Biosignature detection in the atmospheres of Earth-like exoplanets is one of
the most significant and ambitious goals for astronomy, astrobiology, and
humanity. Molecular oxygen is among the ...strongest indicators of life on Earth,
but it will be extremely difficult to detect via transmission spectroscopy. We
used the Bioverse statistical framework to assess the ability to probe
Earth-like O$_{\mathrm{2}}$ levels on hypothetical nearby habitable zone
exoplanets (EECs) using direct imaging and high-resolution spectroscopy on the
Giant Magellan Telescope (GMT) and the Extremely Large Telescope (ELT). We
found that O$_{\mathrm{2}}$ could be probed on up to $\sim$5 and $\sim$15 EECs
orbiting bright M dwarfs within 20 pc in a 10-year survey on the GMT and ELT,
respectively. Earth-like O$_{\mathrm{2}}$ levels could be probed on four known
super-Earth candidates, including Proxima Centauri b, within about one week on
the ELT and a few months on the GMT. We also assessed the ability of the ELT to
test the habitable zone oxygen hypothesis $\unicode{x2013}$ that habitable zone
Earth-sized planets are more likely to have O$_{\mathrm{2}}$ $\unicode{x2013}$
within a 10-year survey using Bioverse. Testing this hypothesis requires either
$\sim$1/2 of the EECs to have O$_{\mathrm{2}}$ or $\sim$1/3 if $\eta_{\oplus}$
is large. A northern hemisphere large-aperture telescope, such as the Thirty
Meter Telescope (TMT), would expand the target star pool by about 25%, reduce
the time to probe biosignatures on individual targets, and provide an
additional independent check on potential biosignature detections.
Molecular oxygen is a strong indicator of life on Earth, and may indicate
biological processes on exoplanets too. Recent studies proposed that Earth-like
O$_\mathrm{2}$ levels might be detectable on ...nearby exoplanets using
high-resolution spectrographs on future extremely large telescopes (ELTs).
However, these studies did not consider constraints like relative velocities,
planet occurrence rates, and target observability. We expanded on past studies
by creating a homogeneous catalog of 286,391 main-sequence stars within 120 pc
using Gaia DR3, and used the Bioverse framework to simulate the likelihood of
finding nearby transiting Earth analogs. We also simulated a survey of M dwarfs
within 20 pc accounting for $\eta_{\oplus}$ estimates, transit probabilities,
relative velocities, and target observability to determine how long ELTs and
theoretical 50-100 meter ground-based telescopes need to observe to probe for
Earth-like O$_\mathrm{2}$ levels with an $R=100,000$ spectrograph. This would
only be possible within 50 years for up to $\sim$21% of nearby M dwarf systems
if a suitable transiting habitable zone Earth-analog was discovered, assuming
signals from every observable partial transit from each ELT can be combined. If
so, Earth-like O$_\mathrm{2}$ levels could be detectable on TRAPPIST-1 d-g
within 16 to 55 years, respectively, and about half that time with an
$R=500,000$ spectrograph. These results have important implications for whether
ELTs can survey nearby habitable zone Earth analogs for O$_\mathrm{2}$ via
transmission spectroscopy. Our work provides the most comprehensive assessment
to date of the ground-based capabilities to search for life beyond the solar
system.
Reliable detections of Earth-sized planets in the habitable zone remain elusive in the Kepler sample, even for M dwarfs. The Kepler sample was once thought to contain a considerable number of M dwarf ...stars (\(T_\mathrm{eff} < 4000\) K), which hosted enough Earth-sized (\(0.5,1.5\) R\(_\oplus\)) planets to estimate their occurrence rate (\(\eta_\oplus\)) in the habitable zone. However, updated stellar properties from Gaia have shifted many Kepler stars to earlier spectral type classifications, with most stars (and their planets) now measured to be larger and hotter than previously believed. Today, only one partially-reliable Earth-sized candidate remains in the optimistic habitable zone, and zero in the conservative zone. Here we performed a new investigation of Kepler's Earth-sized planets orbiting M dwarf stars, using occurrence rate models with considerations of updated parameters and candidate reliability. Extrapolating our models to low instellations, we found an occurrence rate of \(\eta_\oplus={8.58}_{-8.22}^{+17.94}\%\) for the conservative habitable zone (and \({14.22}_{-12.71}^{+24.96}\%\) for the optimistic), consistent with previous works when considering the large uncertainties. Comparing these estimates to those from similarly comprehensive studies of Sun-like stars, we found that the current Kepler sample does not offer evidence to support an increase in \(\eta_\oplus\) from FGK to M stars. While the Kepler sample is too sparse to resolve an occurrence trend between early and mid-to-late M dwarfs for Earth-sized planets, studies including larger planets and/or data from the K2 and TESS missions are well-suited to this task.
