Young exoplanets can offer insight into the evolution of planetary atmospheres, compositions, and architectures. We present the discovery of the young planetary system TOI 451 (TIC 257605131, Gaia ...DR2 4844691297067063424). TOI 451 is a member of the 120-Myr-old Pisces--Eridanus stream (Psc--Eri). We confirm membership in the stream with its kinematics, its lithium abundance, and the rotation and UV excesses of both TOI 451 and its wide binary companion, TOI 451 B (itself likely an M dwarf binary). We identified three candidate planets transiting in the TESS data and followed up the signals with photometry from Spitzer and ground-based telescopes. The system comprises three validated planets at periods of 1.9, 9.2 and 16 days, with radii of 1.9, 3.1, and 4.1 Earth radii, respectively. The host star is near-solar mass with V=11.0 and H=9.3 and displays an infrared excess indicative of a debris disk. The planets offer excellent prospects for transmission spectroscopy with HST and JWST, providing the opportunity to study planetary atmospheres that may still be in the process of evolving.
We present the bright (V\(_{mag} = 9.12\)), multi-planet system TOI-431, characterised with photometry and radial velocities. We estimate the stellar rotation period to be \(30.5 \pm 0.7\) days using ...archival photometry and radial velocities. TOI-431b is a super-Earth with a period of 0.49 days, a radius of 1.28 \(\pm\) 0.04 R\(_{\oplus}\), a mass of \(3.07 \pm 0.35\) M\(_{\oplus}\), and a density of \(8.0 \pm 1.0\) g cm\(^{-3}\); TOI-431d is a sub-Neptune with a period of 12.46 days, a radius of \(3.29 \pm 0.09\) R\(_{\oplus}\), a mass of \(9.90^{+1.53}_{-1.49}\) M\(_{\oplus}\), and a density of \(1.36 \pm 0.25\) g cm\(^{-3}\). We find a third planet, TOI-431c, in the HARPS radial velocity data, but it is not seen to transit in the TESS light curves. It has an \(M \sin i\) of \(2.83^{+0.41}_{-0.34}\) M\(_{\oplus}\), and a period of 4.85 days. TOI-431d likely has an extended atmosphere and is one of the most well-suited TESS discoveries for atmospheric characterisation, while the super-Earth TOI-431b may be a stripped core. These planets straddle the radius gap, presenting an interesting case-study for atmospheric evolution, and TOI-431b is a prime TESS discovery for the study of rocky planet phase curves.
HD 106315 and GJ 9827 are two bright, nearby stars that host multiple super-Earths and sub-Neptunes discovered by K2 that are well suited for atmospheric characterization. We refined the planets' ...ephemerides through Spitzer transits, enabling accurate transit prediction required for future atmospheric characterization through transmission spectroscopy. Through a multi-year high-cadence observing campaign with Keck/HIRES and Magellan/PFS, we improved the planets' mass measurements in anticipation of HST transmission spectroscopy. For GJ 9827, we modeled activity-induced radial velocity signals with a Gaussian process informed from the Calcium II H&K lines in order to more accurately model the effect of stellar noise on our data. We found planet masses of M\(_b\)=\(4.87\pm 0.37\) M\(_\oplus\), M\(_c\)=\(1.92\pm 0.49\) M\(_\oplus\), and M\(_d\)=\(3.42\pm 0.62\) M\(_\oplus\). For HD 106315, we found that such activity-radial velocity decorrelation was not effective due to the reduced presence of spots and speculate that this may extend to other hot stars as well (T\(_{\rm {eff}}>6200\) K). We found planet masses of M\(_b\)=\(10.5\pm 3.1\) M\(_\oplus\) and M\(_c\)=\(12.0\pm 3.8\) M\(_\oplus\). We investigated all of the planets' compositions through comparing their masses and radii to a range of interior models. GJ 9827 b and GJ 9827 c are both consistent with an Earth-like rocky composition, GJ 9827 d and HD 106315 b both require additional volatiles and are consistent with moderate amounts of water or hydrogen/helium, and HD 106315 c is consistent with 10% hydrogen/helium surrounding an Earth-like rock and iron core.
