Gautam Vasisht

Discovering Earth-like planets orbiting Sun-like stars was identified as a priority science goal of the Astronomy 2020 Decadal Survey. It is confounded by many factors, one of which is the high multiplicity of Sun-like stars in the local neighborhood—half of nearby Sun-like stars are in binary or higher-order stellar systems, which are less amenable to the detection of small planets with almost all of the currently productive exoplanet detection techniques. We describe the SHERA (Searc+hing for Habitable Exoplanets with Relative Astrometry) NASA Small Explorer mission concept. SHERA utilizes diffractive-pupil technology on a small, simple optical space telescope to achieve microarcsecond precision relative astrometry on 14 Sun-like stars in seven nearby multistar systems, combining the pupil and stellar binarity to provide a precise reference in the image plane. With this precision, SHERA would enable the following: (i) a search for rocky planets in the habitable zones of the closest Sun-like stars; (ii) an investigation of the impact of binary star formation on small, widely separated planets; and (iii) the performance of crucial precursor observations on a number of high-priority targets of NASA’s future missions to characterize Earth-like planets, such as the Habitable Worlds Observatory. When combined with radial velocity measurements, SHERA relative astrometry will also enable exploration of the three-dimensional orbital structure of planets in binary systems.

At a distance of 5.1 pc, the 70 Oph AB binary star system is one of the most favorable targets for future direct imaging and astrometry missions surveying mature, terrestrial planets. We present new radial velocities (RVs) obtained with the Planet Finder Spectrograph (PFS) on the 6.5 m Magellan II Clay Telescope in Chile. We collected 499 measurements of 70 Oph A and 334 measurements of 70 Oph B during 2023–2025. Combining these data with decades of archival RVs and astrometry, we derive an updated orbital solution for the binary and dynamical masses of 0.88 ± 0.004 and 0.73 ± 0.003 M _⊙ for the primary and secondary components, respectively. We find that the long-term RV variability of both components is consistent with stellar activity modulated by rotation periods, and we detect no coherent planetary signals in either component. We place upper limits on any planets orbit in the plane of the binary. The 27 yr RV baseline for 70 Oph A excludes Jupiter-mass planets interior to 5 au and reaches sensitivity of 0.3 M _Jup at 1 au or 0.5 M _Jup at 2 au. For 70 Oph B, with PFS data we rule out planets more massive than 0.25–0.3 M _Jup inside 0.5 au. We show that stable S-type orbits around 70 Oph A extend to ∼2.5 au, covering the habitable zone. Thus, Saturn-mass planets or smaller on stable orbits in the habitable zone of 70 Oph A are allowed. Overall, our results provide important guidance for future planet searches around this stellar system.

We present a near-infrared spectropolarimetric survey of brown dwarfs conducted with the WIRC+Pol instrument on the Palomar 200 inch Hale Telescope. The primary survey goal was to investigate the presence of polarization signatures arising from cloudy atmospheres in these objects, with possible asymmetries indicating patchy cloud distributions. Our sample includes 22 brown dwarfs spanning M6 to T2 spectral types, a crucial phase in the evolution of substellar objects characterized by suspected drastic changes in atmospheric composition and structure. We find that only three of the brown dwarfs in our sample show a detectable polarization signal: CFHT BD Tau 4 and 2MASS J04221413+1530525, which are both located in or near molecular clouds, complicating interpretation of the polarimetric signal due to likely contamination from interstellar dust scattering; and 2MASS J15074769-1627386, which is a nearby L dwarf with previously confirmed photometric variability and measured optical polarization, here with its first measured near-infrared polarization. For this target, we rule out interstellar polarization and rotationally induced oblateness as possible causes of the measured polarization, potentially suggesting atmospheric asymmetries as the culprit.

TOI-1266 is a benchmark system of two temperate (P = 32.5 days, M _d = 3.68 − 1.11 + 1.05 M ⊕ ), and combine the TTVs with archival radial velocity measurements to improve our knowledge of the planetary masses and radii. We find that, consistent with previous studies, TOI-1266 b (R _b = 2.52 ± 0.08 R _⊕, M _b = 4.46 ± 0.69 M _⊕) has a low bulk density requiring the presence of a hydrogen-rich envelope, while TOI-1266 c (R _c = 1.98 ± 0.10 R _⊕, M _c = 3.17 ± 0.76 M _⊕) has a higher bulk density that can be matched by either a hydrogen-rich or water-rich envelope. Our new dynamical model reveals that this system is arranged in a rare configuration with the inner and outer planets located near the 3:1 period ratio with a nonresonant planet in between them. Our dynamical fits indicate that the inner and outer planet have significantly nonzero eccentricities ( e b + e d = 0.07 6 − 0.019 + 0.029 ), suggesting that TOI-1266 b may have an inflated envelope due to tidal heating. Finally, we explore the corresponding implications for the formation and long-term evolution of the system, which contains two of the most favorable cool (<500 K) sub-Neptunes for atmospheric characterization with JWST.