The NASA K2 mission obtained high precision time-series photometry for four young clusters, including the near-twin 600-800 Myr-old Praesepe and Hyades clusters. Hot sub-Neptunes are highly prone to ...mass-loss mechanisms, given their proximity to the the host star and the weakly bound gaseous envelopes, and analyzing this population at young ages can provide strong constraints on planetary evolution models. Using our automated transit detection pipeline, we recover 15 planet candidates across the two clusters, including 10 previously confirmed planets. We find a hot sub-Neptune occurrence rate of 79-107% for GKM stars in the Praesepe cluster. This is 2.5-3.5 sigma higher than the occurrence rate of 16.54+1.00-0.98% for the same planets orbiting the ~3-9 Gyr-old GKM field stars observed by K2, even after accounting for the slightly super-solar metallicity (Fe/H~0.2 dex) of the Praesepe cluster. We examine the effect of adding ~100 targets from the Hyades cluster, and extending the planet parameter space under examination, and find similarly high occurrence rates in both cases. The high occurrence rate of young, hot sub-Neptunes could indicate either that these planets are undergoing atmospheric evolution as they age, or that planetary systems that formed when the Galaxy was much younger are substantially different than from today. Under the assumption of the atmospheric mass-loss scenario, a significantly higher occurrence rate of these planets at the intermediate ages of Praesepe and Hyades appears more consistent with the core-powered mass loss scenario sculpting the hot sub-Neptune population, compared to the photoevaporation scenario.
We collected near-infrared spectra of 65 cool stars with the NASA InfraRed Telescope Facility (IRTF) and analyze them to calculate accurate metallicities and stellar parameters. The sample of 55 M ...dwarfs and 10 K dwarfs includes 25 systems with confirmed planets and 27 systems with planet candidates identified by the K2 and TESS missions. Three of the 25 confirmed planetary systems host multiple confirmed planets and two of the 27 planet candidate systems host multiple planet candidates. Using the new stellar parameters, we re-fit the K2 and TESS light curves to calculate updated planet properties. In general, our updated stellar properties are more precise than those previously reported and our updated planet properties agree well with those in the literature. Lastly, we briefly examine the relationship between stellar mass, stellar metallicity, and planetary system properties for targets in our sample and for previously characterized planet-hosting low-mass stars. We provide our spectra, stellar parameters, and new planetary fits to the community, expanding the sample available with which to investigate correlations between stellar and planetary properties for low-mass stars.
The demographics of young exoplanets can shed light onto their formation and evolution processes. Exoplanet properties are derived from the properties of their host stars. As such, it is important to ...accurately characterize the host stars since any systematic biases in their derivation can negatively impact the derivation of planetary properties. Here, we present a uniform catalog of photometrically-derived stellar effective temperatures, luminosities, radii, and masses for 4,865 young (<1 Gyr) stars in 31 nearby clusters and moving groups within 200 pc. We compared our photometrically-derived properties to a subset of those derived from spectra, and found them to be in good agreement. We also investigated the effect of stellar properties on the detection efficiency of transiting short-period young planets with TESS as calculated in Fernandes et al. 2022, and found an overall increase in the detection efficiency when the new photometrically derived properties were taken into account. Most notably, there is a 1.5 times increase in the detection efficiencies for sub-Neptunes/Neptunes (1.8-6 Re) implying that, for our sample of young stars, better characterization of host star properties can lead to the recovery of more small transiting planets. Our homogeneously derived catalog of updated stellar properties, along with a larger unbiased stellar sample and more detections of young planets, will be a crucial input to the accurate estimation of the occurrence rates of young short-period planets.
Over the last decade, precise exoplanet transmission spectroscopy has revealed the atmospheres of dozens of exoplanets, driven largely by observatories like the Hubble Space Telescope. One major ...discovery has been the ubiquity of atmospheric aerosols, often blocking access to exoplanet chemical inventories. Tentative trends have been identified, showing that the clarity of planetary atmospheres may depend on equilibrium temperature. Previous work has often grouped dissimilar planets together in order to increase the statistical power of any trends, but it remains unclear from observed transmission spectra whether these planets exhibit the same atmospheric physics and chemistry. We present a re-analysis of a smaller, more physically similar sample of 15 exo-Neptune transmission spectra across a wide range of temperatures (200-1000 K). Using condensation cloud and hydrocarbon haze models, we find that the exo-Neptune population is best described by low cloud sedimentation efficiency (\(\mathrm{f_{sed}}\sim0.1\)) and high metallicity (\(100\times\) Solar). There is an intrinsic scatter of \(\sim0.5\) scale height, perhaps evidence of stochasticity in these planets' formation processes. Observers should expect significant attenuation in transmission spectra of Neptune-size exoplanets, up to 6 scale heights for equilibrium temperatures between 500 and 800 K. With JWST's greater wavelength sensitivity, colder (<500 K) planets should be high-priority targets given their clearer atmospheres, and the need to distinguish between the "super-puffs" and more typical gas-dominated planets.
