93 research outputs found
The California-Kepler survey. X. The radius gap as a function of stellar mass, metallicity, and age
In 2017, the California-Kepler Survey (CKS) published its first data release (DR1) of high-resolution optical spectra of 1305 planet hosts. Refined CKS planet radii revealed that small planets are bifurcated into two distinct populations, super-Earths (smaller than 1.5 Râ) and sub-Neptunes (between 2.0 and 4.0 Râ), with few planets in between (the "radius gap"). Several theoretical models of the radius gap predict variation with stellar mass, but testing these predictions is challenging with CKS DR1 due to its limited Mâ range of 0.8â1.4 Mâ. Here we present CKS DR2 with 411 additional spectra and derived properties focusing on stars of 0.5â0.8 Mâ. We found that the radius gap follows Rp â Pm with m = â0.10 ± 0.03, consistent with predictions of X-ray and ultraviolet- and core-powered mass-loss mechanisms. We found no evidence that m varies with Mâ. We observed a correlation between the average sub-Neptune size and Mâ. Over 0.5â1.4 Mâ, the average sub-Neptune grows from 2.1 to 2.6 Râ, following with α = 0.25 ± 0.03. In contrast, there is no detectable change for super-Earths. These MââRp trends suggest that protoplanetary disks can efficiently produce cores up to a threshold mass of Mc, which grows linearly with stellar mass according to Mc â 10 Mâ(Mâ/Mâ). There is no significant correlation between sub-Neptune size and stellar metallicity (over â0.5 to +0.5 dex), suggesting a weak relationship between planet envelope opacity and stellar metallicity. Finally, there is no significant variation in sub-Neptune size with stellar age (over 1â10 Gyr), which suggests that the majority of envelope contraction concludes after âŒ1 Gyr
Exoplanets and SETI
The discovery of exoplanets has both focused and expanded the search for
extraterrestrial intelligence. The consideration of Earth as an exoplanet, the
knowledge of the orbital parameters of individual exoplanets, and our new
understanding of the prevalence of exoplanets throughout the galaxy have all
altered the search strategies of communication SETI efforts, by inspiring new
"Schelling points" (i.e. optimal search strategies for beacons). Future efforts
to characterize individual planets photometrically and spectroscopically, with
imaging and via transit, will also allow for searches for a variety of
technosignatures on their surfaces, in their atmospheres, and in orbit around
them. In the near-term, searches for new planetary systems might even turn up
free-floating megastructures.Comment: 9 page invited review. v2 adds some references and v3 has other minor
additions and modification
Planet Populations as a Function of Stellar Properties
Exoplanets around different types of stars provide a window into the diverse
environments in which planets form. This chapter describes the observed
relations between exoplanet populations and stellar properties and how they
connect to planet formation in protoplanetary disks. Giant planets occur more
frequently around more metal-rich and more massive stars. These findings
support the core accretion theory of planet formation, in which the cores of
giant planets form more rapidly in more metal-rich and more massive
protoplanetary disks. Smaller planets, those with sizes roughly between Earth
and Neptune, exhibit different scaling relations with stellar properties. These
planets are found around stars with a wide range of metallicities and occur
more frequently around lower mass stars. This indicates that planet formation
takes place in a wide range of environments, yet it is not clear why planets
form more efficiently around low mass stars. Going forward, exoplanet surveys
targeting M dwarfs will characterize the exoplanet population around the lowest
mass stars. In combination with ongoing stellar characterization, this will
help us understand the formation of planets in a large range of environments.Comment: Accepted for Publication in the Handbook of Exoplanet
Planetary population synthesis
In stellar astrophysics, the technique of population synthesis has been
successfully used for several decades. For planets, it is in contrast still a
young method which only became important in recent years because of the rapid
increase of the number of known extrasolar planets, and the associated growth
of statistical observational constraints. With planetary population synthesis,
the theory of planet formation and evolution can be put to the test against
these constraints. In this review of planetary population synthesis, we first
briefly list key observational constraints. Then, the work flow in the method
and its two main components are presented, namely global end-to-end models that
predict planetary system properties directly from protoplanetary disk
properties and probability distributions for these initial conditions. An
overview of various population synthesis models in the literature is given. The
