Diversity of Exoplanets

Abstract

This review article delves into the study of low-mass exoplanets: super-Earths, mini-Neptunes and the new categories within and between that we are starting to discover. We provide an overview of current exoplanet observational capabilities, their limitations, and what they are allowing us to learn about low-mass planets. We briefly summarize the most important aspects of planet formation, with an emphasis on processes that may be testable with small exoplanets, in particular those that affect their composition. We also describe the observed compositional diversity of low-mass exoplanets and what it teaches us about planet formation pathways. We finish this review summarizing the study of the composition of small exoplanets during the very last stage of stellar evolution, by studying white dwarfs. This review is written as the JWST is making its first contributions to small planet characterization, rapidly opening new lines of inquiry.

50 pages, 9 figures

References in corpus (190)

• The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
• Detection of Planetary Transits Across a Sun-like Star
• Detection of an Extrasolar Planet Atmosphere
• Rapid planetesimal formation in turbulent circumstellar discs
• A low mass for Mars from Jupiter's early gas-driven migration
• Streaming Instabilities in Protoplanetary Disks
• The false positive rate of Kepler and the occurrence of planets
• The Disk Substructures at High Angular Resolution Project (DSHARP): I. Motivation, Sample, Calibration, and Overview
• Clouds in the atmosphere of the super-Earth exoplanet GJ 1214b
• Rapid growth of gas-giant cores by pebble accretion
• Prevalence of Earth-size planets orbiting Sun-like stars
• Kepler planets: a tale of evaporation
• Theoretical Transmission Spectra During Extrasolar Giant Planet Transits
• Atmospheric Escape from Hot Jupiters
• The effect of gas drag on the growth of protoplanets -- Analytical expressions for the accretion of small bodies in laminar disks
• A Tidally-Disrupted Asteroid Around the White Dwarf G29-38
• The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals? II. Introducing the bouncing barrier
• Habitable planets around the star Gl 581?
• Core-powered mass loss and the radius distribution of small exoplanets
• Planetesimal Formation by Gravitational Instability
• Building Terrestrial Planets
• Roche lobe effects on the atmospheric loss of "Hot Jupiters"
• A disintegrating minor planet transiting a white dwarf
• Forming the cores of giant planets from the radial pebble flux in protoplanetary discs
• Building the Terrestrial Planets: Constrained Accretion in the Inner Solar System
• A $Gaia$ Data Release 2 catalogue of white dwarfs and a comparison with SDSS
• Sculpting the Valley in the Radius Distribution of Small Exoplanets as a by-product of Planet Formation: The Core-Powered Mass-Loss Mechanism
• The Disk Substructures at High Angular Resolution Project (DSHARP): VII. The Planet-Disk Interactions Interpretation
• The Disk Substructures at High Angular Resolution Project (DSHARP) VI: Dust trapping in thin-ringed protoplanetary disks
• Towards planetesimals: dense chondrule clumps in the protoplanetary nebula
• Migration and the formation of systems of hot super-Earths and Neptunes
• Statistical Study of the Early Solar System's Instability with 4, 5 and 6 Giant Planets
• A gaseous metal disk around a white dwarf
• The Frequency of Hot Jupiters Orbiting Nearby Solar-Type Stars
• Ancient planetary systems are orbiting a large fraction of white dwarf stars
• Atmospheres of protoplanetary cores: critical mass for nucleated instability
• Planet Formation by Coagulation: A Focus on Uranus and Neptune
• Debris Disks: Structure, Composition, and Variability
• A stellar-mass-dependent drop in planet occurrence rates
