Context.
TOI-2076 is a transiting three-planet system of sub-Neptunes orbiting a bright (
G
= 8.9 mag), young (340 ± 80 Myr) K-type star. Although a validated planetary system, the orbits of the two ...outer planets were unconstrained as only two non-consecutive transits were seen in TESS photometry. This left 11 and 7 possible period aliases for each.
Aims.
To reveal the true orbits of these two long-period planets, precise photometry targeted on the highest-probability period aliases is required. Long-term monitoring of transits in multi-planet systems can also help constrain planetary masses through TTV measurements.
Methods.
We used the MonoTools package to determine which aliases to follow, and then performed space-based and ground-based photometric follow-up of TOI-2076 c and d with CHEOPS, SAINT-EX, and LCO telescopes.
Results.
CHEOPS observations revealed a clear detection for TOI-2076 c at $P = 21.02538_{ - 0.00074}^{ + 0.00084}$ d, and allowed us to rule out three of the most likely period aliases for TOI-2076 d. Ground-based photometry further enabled us to rule out remaining aliases and confirm the
P
= 35.12537 ± 0.00067 d alias. These observations also improved the radius precision of all three sub-Neptunes to 2.518 ± 0.036, 3.497 ± 0.043, and 3.232 ± 0.063
R
⊕
. Our observations also revealed a clear anti-correlated TTV signal between planets b and c likely caused by their proximity to the 2:1 resonance, while planets c and d appear close to a 5:3 period commensurability, although model degeneracy meant we were unable to retrieve robust TTV masses. Their inflated radii, likely due to extended H-He atmospheres, combined with low insolation makes all three planets excellent candidates for future comparative transmission spectroscopy with JWST.
ABSTRACT
We present the discovery of an exoplanet transiting TOI-908 (TIC-350153977) using data from TESS sectors 1, 12, 13, 27, 28, and 39. TOI-908 is a T = 10.7 mag G-dwarf (Teff = 5626 ± 61 K) ...solar-like star with a mass of 0.950 ± 0.010 M⊙ and a radius of 1.028 ± 0.030 R⊙. The planet, TOI-908 b, is a 3.18 ± 0.16 R⊕ planet in a 3.18 d orbit. Radial velocity measurements from HARPS reveal TOI-908 b has a mass of approximately 16.1 ± 4.1 M⊕, resulting in a bulk planetary density of $2.7^{+0.2}_{-0.4}$ g cm−3. TOI-908 b lies in a sparsely populated region of parameter space known as the Neptune desert. The planet likely began its life as a sub-Saturn planet before it experienced significant photoevaporation due to X-rays and extreme ultraviolet radiation from its host star, and is likely to continue evaporating, losing a significant fraction of its residual envelope mass.
We present the discovery and characterization of two warm mini-Neptunes transiting the K3V star TOI-815 in a K-M binary system. Analysis of the spectra and rotation period reveal it to be a young ...star with an age of \(200^{+400}_{-200}\)Myr. TOI-815b has a 11.2-day period and a radius of 2.94\(\pm\)0.05\(\it{R_{\rm\mathrm{\oplus}}}\) with transits observed by TESS, CHEOPS, ASTEP, and LCOGT. The outer planet, TOI-815c, has a radius of 2.62\(\pm\)0.10\(\it{R_{\rm\mathrm{\oplus}}}\), based on observations of three non-consecutive transits with TESS, while targeted CHEOPS photometry and radial velocity follow-up with ESPRESSO were required to confirm the 35-day period. ESPRESSO confirmed the planetary nature of both planets and measured masses of 7.6\(\pm\)1.5 \(\it{M_{\rm \mathrm{\oplus}}}\) (\(\rho_\mathrm{P}\)=1.64\(^{+0.33}_{-0.31}\)gcm\(^{-3}\)) and 23.5\(\pm\)2.4\(\it{M_{\rm\mathrm{\oplus}}}\) (\(\rho_\mathrm{P}\)=7.2\(^{+1.1}_{-1.0}\)gcm\(^{-3}\)) respectively. Thus, the planets have very different masses, unlike the usual similarity of masses in compact multi-planet systems. Moreover, our statistical analysis of mini-Neptunes orbiting FGK stars suggests that weakly irradiated planets tend to have higher bulk densities compared to those suffering strong irradiation. This could be ascribed to their cooler atmospheres, which are more compressed and denser. Internal structure modeling of TOI-815b suggests it likely has a H-He atmosphere constituting a few percent of the total planet mass, or higher if the planet is assumed to have no water. In contrast, the measured mass and radius of TOI-815c can be explained without invoking any atmosphere, challenging planetary formation theories. Finally, we infer from our measurements that the star is viewed close to pole-on, which implies a spin-orbit misalignment at the 3\(\sigma\) level.