sub-models for the physical processes considered in global models are
described: the evolution of the protoplanetary disk, the planets' accretion of
solids and gas, orbital migration, and N-body interactions among concurrently
growing protoplanets. Next, typical population synthesis results are
illustrated in the form of new syntheses obtained with the latest generation of
the Bern model. Planetary formation tracks, the distribution of planets in the
mass-distance and radius-distance plane, the planetary mass function, and the
distributions of planetary radii, semimajor axes, and luminosities are shown,
linked to underlying physical processes, and compared with their observational
counterparts. We finish by highlighting the most important predictions made by
population synthesis models and discuss the lessons learned from these
predictions - both those later observationally confirmed and those rejected.Comment: 47 pages, 12 figures. Invited review accepted for publication in the
'Handbook of Exoplanets', planet formation section, section editor: Ralph
Pudritz, Springer reference works, Juan Antonio Belmonte and Hans Deeg, Ed
Mass constraints of the WASP-47 planetary system from radial velocities
We report precise radial velocity (RV) measurements of WASP-47, a G star that hosts three transiting planets in close proximity (a hot Jupiter, a super-Earth, and a Neptune-sized planet) and a non-transiting planet at 1.4 au. Through a joint analysis of previously published RVs and our own Keck-HIRES RVs, we significantly improve the planet mass and bulk density measurements. For the super-Earth WASP-47e (P = 0.79 days), we measure a mass of 9.11 ± 1.17 áč, and a bulk density of 7.63 ± 1.90 g cm-3, consistent with a rocky composition. For the hot Jupiter WASP-47b (P = 4.2 days), we measure a mass of 356 ± 12áč(1.12 ± 0.04 MJup) and constrain its eccentricity to at 3Ï confidence. For the Neptune-size planet WASP-47d (P = 9.0 days), we measure a mass of 12.75 ± 50.0 and a bulk density of g cm-3, suggesting that it has a thick H/He envelope. For the outer non-transiting planet, we measure a minimum mass of 411 ±18áč(1.29 ± 0.06 MJup), an orbital period of days, and an orbital eccentricity of . Our new measurements are consistent with but two to four times more precise than previous mass measurements
A Neptune-sized transiting planet closely orbiting a 5â10-million-year-old star
Theories of the formation and early evolution of planetary systems postulate that planets are born in circumstellar disks, and undergo radial migration during and after dissipation of the dust and gas disk from which they formed^1, 2. The precise ages of meteorites indicate that planetesimalsâthe building blocks of planetsâare produced within the first million years of a starâs life^3. Fully formed planets are frequently detected on short orbital periods around mature stars. Some theories suggest that the in situ formation of planets close to their host stars is unlikely and that the existence of such planets is therefore evidence of large-scale migration^4, 5. Other theories posit that planet assembly at small orbital separations may be common^6, 7, 8. Here we report a newly born, transiting planet orbiting its star with a period of 5.4 days. The planet is 50 per cent larger than Neptune, and its mass is less than 3.6 times that of Jupiter (at 99.7 per cent confidence), with a true mass likely to be similar to that of Neptune. The star is 5â10 million years old and has a tenuous dust disk extending outward from about twice the EarthâSun separation, in addition to the fully formed planet located at less than one-twentieth of the EarthâSun separation
Dynamical Evolution of Planetary Systems
Planetary systems can evolve dynamically even after the full growth of the
planets themselves. There is actually circumstantial evidence that most
planetary systems become unstable after the disappearance of gas from the
protoplanetary disk. These instabilities can be due to the original system
being too crowded and too closely packed or to external perturbations such as
tides, planetesimal scattering, or torques from distant stellar companions. The
Solar System was not exceptional in this sense. In its inner part, a crowded
system of planetary embryos became unstable, leading to a series of mutual
impacts that built the terrestrial planets on a timescale of ~100 My. In its
outer part, the giant planets became temporarily unstable and their orbital
configuration expanded under the effect of mutual encounters. A planet might
have been ejected in this phase. Thus, the orbital distributions of planetary
systems that we observe today, both solar and extrasolar ones, can be different
from the those emerging from the formation process and it is important to
consider possible long-term evolutionary effects to connect the two.Comment: Review to appear as a chapter in the "Handbook of Exoplanets", ed. H.