We use TESS, Spitzer, ground-based light curves and HARPS spectrograph radial velocity measurements to establish the physical properties of the transiting exoplanet candidate TOI-674b. We perform a ...joint fit of the light curves and radial velocity time series to measure the mass, radius, and orbital parameters of the candidate. We confirm and characterize TOI-674b, a low-density super-Neptune transiting a nearby M dwarf. The host star (TIC 158588995, \(V = 14.2\) mag, \(J = 10.3\) mag) is characterized by its M2V spectral type with \(\mathrm{M}_\star=0.420\pm 0.010\) M\(_\odot\), \(\mathrm{R}_\star = 0.420\pm 0.013\) R\(_\odot\), and \(\mathrm{T}_{\mathrm{eff}} = 3514\pm 57\) K, and is located at a distance \(d=46.16 \pm 0.03\) pc. Combining the available transit light curves plus radial velocity measurements and jointly fitting a circular orbit model, we find an orbital period of \(1.977143 \pm 3\times 10^{-6}\) days, a planetary radius of \(5.25 \pm 0.17\) \(\mathrm{R}_\oplus\), and a mass of \(23.6 \pm 3.3\) \(\mathrm{M}_\oplus\) implying a mean density of \(\rho_\mathrm{p} = 0.91 \pm 0.15\) g cm\(^{-3}\). A non-circular orbit model fit delivers similar planetary mass and radius values within the uncertainties. Given the measured planetary radius and mass, TOI-674b is one of the largest and most massive super-Neptune class planets discovered around an M type star to date. It is also a resident of the so-called Neptunian desert and a promising candidate for atmospheric characterisation using the James Webb Space Telescope.
We present a transmission spectrum for the Neptune-size exoplanet HD 106315 c from optical to infrared wavelengths based on transit observations from the Hubble Space Telescope/Wide Field Camera 3, ...K2, and Spitzer. The spectrum shows tentative evidence for a water absorption feature in the \(1.1 - 1.7\mu\)m wavelength range with a small amplitude of 30 ppm (corresponding to just \(0.8 \pm 0.04\) atmospheric scale heights). Based on an atmospheric retrieval analysis, the presence of water vapor is tentatively favored with a Bayes factor of 1.7 - 2.6 (depending on prior assumptions). The spectrum is most consistent with either enhanced metallicity, high altitude condensates, or both. Cloud-free solar composition atmospheres are ruled out at \(>5\sigma\) confidence. We compare the spectrum to grids of cloudy and hazy forward models and find that the spectrum is fit well by models with moderate cloud lofting or haze formation efficiency, over a wide range of metallicities (\(1 - 100\times\) solar). We combine the constraints on the envelope composition with an interior structure model and estimate that the core mass fraction is \(\gtrsim0.3\). With a bulk composition reminiscent of that of Neptune and an orbital distance of 0.15 AU, HD 106315 c hints that planets may form out of broadly similar material and arrive at vastly different orbits later in their evolution.
We present a new {\em Spitzer} transit observation of K2\(-\)28b, a sub-Neptune (\(R_{\rm p} = 2.45\pm0.28 R_{Earth}\)) orbiting a relatively bright (\(V_{\rm mag} = 16.06\), \(K_{\rm mag} = 10.75\)) ...metal-rich M4 dwarf (EPIC 206318379). This star is one of only seven with masses less than 0.2 M_{Sun} known to host transiting planets, and the planet appears to be a slightly smaller analogue of GJ 1214b (\(2.85\pm0.20 R_{Earth}\); \citealt{2013A&A...549A..10H}). Our new {\em Spitzer} observations were taken two years after the original K2 discovery data and have a significantly higher cadence, allowing us to derive improved estimates for this planet's radius, semi-major axis, and orbital period, which greatly reduce the uncertainty in the prediction of near future transit times for the {\em James Webb Space Telescope} ({\em JWST}) observations. We also evaluate the system's suitability for atmospheric characterization with {\em JWST} and find that it is currently the only small (< 3 R_{Earth}) and cool (\(< 600\) K) planet aside from GJ 1214b with a potentially detectable secondary eclipse. We also note that this system is a favorable target for near-infrared radial velocity instruments on larger telescopes (e.g., the Habitable Planet Finder on the Hobby-Eberly Telescope), making it one of only a handful of small, cool planets accessible with this technique. Finally, we compare our results with the simulated catalog of the Transiting Exoplanet Survey Satellite ({\em TESS}) and find K2\(-\)28b to be representative of the kind of mid-M systems that should be detectible in the {\em TESS} sample.