We present the discovery of an exoplanet transiting TOI-908 (TIC-350153977) using data from TESS sectors 1, 12, 13, 27, 28 and 39. TOI-908 is a T = 10.7 mag G-dwarf (\(T_{eff}\) = 5626 \(\pm\) 61 K) ...solar-like star with a mass of 0.950 \(\pm\) 0.010 \(M_{\odot}\) and a radius of 1.028 \(\pm\) 0.030 \(R_{\odot}\). The planet, TOI-908 b, is a 3.18 \(\pm\) 0.16 \(R_{\oplus}\) planet in a 3.18 day orbit. Radial velocity measurements from HARPS reveal TOI-908 b has a mass of approximately 16.1 \(\pm\) 4.1 \(M_{\oplus}\) , resulting in a bulk planetary density of 2.7+0.2-0.4 g cm-3. TOI-908 b lies in a sparsely-populated region of parameter space known as the Neptune desert. The planet likely began its life as a sub-Saturn planet before it experienced significant photoevaporation due to X-rays and extreme ultraviolet radiation from its host star, and is likely to continue evaporating, losing a significant fraction of its residual envelope mass.
To date, thousands of planets have been discovered, but there are regions of the orbital parameter space that are still bare. An example is the short period and intermediate mass/radius space known ...as the Neptunian desert, where planets should be easy to find but discoveries remain few. This suggests unusual formation and evolution processes are responsible for the planets residing here. We present the discovery of TOI-332 b, a planet with an ultra-short period of \(0.78\) d that sits firmly within the desert. It orbits a K0 dwarf with an effective temperature of \(5251 \pm 71\) K. TOI-332 b has a radius of \(3.20^{+0.16}_{-0.12}\) R\(_{\oplus}\), smaller than that of Neptune, but an unusually large mass of \(57.2 \pm 1.6\) M\(_{\oplus}\). It has one of the highest densities of any Neptune-sized planet discovered thus far at \(9.6^{+1.1}_{-1.3}\) gcm\(^{-3}\). A 4-layer internal structure model indicates it likely has a negligible hydrogen-helium envelope, something only found for a small handful of planets this massive, and so TOI-332 b presents an interesting challenge to planetary formation theories. We find that photoevaporation cannot account for the mass loss required to strip this planet of the Jupiter-like envelope it would have been expected to accrete. We need to look towards other scenarios, such as high-eccentricity migration, giant impacts, or gap opening in the protoplanetary disc, to try and explain this unusual discovery.
HIP 9618 (HD 12572, TOI-1471, TIC 306263608) is a bright (\(G=9.0\) mag) solar analogue. TESS photometry revealed the star to have two candidate planets with radii of \(3.9 \pm 0.044\) \(R_\oplus\) ...(HIP 9618 b) and \(3.343 \pm 0.039\) \(R_\oplus\) (HIP 9618 c). While the 20.77291 day period of HIP 9618 b was measured unambiguously, HIP 9618 c showed only two transits separated by a 680-day gap in the time series, leaving many possibilities for the period. To solve this issue, CHEOPS performed targeted photometry of period aliases to attempt to recover the true period of planet c, and successfully determined the true period to be 52.56349 d. High-resolution spectroscopy with HARPS-N, SOPHIE and CAFE revealed a mass of \(10.0 \pm 3.1 M_\oplus\) for HIP 9618 b, which, according to our interior structure models, corresponds to a \(6.8\pm1.4\%\) gas fraction. HIP 9618 c appears to have a lower mass than HIP 9618 b, with a 3-sigma upper limit of \(< 18M_\oplus\). Follow-up and archival RV measurements also reveal a clear long-term trend which, when combined with imaging and astrometric information, reveal a low-mass companion (\(0.08^{+0.12}_{-0.05} M_\odot\)) orbiting at \(26^{+19}_{-11}\) au. This detection makes HIP 9618 one of only five bright (\(K<8\) mag) transiting multi-planet systems known to host a planet with \(P>50\) d, opening the door for the atmospheric characterisation of warm (\(T_{\rm eq}<750\) K) sub-Neptunes.