• Pebble isolation mass --- scaling law and implications for the formation of super-Earths and gas giants
• Formation of the Terrestrial Planets from a Narrow Annulus
• A map of the large day-night temperature gradient of a super-Earth exoplanet
• Water Vapor and Clouds on the Habitable-Zone Sub-Neptune Exoplanet K2-18b
• Breeding Super-Earths and Birthing Super-Puffs in Transitional Disks
• Water vapour in the atmosphere of the habitable-zone eight Earth-mass planet K2-18 b
• A Young White Dwarf Companion to Pulsar 1620-26: Evidence for Early Planet Formation
• A Super-Earth Transiting a Naked-Eye Star
• Water Delivery and Giant Impacts in the 'Grand Tack' Scenario
• Finding extraterrestrial life using ground-based high-resolution spectroscopy
• The Link Between Planetary Systems, Dusty White Dwarfs, and Metal Polluted White Dwarfs
• Circumstellar Debris and Pollution at White Dwarf Stars
• Exploring Exoplanet Populations with NASA's Kepler Mission
• Absence of a thick atmosphere on the terrestrial exoplanet LHS 3844b
• Thermal emission from the Earth-sized exoplanet TRAPPIST-1 b using JWST
• Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1
• Constraints on the mass of a habitable planet with water of nebular origin
• Dynamics of pebbles in the vicinity of a growing planetary embryo: hydro-dynamical simulations
• The Complete Census of 70-um-Bright Debris Disks within the FEPS (Formation and Evolution of Planetary Systems) Spitzer Legacy Survey of Sun-like Stars
• Density and Eccentricity of Kepler Planets
• Fossilized condensation lines in the Solar System protoplanetary disk
• No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c
• Detection of a transit of the super-Earth 55 Cnc e with Warm Spitzer
• Bifurcation of planetary building blocks during Solar System formation
• Atmospheric Mass Loss During Planet Formation: The Importance of Planetesimal Impacts
• Hydrodynamics of Embedded Planets' First Atmospheres. II. A Rapid Recycling of Atmospheric Gas
• An Excess Due to Small Grains Around The Nearby K0V Star HD69830: Asteroid or Cometary Debris?
• Chemical Abundances in the Externally Polluted White Dwarf GD 40: Evidence of a Rocky Extrasolar Minor Planet
• Evidence for Water in the Rocky Debris of a Disrupted Extrasolar Minor Planet
• Discovery of a Candidate for the Coolest Known Brown Dwarf
• Externally-polluted white dwarfs with dust disks
• A Giant Planet Candidate Transiting a White Dwarf
• Growing the terrestrial planets from the gradual accumulation of sub-meter sized objects
• Numerical Modeling of the Coagulation and Porosity Evolution of Dust Aggregates
• Atmospheric Reconnaissance of TRAPPIST-1 b with JWST/NIRISS: Evidence for Strong Stellar Contamination in the Transmission Spectra
• Terrestrial Planet Formation I. The Transition from Oligarchic Growth to Chaotic Growth
• A planetesimal orbiting within the debris disc around a white dwarf star
• Evolution of the radius valley around low mass stars from $Kepler$ and $K2$
• The Carbon-Rich Gas in the Beta Pictoris Circumstellar Disk
• A reflective, metal-rich atmosphere for GJ 1214b from its JWST phase curve
• Elemental Compositions of Two Extrasolar Rocky Planetesimals
• Evidence for multiple magma ocean outgassing and atmospheric loss episodes from mantle noble gases
• Accretion of a giant planet onto a white dwarf
• HELIOS-Retrieval: An Open-source, Nested Sampling Atmospheric Retrieval Code, Application to the HR 8799 Exoplanets and Inferred Constraints for Planet Formation
• Cool DZ white dwarfs II: Compositions and evolution of old remnant planetary systems
• How formation time-scales affect the period dependence of the transition between rocky super-Earths and gaseous sub-Neptunes and implications for ${\eta }_{\oplus }$
• Reduced gas accretion on super-Earths and ice giants