Deeg & J.A. Belmont
The Multiplanet System TOI-421*: A Warm Neptune and a Super Puffy Mini-Neptune Transiting a G9 V Star in a Visual Binary*
We report the discovery of a warm Neptune and a hot sub-Neptune transiting TOI-421 (BD-14 1137, TIC 94986319), a bright (V = 9.9) G9 dwarf star in a visual binary system observed by the Transiting Exoplanet Survey Satellite (TESS) space mission in Sectors 5 and 6. We performed ground-based follow-up observationsâcomprised of Las Cumbres Observatory Global Telescope transit photometry, NIRC2 adaptive optics imaging, and FIbre-fed EchellĂ© Spectrograph, CORALIE, High Accuracy Radial velocity Planet Searcher, High Resolution Ăchelle Spectrometer, and Planet Finder Spectrograph high-precision Doppler measurementsâand confirmed the planetary nature of the 16 day transiting candidate announced by the TESS team. We discovered an additional radial velocity signal with a period of five days induced by the presence of a second planet in the system, which we also found to transit its host star. We found that the inner mini-Neptune, TOI-421 b, has an orbital period of Pb = 5.19672 ± 0.00049 days, a mass of Mb = 7.17 ± 0.66 Mâ, and a radius of Rb = Râ, whereas the outer warm Neptune, TOI-421 c, has a period of Pc = 16.06819 ± 0.00035 days, a mass of Mc = Mâ, a radius of Rc = Râ, and a density of Ïc = g cmâ3. With its characteristics, the outer planet (Ïc = g cmâ3) is placed in the intriguing class of the super-puffy mini-Neptunes. TOI-421 b and TOI-421 c are found to be well-suited for atmospheric characterization. Our atmospheric simulations predict significant Lyα transit absorption, due to strong hydrogen escape in both planets, as well as the presence of detectable CH4 in the atmosphere of TOI-421 c if equilibrium chemistry is assumed
Populations of planets in multiple star systems
Astronomers have discovered that both planets and binaries are abundant
throughout the Galaxy. In combination, we know of over 100 planets in binary
and higher-order multi-star systems, in both circumbinary and circumstellar
configurations. In this chapter we review these findings and some of their
implications for the formation of both stars and planets. Most of the planets
found have been circumstellar, where there is seemingly a ruinous influence of
the second star if sufficiently close (<50 AU). Hosts of hot Jupiters have been
a particularly popular target for binary star studies, showing an enhanced rate
of stellar multiplicity for moderately wide binaries (>100 AU). This was
thought to be a sign of Kozai-Lidov migration, however recent studies have
shown this mechanism to be too inefficient to account for the majority of hot
Jupiters. A couple of dozen circumbinary planets have been proposed around both
main sequence and evolved binaries. Around main sequence binaries there are
preliminary indications that the frequency of gas giants is as high as those
around single stars. There is however a conspicuous absence of circumbinary
planets around the tightest main sequence binaries with periods of just a few
days, suggesting a unique, more disruptive formation history of such close
stellar pairs.Comment: Invited review chapter, accepted for publication in "Handbook of
Exoplanets", ed. H. Deeg & J. A. Belmont
Liverpool telescope 2: a new robotic facility for rapid transient follow-up
The Liverpool Telescope is one of the world's premier facilities for time domain astronomy. The time domain landscape is set to radically change in the coming decade, with surveys such as LSST providing huge numbers of transient detections on a nightly basis; transient detections across the electromagnetic spectrum from other facilities such as SVOM, SKA and CTA; and the era of `multi-messenger astronomy', wherein events are detected via non-electromagnetic means, such as gravitational wave emission. We describe here our plans for Liverpool Telescope 2: a new robotic telescope designed to capitalise on this new era of time domain astronomy. LT2 will be a 4-metre class facility co-located with the LT at the Observatorio del Roque de Los Muchachos on the Canary island of La Palma. The telescope will be designed for extremely rapid response: the aim is that the telescope will take data within 30 seconds of the receipt of a trigger from another facility. The motivation for this is twofold: firstly it will make it a world-leading facility for the study of fast fading transients and explosive phenomena discovered at early times. Secondly, it will enable large-scale programmes of low-to-intermediate resolution spectral classification of transients to be performed with great efficiency. In the target-rich environment of the LSST era, minimising acquisition overheads will be key to maximising the science gains from any follow-up programme. The telescope will have a diverse instrument suite which is simultaneously mounted for automatic changes, but it is envisaged that the primary instrument will be an intermediate resolution, optical/infrared spectrograph for scientific exploitation of transients discovered with the next generation of synoptic survey facilities. In this paper we outline the core science drivers for the telescope, and the requirements for the optical and mechanical design
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