We report the discovery of two warm sub-Neptunes transiting the bright (G = 9.5 mag) K-dwarf HD 15906 (TOI 461, TIC 4646810). This star was observed by the Transiting Exoplanet Survey Satellite ...(TESS) in sectors 4 and 31, revealing two small transiting planets. The inner planet, HD 15906 b, was detected with an unambiguous period but the outer planet, HD 15906 c, showed only two transits separated by \(\sim\) 734 days, leading to 36 possible values of its period. We performed follow-up observations with the CHaracterising ExOPlanet Satellite (CHEOPS) to confirm the true period of HD 15906 c and improve the radius precision of the two planets. From TESS, CHEOPS and additional ground-based photometry, we find that HD 15906 b has a radius of 2.24 \(\pm\) 0.08 R\(_\oplus\) and a period of 10.924709 \(\pm\) 0.000032 days, whilst HD 15906 c has a radius of 2.93\(^{+0.07}_{-0.06}\) R\(_\oplus\) and a period of 21.583298\(^{+0.000052}_{-0.000055}\) days. Assuming zero bond albedo and full day-night heat redistribution, the inner and outer planet have equilibrium temperatures of 668 \(\pm\) 13 K and 532 \(\pm\) 10 K, respectively. The HD 15906 system has become one of only six multiplanet systems with two warm (\(\lesssim\) 700 K) sub-Neptune sized planets transiting a bright star (G \(\leq\) 10 mag). It is an excellent target for detailed characterisation studies to constrain the composition of sub-Neptune planets and test theories of planet formation and evolution.
The Kepler mission has made an important observation: the first detection of photons from a terrestrial planet by observing its phase curve (Kepler-10b). This opens a new field in exoplanet science: ...the possibility of obtaining information about the atmosphere and surface of rocky planets, objects of prime interest. In this Letter, we apply the Lava-ocean model to interpret the observed phase curve. The model, a planet without atmosphere and a surface partially made of molten rocks, has been proposed for planets of the class of CoRoT-7b, i.e., rocky planets very close to their star (at a few stellar radii). Kepler-10b is a typical member of this family. It predicts that the light from the planet has an important emission component in addition to the reflected one, even in the Kepler spectral band. Assuming an isotropical reflection of light by the planetary surface (Lambertian-like approximation), we find that a Bond albedo of ~50% can account for the observed amplitude of the phase curve, as opposed to a first attempt where an unusually high value was found. We propose a physical process to explain this still large value of the albedo. The overall interpretation can be tested in the future with instruments such as the James Webb Space Telescope or the Exoplanet Characterization Observatory. Our model predicts a spectral dependence that is clearly distinguishable from that of purely reflected light and from that of a planet at a uniform temperature.
High precision time series photometry from space is being used for a number of scientific cases. In this context, the recently launched CHEOPS (ESA) mission promises to bring 20 ppm precision over an ...exposure time of 6 hours, when targeting nearby bright stars, having in mind the detailed characterization of exoplanetary systems through transit measurements. However, the official CHEOPS (ESA) mission pipeline only provides photometry for the main target (the central star in the field). In order to explore the potential of CHEOPS photometry for all stars in the field, in this paper we present archi, an additional open-source pipeline module{\dag}to analyse the background stars present in the image. As archi uses the official Data Reduction Pipeline data as input, it is not meant to be used as independent tool to process raw CHEOPS data but, instead, to be used as an add-on to the official pipeline. We test archi using CHEOPS simulated images, and show that photometry of background stars in CHEOPS images is only slightly degraded (by a factor of 2 to 3) with respect to the main target. This opens a potential for the use of CHEOPS to produce photometric time series of several close-by targets at once, as well as to use different stars in the image to calibrate systematic errors. We also show one clear scientific application where the study of the companion light curve can be important for the understanding of the contamination on the main target.
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