space, and with the help of visualized water equations of state in the temperature–density (
T
–
ρ
) graph and the entropy–pressure (
s
–
P
) graph. We show that the radius valley can be explained by a compositional difference between smaller, predominantly rocky planets (<2 ×
R
⊕
) and larger planets (>2 ×
R
⊕
) that exhibit greater compositional diversity including cosmic ices (water, ammonia, methane, etc.) and gaseous envelopes. In particular, among the larger planets (>2 ×
R
⊕
), when viewed from the perspective of planet equilibrium temperature (
T
eq
), the hot ones (
T
eq
≳ 900 K) are consistent with ice-dominated composition without significant gaseous envelopes, while the cold ones (
T
eq
≲ 900 K) have more diverse compositions, including various amounts of gaseous envelopes.
Abstract
K2-136 is a late-K dwarf (0.742 ± 0.039
M
⊙
) in the Hyades open cluster with three known, transiting planets and an age of 650 ± 70 Myr. Analyzing K2 photometry, we found that planets ...K2-136b, c, and d have periods of 8.0, 17.3, and 25.6 days and radii of 1.014 ± 0.050
R
⊕
, 3.00 ± 0.13
R
⊕
, and 1.565 ± 0.077
R
⊕
, respectively. We collected 93 radial velocity (RV) measurements with the High-Accuracy Radial-velocity Planet Searcher for the Northern hemisphere (HARPS-N) spectrograph (Telescopio Nazionale Galileo) and 22 RVs with the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) spectrograph (Very Large Telescope). Analyzing HARPS-N and ESPRESSO data jointly, we found that K2-136c induced a semi-amplitude of 5.49 ± 0.53 m s
−1
, corresponding to a mass of 18.1 ± 1.9
M
⊕
. We also placed 95% upper mass limits on K2-136b and d of 4.3 and 3.0
M
⊕
, respectively. Further, we analyzed Hubble Space Telescope and XMM-Newton observations to establish the planetary high-energy environment and investigate possible atmospheric loss. K2-136c is now the smallest planet to have a measured mass in an open cluster and one of the youngest planets ever with a mass measurement. K2-136c has ∼75% the radius of Neptune but is similar in mass, yielding a density of
3.69
−
0.56
+
0.67
g cm
−3
(∼2–3 times denser than Neptune). Mass estimates for K2-136b (and possibly d) may be feasible with more RV observations, and insights into all three planets’ atmospheres through transmission spectroscopy would be challenging but potentially fruitful. This research and future mass measurements of young planets are critical for investigating the compositions and characteristics of small exoplanets at very early stages of their lives and providing insights into how exoplanets evolve with time.
Recent astronomical observations obtained with the Kepler and TESS missions and their related ground-based follow-ups revealed an abundance of exoplanets with a size intermediate between Earth and ...Neptune. A low occurrence rate of planets has been identified at around twice the size of Earth, known as the exoplanet radius gap or radius valley. We explore the geometry of this gap in the mass-radius diagram, with the help of a Mathematica plotting tool developed with the capability of manipulating exoplanet data in multidimensional parameter space, and with the help of visualized water equations of state in the temperature-density graph and the entropy-pressure graph. We show that the radius valley can be explained by a compositional difference between smaller, predominantly rocky planets and larger planets that exhibit greater compositional diversity including cosmic ices (water, ammonia, methane) and gaseous envelopes. In particular, among the larger planets, when viewed from the perspective of planet equilibrium temperature, the hot ones are consistent with ice-dominated composition without significant gaseous envelopes, while the cold ones have more diverse compositions, including various amounts of gaseous envelopes.