We describe a joint high contrast imaging survey for planets at Keck and VLT of the last large sample of debris disks identified by the Spitzer Space Telescope. No new substellar companions were ...discovered in our survey of 30 Spitzer-selected targets. We combine our observations with data from four published surveys to place constraints on the frequency of planets around 130 debris disk single stars, the largest sample to date. For a control sample, we assembled contrast curves from several published surveys targeting 277 stars which do not show infrared excesses. We assumed a double power law distribution in mass and semi-major axis of the form f(m,a) = \(Cm^{\alpha}a^{\beta}\), where we adopted power law values and logarithmically flat values for the mass and semi-major axis of planets. We find that the frequency of giant planets with masses 5-20 \(M_{\rm Jup}\) and separations 10-1000 AU around stars with debris disks is 6.27% (68% confidence interval 3.68 - 9.76%), compared to 0.73% (68% confidence interval 0.20 - 1.80%) for the control sample of stars without disks. These distributions differ at the 88% confidence level, tentatively suggesting distinctness of these samples.
Observations from the Kepler and K2 missions have provided the astronomical community with unprecedented amounts of data to search for transiting exoplanets and other astrophysical phenomena. Here, ...we present K2-288, a low-mass binary system (M2.0 +/- 1.0; M3.0 +/- 1.0) hosting a small (Rp = 1.9 REarth), temperate (Teq = 226 K) planet observed in K2 Campaign 4. The candidate was first identified by citizen scientists using Exoplanet Explorers hosted on the Zooniverse platform. Follow-up observations and detailed analyses validate the planet and indicate that it likely orbits the secondary star on a 31.39-day period. This orbit places K2-288Bb in or near the habitable zone of its low-mass host star. K2-288Bb resides in a system with a unique architecture, as it orbits at >0.1 au from one component in a moderate separation binary (aproj approximately 55 au), and further follow-up may provide insight into its formation and evolution. Additionally, its estimated size straddles the observed gap in the planet radius distribution. Planets of this size occur less frequently and may be in a transient phase of radius evolution. K2-288 is the third transiting planet system identified by the Exoplanet Explorers program and its discovery exemplifies the value of citizen science in the era of Kepler, K2, and the Transiting Exoplanet Survey Satellite.
Given that Campaign 16 of the K2 mission is one of just two K2 campaigns observed so far in "forward-facing" mode, which enables immediate follow-up observations from the ground, we present a catalog ...of interesting targets identified through photometry alone. Our catalog includes 30 high-quality planet candidates (showing no signs of being non-planetary in nature), 48 more ambiguous events that may be either planets or false positives, 164 eclipsing binaries, and 231 other regularly periodic variable sources. We have released light curves for all targets in C16, and have also released system parameters and transit vetting plots for all interesting candidates identified in this paper. Of particular interest is a candidate planet orbiting the bright F dwarf HD 73344 (V=6.9, K=5.6) with an orbital period of 15 days. If confirmed, this object would correspond to a \(2.56 \pm 0.18 \ R_\oplus\) planet and would likely be a favorable target for radial velocity characterization. This paper is intended as a rapid release of planet candidates, eclipsing binaries and other interesting periodic variables to maximize the scientific yield of this campaign, and as a test run for the upcoming TESS mission, whose frequent data releases call for similarly rapid candidate identification and efficient follow-up.
We report improved masses, radii, and densities for four planets in two bright M-dwarf systems, K2-3 and GJ3470, derived from a combination of new radial velocity and transit observations. ...Supplementing K2 photometry with follow-up Spitzer transit observations refined the transit ephemerides of K2-3 b, c, and d by over a factor of 10. We analyze ground-based photometry from the Evryscope and Fairborn Observatory to determine the characteristic stellar activity timescales for our Gaussian Process fit, including the stellar rotation period and activity region decay timescale. The stellar rotation signals for both stars are evident in the radial velocity data and are included in our fit using a Gaussian process trained on the photometry. We find the masses of K2-3 b, K2-3 c and GJ3470 b to be 6.48\(^{+0.99}_{-0.93}\), 2.14\(^{+1.08}_{-1.04}\), and 12.58\(^{+1.31}_{-1.28}\) M\(_\oplus\) respectively. K2-3 d was not significantly detected and has a 3-\(\sigma\) upper limit of 2.80 M\(_\oplus\). These two systems are training cases for future TESS systems; due to the low planet densities (\(\rho\) \(<\) 3.7 g cm\(^{-3}\)) and bright host stars (K \(<\) 9 mag), they are among the best candidates for transmission spectroscopy in order to characterize the atmospheric compositions of small planets.