Neptune-size exoplanets are less studied as characterizing their atmospheres presents challenges due to their relatively small radius and atmospheric scale height. As the most common outcome of planet formation, these planets are crucial for understanding planetary formation, migration theories, atmospheric composition, and potential habitability. Their diverse atmospheres, influenced by equilibrium temperature, composition, and cloud presence, offer unique opportunities to study atmospheric dynamics and chemistry. While low-resolution spectroscopy struggles with atmospheric characterization due to clouds, high-resolution observations provide detailed analysis of the atmospheres by detecting molecular lines beyond the cloud deck. This study investigates four subclasses of Neptune atmospheres: HAT-P-11 b (warm Neptune), HD 63433c (warm sub-Neptune), K2-25 b (temperate Neptune), and TOI-270 d (temperate sub-Neptune), using six ground-based spectrographs: GIANO-B, CARMENES, IGRINS, HISPEC, MODHIS, and ANDES over one and three transits. Our simulation integrates the chemical kinetics model VULCAN with the 1D line-by-line radiative transfer model petitRADTRANS, and estimates detection significance using the ground-based noise simulator SPECTR. We aim to predict how future extremely large telescopes (ELTs) such as TMT (MODHIS) and E-ELT (ANDES) can utilize their higher resolving powers and larger collecting areas to surpass current observatories in detecting molecular bands. We highlight the importance of photochemistry in these atmospheres and demonstrate how ELTs will help further in constraining nitrogen and sulfur chemistry. Finally, we present a comprehensive picture of cloud presence in the atmospheres and its impact on molecular detectability in Neptune-class atmospheres.

We report the confirmation of TOI-6324 b, an Earth-sized (1.059 ± 0.041 R _⊕) ultra-short-period (USP) planet orbiting a nearby (∼20 pc) M dwarf. Using the newly commissioned Keck Planet Finder spectrograph, we have measured the mass of TOI-6324 b 1.17 ± 0.22 M _⊕. Because of its extremely short orbit of just ∼6.7 hr, TOI-6324 b is intensely irradiated by its M dwarf host and is expected to be stripped of any thick H/He envelope. We were able to constrain its interior composition and found an iron-core mass fraction (CMF = 27% ± 37%) consistent with that of Earth (∼33%) and other confirmed USPs. TOI-6324 b is the closest to an Earth-sized USP confirmed to date. TOI-6324 b is a promising target for JWST phase-curve and secondary eclipse observations (emission spectroscopy metric = 25), which may reveal its surface mineralogy, day–night temperature contrast, and possible tidal deformation. From seven sectors of TESS data, we report a tentative detection of the optical phase-curve variation with an amplitude of 42 ± 28 ppm.

We present a catalog of uniformly processed 3.6 μm and 4.5 μm band exoplanet thermal phase curves based on Infrared Array Camera observations obtained from the Spitzer Heritage Archive. The catalog includes phase curve measurements for 34 planets, 16 of which contain full orbit coverage and have detectable secondary eclipses in both channels. The data are processed in the EXCALIBUR pipeline using a uniform analysis consisting of aperture photometry and modeling of instrument effects along with the exoplanet signal. Nearest-neighbor regression with a Gaussian kernel is used to correct for instrumental systematics correlated to the star’s centroid position and shape in conjunction with a novel test to avoid overfitting. These methods may have utility in addressing subpixel gain variations present in modern infrared detectors. We analyze the 3.6 μm and 4.5 μm phase curve properties and find a strong wavelength-dependent difference in how the properties correlate with physical parameters as well as evidence that the phase curve properties are determined by multiple physical parameters. We suggest that differences between the 3.6 μm and 4.5 μm phase curve properties are due to 3.6 μm observations probing regions of the atmosphere which could include a cloud layer. Taken together, the observed phase curve behavior suggests that different physical processes are responsible for establishing the thermal phase curve at different pressures, which are probed by different wavelengths, and that further 3D Global Circulation Model modeling is required to investigate the reason for this complex dependence on planetary properties.