K2-136 is a late-K dwarf (\(0.742\pm0.039\) M\(_\odot\)) in the Hyades open cluster with three known, transiting planets and an age of \(650\pm70\) Myr. Analyzing K2 photometry, we found that planets ...K2-136b, c, and d have periods of \(8.0\), \(17.3\), and \(25.6\) days and radii of \(1.014\pm0.050\) R\(_\oplus\), \(3.00\pm0.13\) R\(_\oplus\), and \(1.565\pm0.077\) R\(_\oplus\), respectively. We collected 93 radial velocity measurements (RVs) with the HARPS-N spectrograph (TNG) and 22 RVs with the ESPRESSO spectrograph (VLT). Analyzing HARPS-N and ESPRESSO data jointly, we found K2-136c induced a semi-amplitude of \(5.49\pm0.53\) m s\(^{-1}\), corresponding to a mass of \(18.1\pm1.9\) M\(_\oplus\). We also placed \(95\)% upper mass limits on K2-136b and d of \(4.3\) and \(3.0\) M\(_\oplus\), respectively. Further, we analyzed HST and XMM-Newton observations to establish the planetary high-energy environment and investigate possible atmospheric loss. K2-136c is now the smallest planet to have a measured mass in an open cluster and one of the youngest planets ever with a mass measurement. K2-136c has \(\sim\)75% the radius of Neptune but is similar in mass, yielding a density of \(3.69^{+0.67}_{-0.56}\) g cm\(^{-3}\) (\(\sim\)2-3 times denser than Neptune). Mass estimates for K2-136b (and possibly d) may be feasible with more RV observations, and insights into all three planets' atmospheres through transmission spectroscopy would be challenging but potentially fruitful. This research and future mass measurements of young planets are critical for investigating the compositions and characteristics of small exoplanets at very early stages of their lives and providing insights into how exoplanets evolve with time.
We present the discovery of two exoplanets transiting TOI-836 (TIC 440887364) using data from TESS Sector 11 and Sector 38. TOI-836 is a bright (\(T = 8.5\) mag), high proper motion (\(\sim\,200\) ...mas yr\(^{-1}\)), low metallicity (Fe/H\(\approx\,-0.28\)) K-dwarf with a mass of \(0.68\pm0.05\) M\(_{\odot}\) and a radius of \(0.67\pm0.01\) R\(_{\odot}\). We obtain photometric follow-up observations with a variety of facilities, and we use these data-sets to determine that the inner planet, TOI-836 b, is a \(1.70\pm0.07\) R\(_{\oplus}\) super-Earth in a 3.82 day orbit, placing it directly within the so-called 'radius valley'. The outer planet, TOI-836 c, is a \(2.59\pm0.09\) R\(_{\oplus}\) mini-Neptune in an 8.60 day orbit. Radial velocity measurements reveal that TOI-836 b has a mass of \(4.5\pm0.9\) M\(_{\oplus}\) , while TOI-836 c has a mass of \(9.6\pm2.6\) M\(_{\oplus}\). Photometric observations show Transit Timing Variations (TTVs) on the order of 20 minutes for TOI-836 c, although there are no detectable TTVs for TOI-836 b. The TTVs of planet TOI-836 c may be caused by an undetected exterior planet.
We report the discovery and characterisation of a pair of sub-Neptunes transiting the bright K-dwarf TOI-1064 (TIC 79748331), initially detected in TESS photometry. To characterise the system, we ...performed and retrieved CHEOPS, TESS, and ground-based photometry, HARPS high-resolution spectroscopy, and Gemini speckle imaging. We characterise the host star and determine \(T_{\rm eff, \star}=4734\pm67\) K, \(R_{\star}=0.726\pm0.007\) \(R_{\odot}\), and \(M_{\star}=0.748\pm0.032\) \(M_{\odot}\). We present a novel detrending method based on PSF shape-change modelling and demonstrate its suitability to correct flux variations in CHEOPS data. We confirm the planetary nature of both bodies and find that TOI-1064 b has an orbital period of \(P_{\rm b}=6.44387\pm0.00003\) d, a radius of \(R_{\rm b}=2.59\pm0.04\) \(R_{\oplus}\), and a mass of \(M_{\rm b}=13.5_{-1.8}^{+1.7}\) \(M_{\oplus}\), whilst TOI-1064 c has an orbital period of \(P_{\rm c}=12.22657^{+0.00005}_{-0.00004}\) d, a radius of \(R_{\rm c}=2.65\pm0.04\) \(R_{\oplus}\), and a 3\(\sigma\) upper mass limit of 8.5 \({\rm M_{\oplus}}\). From the high-precision photometry we obtain radius uncertainties of \(\sim\)1.6%, allowing us to conduct internal structure and atmospheric escape modelling. TOI-1064 b is one of the densest, well-characterised sub-Neptunes, with a tenuous atmosphere that can be explained by the loss of a primordial envelope following migration through the protoplanetary disc. It is likely that TOI-1064 c has an extended atmosphere due to the tentative low density, however further RVs are needed to confirm this scenario and the similar radii, different masses nature of this system. The high-precision data and modelling of TOI-1064 b are important for planets in this region of mass-radius space, and it allows us to identify a trend in bulk density-stellar metallicity for massive sub-Neptunes that may hint at the formation of this population of planets.
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