• Breaking through: The effects of a velocity distribution on barriers to dust growth
• The nature and origins of sub-Neptune size planets
• Predictions for the secondary CO, C and O gas content of debris discs from the destruction of volatile-rich planetesimals
• Ice Lines, Planetesimal Composition and Solid Surface Density in the Solar Nebula
• Spectral Energy Distributions of Passive T Tauri Disks: Inclination
• Atmosphere-interior exchange on hot rocky exoplanets
• Abundant Circumstellar Silica Dust and SiO Gas Created by a Giant Hypervelocity Collision in the ~12 Myr HD172555 System
• Six White Dwarfs with Circumstellar Silicates
• High Metallicity and Non-equilibrium Chemistry in the Dayside Atmosphere of hot-Neptune GJ 436b
• Simulations of dust-trapping vortices in protoplanetary discs
• Theoretical Spectra of Terrestrial Exoplanet Surfaces
• Terrestrial planet formation from lost inner solar system material
• The interior and atmosphere of the habitable-zone exoplanet K2-18b
• The 55 Cnc system reassessed
• Earth shaped by primordial H${}_2$ atmospheres
• Debris Disks in the Scorpius-Centaurus OB Association Resolved by ALMA
• The Near-Infrared Transmission Spectra of TRAPPIST-1 Planets b, c, d, e, f, and g and Stellar Contamination in Multi-Epoch Transit Spectra
• Likely detection of water-rich asteroid debris in a metal-polluted white dwarf
• Polluted White Dwarfs: Constraints on the Origin and Geology of Exoplanetary Material
• Molecular gas in debris disks around young A-type stars
• A secondary atmosphere on the rocky exoplanet 55 Cancri e
• I Spy Transits and Pulsations: Empirical Variability in White Dwarfs Using Gaia and the Zwicky Transient Facility
• The unbiased frequency of planetary signatures around single and binary white dwarfs using $Spitzer$ and $Hubble$
• Retention of water in terrestrial magma oceans and carbon-rich early atmospheres
• Large Impacts around a Solar Analog Star in the Era of Terrestrial Planet Formation
• ALMA observations of the $\eta$ Corvi debris disc: inward scattering of CO-rich exocomets by a chain of 3-30 M${}_{\oplus }$ planets?
• Catching drifting pebbles I. Enhanced pebble accretion efficiencies for eccentric planets
• A Metallicity Recipe for Rocky Planets
• Spitzer Evidence for a Late Heavy Bombardment and the Formation of Urelites in η Corvi at ~1 Gyr
• A new class of Super-Earths formed from high-temperature condensates: HD219134 b, 55 Cnc e, WASP-47 e
• Water on Hot Rocky Exoplanets
• Chemical Fingerprints of Formation in Rocky Super-Earths' Data
• Homogeneous Analysis of Hot Earths: Masses, Sizes, and Compositions
• Masses and compositions of three small planets orbiting the nearby M dwarf L231-32 (TOI-270) and the M dwarf radius valley
• Constraining the giant planets' initial configuration from their evolution: implications for the timing of the planetary instability
• Formation and structure of the three Neptune-mass planets system around HD69830
• Compositions of Planetary Debris around Dusty White Dwarfs
• Search for water in a super-Earth atmosphere: High-resolution optical spectroscopy of 55 Cancri e
• A Case for an Atmosphere on Super-Earth 55 Cancri e
• Search for an exosphere in sodium and calcium in the transmission spectrum of exoplanet 55 Cancri e
• Distinguishing oceans of water from magma on mini-Neptune K2-18b
• Trace hydrogen in helium atmosphere white dwarfs as a possible signature of water accretion
• Inference on the Nature and the Mass of Earth's Late Veneer from Noble Metals and Gases
• On the Role of Dissolved Gases in the Atmosphere Retention of Low-Mass Low-Density Planets
• GJ 1252b: A Hot Terrestrial Super-Earth With No Atmosphere
• Photophoretic separation of metals and silicates: the formation of Mercury like planets and metal depletion in chondrites
• Are exoplanetesimals differentiated?