Neptune-size exoplanets are less studied as characterizing their atmospheres presents challenges due to their relatively small radius and atmospheric scale height. As the most common outcome of planet formation, these planets are crucial for understanding planetary formation, migration theories, atmospheric composition, and potential habitability. Their diverse atmospheres, influenced by equilibrium temperature, composition, and cloud presence, offer unique opportunities to study atmospheric dynamics and chemistry. While low-resolution spectroscopy struggles with atmospheric characterization due to clouds, high-resolution observations provide detailed analysis of the atmospheres by detecting molecular lines beyond the cloud deck. This study investigates four subclasses of Neptune atmospheres: HAT-P-11 b (warm Neptune), HD 63433 c (warm sub-Neptune), K2-25 b (temperate Neptune), and TOI-270 d (temperate sub-Neptune), using six ground-based spectrographs: GIANO-B, CARMENES, IGRINS, HISPEC, MODHIS, and ANDES over one and three transits. Our simulation integrates the chemical kinetics model, VULCAN with the 1-D line-by-line radiative transfer model, petitRADTRANS, and estimates detection significance using the ground-based noise simulator, SPECTR. We aim to predict how future Extremely Large Telescopes (ELTs) such as TMT (MODHIS) and E-ELT (ANDES) can utilize their higher resolving powers and larger collecting areas to surpass current observatories in detecting molecular bands. We highlight the importance of photochemistry in these atmospheres and demonstrate how ELTs will help further in constraining nitrogen and sulfur chemistry. Finally, we present a comprehensive picture of cloud presence in the atmospheres and its impact on molecular detectability in Neptune-class atmospheres.

The Palomar Radial Velocity Instrument (PARVI) is a J & H band, high resolution (R∼80,000) spectrograph on the Hale 5.08-m telescope at Palomar Observatory. PARVI is a stabilized, single-mode fiber-fed spectrometer designed to search for small rocky planets in the habitable zones of late-type stars. PARVI is one of the first radial velocity (RV) instruments to employ single-mode fibers (SMF) instead of the more common multi-mode fibers (MMF). SMFs provide a number of advantages over MMFs. In particular, they only allow one spatial mode of light to propagate, resulting in a time-invariant, near-Gaussian output beam that eliminates speckle noise present in MMFs. A challenge with SMFs is that the single spatial mode is composed of two polarization states (PS) and the output PS is completely decoupled from the input PS. During the commissioning of PARVI, we discovered a source of polarization noise that manifested as a time-dependent, wavelength-dependent, PSF displacement responsible for RV errors on the order of 10 ms−1 that we call the accordion effect. We set up a warm testbed to test the polarization response of the PARVI optics and determined the cross-dispersing prism and variable PS of injected light were responsible for the polarization noise. We used raytracing software to simulate birefringence in the prism and found it to be consistent with the observed effect. We designed a number of tests with the PARVI spectrometer operating cold and under vacuum to evaluate how modifications to the prism impacted the effect. After mitigating stress-induced birefringence in the prism and installing fiber polarization scramblers before light is injected into the spectrograph, we are able to demonstrate an instrument RV precision of <1 ms−1.

Asteroseismology of dwarf stars cooler than the Sun is very challenging owing to the low amplitudes and rapid timescales of oscillations. Here we present the asteroseismic detection of solar-like oscillations at 4-minute timescales ( ν max ∼ 4300 μHz) in the nearby K dwarf σ Draconis using extreme-precision Doppler velocity observations from the Keck Planet Finder and 20 s cadence photometry from NASA’s Transiting Exoplanet Survey Satellite. The star is the coolest dwarf star to date with both velocity and luminosity observations of solar-like oscillations, having amplitudes of 5.9 ± 0.8 cm s⁻¹ and 0.8 ± 0.2 ppm, respectively. These measured values are in excellent agreement with established luminosity−velocity amplitude relations for oscillations and provide further evidence that mode amplitudes for stars with T _eff < 5500 K diminish in scale following an (L/M)^1.5 relation. By modeling the star’s oscillation frequencies from photometric data, we measure an asteroseismic age of 4.5 ± 0.9 (ran) ± 1.2 (sys) Gyr. The observations demonstrate the capability of next-generation spectrographs and precise space-based photometry to extend observational asteroseismology to nearby cool dwarfs, which are benchmarks for stellar astrophysics and prime targets for directly imaging planets using future space-based telescopes.