Recent work on the characterization of small exoplanets has allowed us to accumulate growing evidence that the sub-Neptunes with radii greater than \(\sim2.5\,R_\oplus\) often host ...H\(_2\)/He-dominated atmospheres both from measurements of their low bulk densities and direct detections of their low mean-molecular-mass atmospheres. However, the smaller sub-Neptunes in the 1.5-2.2 R\(_\oplus\) size regime are much less understood, and often have bulk densities that can be explained either by the H\(_2\)/He-rich scenario, or by a volatile-dominated composition known as the "water world" scenario. Here, we report the detection of water vapor in the transmission spectrum of the \(1.96\pm0.08\) R\(_\oplus\) sub-Neptune GJ9827d obtained with the Hubble Space Telescope. We observed 11 HST/WFC3 transits of GJ9827d and find an absorption feature at 1.4\(\mu\)m in its transit spectrum, which is best explained (at 3.39\(\sigma\)) by the presence of water in GJ9827d's atmosphere. We further show that this feature cannot be caused by unnoculted star spots during the transits by combining an analysis of the K2 photometry and transit light-source effect retrievals. We reveal that the water absorption feature can be similarly well explained by a small amount of water vapor in a cloudy H\(_2\)/He atmosphere, or by a water vapor envelope on GJ9827d. Given that recent studies have inferred an important mass-loss rate (\(>0.5\,\)M\(_\oplus\)/Gyr) for GJ9827d making it unlikely to retain a H-dominated envelope, our findings highlight GJ9827d as a promising water world candidate that could host a volatile-dominated atmosphere. This water detection also makes GJ9827d the smallest exoplanet with an atmospheric molecular detection to date.
In this study, we performed a homogeneous analysis of the planets around FGK dwarf stars observed by the Kepler and K2 missions, providing spectroscopic parameters for 310 K2 targets -- including 239 ...Scaling K2 hosts -- observed with Keck/HIRES. For orbital periods less than 40 days, we found that the distribution of planets as a function of orbital period, stellar effective temperature, and metallicity was consistent between K2 and Kepler, reflecting consistent planet formation efficiency across numerous ~1 kpc sight-lines in the local Milky Way. Additionally, we detected a 3X excess of sub-Saturns relative to warm Jupiters beyond 10 days, suggesting a closer association between sub-Saturn and sub-Neptune formation than between sub-Saturn and Jovian formation. Performing a joint analysis of Kepler and K2 demographics, we observed diminishing super-Earth, sub-Neptune, and sub-Saturn populations at higher stellar effective temperatures, implying an inverse relationship between formation and disk mass. In contrast, no apparent host-star spectral-type dependence was identified for our population of Jupiters, which indicates gas-giant formation saturates within the FGK mass regimes. We present support for stellar metallicity trends reported by previous Kepler analyses. Using GAIA DR3 proper motion and RV measurements, we discovered a galactic location trend: stars that make large vertical excursions from the plane of the Milky Way host fewer super-Earths and sub-Neptunes. While oscillation amplitude is associated with metallicity, metallicity alone cannot explain the observed trend, demonstrating that galactic influences are imprinted on the planet population. Overall, our results provide new insights into the distribution of planets around FGK dwarf stars and the factors that influence their formation and evolution.
Biosignature detection in the atmospheres of Earth-like exoplanets is one of the most significant and ambitious goals for astronomy, astrobiology, and humanity. Molecular oxygen is among the ...strongest indicators of life on Earth, but it will be extremely difficult to detect via transmission spectroscopy. We used the Bioverse statistical framework to assess the ability to probe Earth-like O\(_{\mathrm{2}}\) levels on hypothetical nearby habitable zone exoplanets (EECs) using direct imaging and high-resolution spectroscopy on the Giant Magellan Telescope (GMT) and the Extremely Large Telescope (ELT). We found that O\(_{\mathrm{2}}\) could be probed on up to \(\sim\)5 and \(\sim\)15 EECs orbiting bright M dwarfs within 20 pc in a 10-year survey on the GMT and ELT, respectively. Earth-like O\(_{\mathrm{2}}\) levels could be probed on four known super-Earth candidates, including Proxima Centauri b, within about one week on the ELT and a few months on the GMT. We also assessed the ability of the ELT to test the habitable zone oxygen hypothesis \(\unicode{x2013}\) that habitable zone Earth-sized planets are more likely to have O\(_{\mathrm{2}}\) \(\unicode{x2013}\) within a 10-year survey using Bioverse. Testing this hypothesis requires either \(\sim\)1/2 of the EECs to have O\(_{\mathrm{2}}\) or \(\sim\)1/3 if \(\eta_{\oplus}\) is large. A northern hemisphere large-aperture telescope, such as the Thirty Meter Telescope (TMT), would expand the target star pool by about 25%, reduce the time to probe biosignatures on individual targets, and provide an additional independent check on potential biosignature detections.