• GJ 367b is a dark, hot, airless sub-Earth
• Planet populations inferred from debris discs: insights from 178 debris systems in the ISPY, LEECH and LIStEN planet-hunting surveys
• The Role of Early Giant Planet Instability in the Terrestrial Planet Formation
• Constraining the Thickness of the Atmosphere of TRAPPIST-1 b from its JWST Secondary Eclipse Observation
• Seeing above the Clouds with High Resolution Spectroscopy
• Solubility of water in peridotite liquids and the prevalence of steam atmospheres on rocky planets
• Chemical Diversity of Super-Earths As a Consequence of Formation
• Prospects for Characterizing the Haziest Sub-Neptune Exoplanets with High Resolution Spectroscopy
• Exoplanet phase curves: observations and theory
• Alkali metals in white dwarf atmospheres as tracers of ancient planetary crusts
• Mass, radius, and composition of the transiting planet 55 Cnc e : using interferometry and correlations
• Nucleation and growth of iron pebbles explains the formation of iron-rich planets akin to Mercury
• HST/WFC3 transmission spectroscopy of the cold rocky planet TRAPPIST-1h
• Polluted White Dwarfs Reveal Exotic Mantle Rock Types on Exoplanets in our Solar Neighborhood
• Confirming Variability in the Secondary Eclipse Depth of the Super-Earth 55 Cancri e
• Volatile-Rich Circumstellar Gas in the Unusual 49 Ceti Debris Disk
• Extreme Debris Disk Variability -- Exploring the Diverse Outcomes of Large Asteroid Impacts During the Era of Terrestrial Planet Formation
• Exocomets: A spectroscopic survey
• Arid or Cloudy: Characterizing the Atmosphere of the super-Earth 55 Cancri e using High-Resolution Spectroscopy
• Rocklines as Cradles for Refractory Solids in the Protosolar Nebula
• Post main sequence evolution of icy minor planets: Implications for water retention and white dwarf pollution
• TKS X: Confirmation of TOI-1444b and a Comparative Analysis of the Ultra-short-period Planets with Hot Neptunes
• A cool runaway greenhouse without surface magma ocean
• The Magellan-TESS Survey I: Survey Description and Mid-Survey Results
• TOI-561 b: A Low Density Ultra-Short Period "Rocky" Planet around a Metal-Poor Star
• The co-existence of hot and cold gas in debris discs
• A Near-Infrared Chemical Inventory of the Atmosphere of 55 Cancri e
• Debris Disks Around Nearby Stars with Circumstellar Gas
• The PEPSI Exoplanet Transit Survey (PETS) I: Investigating the presence of a silicate atmosphere on the super-Earth 55 Cnc e
• Icy Exomoons Evidenced by Spallogenic Nuclides in Polluted White Dwarfs
• Cleaning our Hazy Lens: Exploring Trends in Transmission Spectra of Warm Exoplanets
• Revisiting the Iconic Spitzer Phase Curve of 55 Cancri e: Hotter Dayside, Cooler Nightside and Smaller Phase Offset
• Lack of other molecules in CO-rich debris discs: is it primordial or secondary gas?
• Anatomy of rocky planets formed by rapid pebble accretion I. How icy pebbles determine the core fraction and FeO contents
• Anatomy of rocky planets formed by rapid pebble accretion II. Differentiation by accretion energy and thermal blanketing
• Terrestrial planet formation from a ring
• Where are the Extrasolar Mercuries?
• Helium Enhanced Planets Along the Upper Edge of the Radius Valley
• Primordial or Secondary? Testing models of debris disk gas with ALMA
• Effects of photophoresis on the dust distribution in a 3D protoplanetary disc
• Asynchronous accretion can mimic diverse white dwarf pollutants I: core and mantle fragments
• The effect of a small amount of hydrogen in the atmosphere of ultrahot magma-ocean planets: atmospheric composition and escape
• A Chondritic Solar Neighborhood
• Observation uncertainty effects on the precision of interior planetary parameters
• Dynamical avenues for Mercury's origin I: The lone survivor of a primordial generation of short-period proto-planets
• Searching for gas giant planets on Solar System scales - A NACO/APP L'-band survey of A- and F-type Main Sequence stars
• Detailed chemical compositions of planet-hosting stars: II. Exploration of the interiors of terrestrial-type exoplanets
• A Self-Consistent Model for Dust-Gas Coupling in Protoplanetary Disks
• Dynamical avenues for Mercury's origin II: in-situ formation in the inner terrestrial disk
• Rapid Formation of Exoplanetesimals Revealed by White Dwarfs
• A Star-sized Impact-Produced Dust Clump in the Terrestrial Zone of the HD 166191 System
• Habitability from Tidally-Induced Tectonics
• Prevalence of short-lived radioactive isotopes across exoplanetary systems inferred from polluted white dwarfs
• Anatomy of rocky planets formed by rapid pebble accretion III. Partitioning of volatiles between planetary core, mantle, and atmosphere
• Effects of pebble accretion on the growth and composition of planetesimals in the inner Solar System
• A Distinct Population of Small Planets: Sub-Earths
• Rocky histories: The effect of high excitations on the formation of rocky planets

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