Sara Seager on Exoplanet Geology
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During the past couple of decades, we have discovered that stars with planetary systems are not rare, exceptional cases, as we once assumed, but actually quite commonplace. However, because exoplanets are like fireflies next to blinding searchlights, they are incredibly difficult to study. Yet, as Sara Seager explains, we are making astonishing progress. Various ingenious methods and the use of powerful space telescopes enable us to learn about exoplanet atmospheres and even, in some cases, what their surfaces consist of.
Sara Seager’s research concentrates on the detection and analysis of exoplanet atmospheres, and she has just won the prestigious Kavli Prize for this work. She has had leadership roles in space missions designed to discover new exoplanets and find Earth analogs orbiting a sun-like star. She is a Professor of Aeronautics and Astronautics, Professor of Planetary Science, and Professor of Physics at the Massachusetts Institute of Technology.
Photo courtesy of Justin Knight & Harper Collins
Podcast Illustrations
In May, 2022 the count of confirmed exoplanets passed the 5,000 mark. As the infographic shows, larger planets make up the bulk of these, but this probably reflects the biases of our detection methods, rather than the true population distribution. Sara Seager describes new detection methods, such as gravitational lensing, that are not subject to the same biases.
NASA/JPL-Caltech
Detecting Exoplanets Using the Transit Method
When a planet moves in front of its star, it blocks out some of the star’s light. The plot at bottom illustrates the light curve that characterizes such a transit.
NASA Ames
Light-curve measurement of an ultra-hot gas-giant exoplanet, WASP-18b. It orbits its star, WASP-18, every 23 hours, is 400 light-years from Earth, and is 10 times more massive than Jupiter. The bottom curve shows the light curve for one of the 29 transits shown in the top plot.
NASA/TESS
Detecting Exoplanets Using Radial Velocity
As Seager explains in the podcast, exoplanets can be detected by measuring tiny periodic variations in a star’s radial velocity. This motion makes the light from the star appear slightly bluer when it is moving toward the observer, and slightly redder when moving away. Since no transit is needed, the radial velocity (as well as the plane-of-the-sky movement, or astrometry method; see below) can detect planets whose orbits are not fortuitously aligned with the star along our line of sight.
ESA
Radial velocity measurement of an exoplanet named 51 Pegasi b, located about 50 light years away. It is the prototype of the “hot Jupiter” class of planets.
ELODIE Spectrograph
Detecting Exoplanets Using Astrometry
The “wobble” in a star's motion caused by the gravitational pull of a planet (Gliese 876b) as the planet and star orbit around a common center of mass can reveal the presence of an exoplanet.
NASA/ESA, Feild (STScl)
Detecting Exoplanet Atmospheres
During transit, we can look for signs of a planetary atmosphere by detecting a change in the light from the star caused by a contribution from light transmitted through the planetary atmosphere. The gases in the atmosphere impose a transmission spectrum on the light; characteristic features of such a spectrum reveal the atmosphere’s composition.
NASA/ESA/CSA
At the beginning and end of an exoplanet transit, its atmosphere blocks some light from the star just before and just after the exoplanet disk respectively, causing a slope at the edges of the transit light curve. The light curve shown was captured over 6.5 hours of observation of the gas-giant planet WASP-96b 1,1150 light years from Earth, by the James Webb Space Telescope. The atmosphere’s transmission spectrum indicates the presence of sodium in the atmosphere.
NASA, ESA, CSA, STScl
The graph cycles through measurements of a transit light curve over a range of wavelengths shown at top right (in microns). This shows an increased planet size around 4.3 microns, indicating the presence of a strongly absorbing gas active at this wavelength. This turns out to be CO2, which, while not the dominant form of carbon in the WASP 39b atmosphere, is a strongly absorbing component.
Data from the JWST Transiting Exoplanet Community Early Release Science Team.
Animation by Pat Wachiraphan, William Waalkes, and Zach Berta-Thompson.
The figure shows the transmission spectrum of K2-18b’s atmosphere obtained with the Webb telescope. In the podcast, Seager says that this exoplanet provides her favorite example of the detection of an exoplanet atmosphere and of what it can tell us about an exoplanet surface. She points out that the presence of methane but surprising absence of ammonia suggests that ammonia, being much more soluble than methane, has dissolved out into a liquid covering the planet’s surface. That liquid could be a water ocean or liquid magma and there is an ongoing debate as to whether or not the hydrogen, water, and methane atmospheres of K2-18b and its ilk overly a massive rocky planet with a deep magma ocean. K2-18b, 8.6 times as massive as Earth, orbits the cool dwarf star K2-18 in the habitable zone and lies 120 light-years from Earth.
NASA, CSA, ESA, R. Crawford (STScI), J. Olmsted (STScI), Science: N. Madhusudhan (Cambridge University)
Gravitational Lensing
As Seager says in the podcast, gravitational lensing is a good way to obtain exoplanet statistics, as a great many stars can be monitored in parallel. Such studies suggest that the Milky Way contains at least 100 billion planets, which corresponds to at least one planet for every star on average. This would imply that there are about 1,500 exoplanets with just 50 light years from Earth.
NASA, ESA, and K. Sahu (STScI)
Nancy Grace Roman Space Telescope
The Roman Space Telescope is designed with a wide field of view so as to monitor a large number of stars for gravitational-lensing events. It is scheduled to launch in 2027, when it is expected to dramatically increase the number of detected exoplanets. It is also equipped with a small field-of-view camera and a spectrometer with a coronagraphic instrument to block out starlight and directly image exoplanets.
Spectra of Exoplanet Surfaces
The surface of a rocky exoplanet is expected to show an emission spectrum that has weak absorption features imposed on it. The absorption features are specific to each rock type, which gives us a means of determining what rock type(s) dominate an exoplanetary surface. The plot at right shows a spectrum for exoplanet Kepler-20f modeled by Seager and her colleagues under various assumptions for the dominant surface rock type. The modeled depth of the dip in brightness of the star+planet system (y-axis) is plotted as a function of wavelength (x-axis) when the planet transits behind the star.
Hu, R., et al. (2012), The Astrophysical Journal 752, 7
Thermal Phase Curves of Transiting Exoplanets
The diagram explains what causes us to see a thermal phase curve in a star/exoplanet system where the exoplanet is tidally locked to the star, i.e., always has the same side facing the star. The principle is similar to that of the phases of our Moon, except we see the phases in the thermal infrared rather than in visible light. At top, the figure shows the phases of the exoplanet as it orbits its star. The middle diagram illustrates the overall brightness (line position) and overall thermal brightness (color) of the star and planet as the planet traces out its orbit. When the planet is on the far side of the star, the hot side of the planet, i.e., the one facing the star, is fully visible to us, and we see the maximum amount of thermal infrared from its surface. Conversely, when it is on the near side of the star, we see only the dark, cold side, and the thermal infrared is at a minimum. If there is no atmosphere on the planet, we expect the thermal infrared phase to be exactly synchronized with the orbital phase. But, as Seager explains in the podcast, if there is an atmosphere, the heat on the illuminated side will be partially smeared out by strong winds, and we may see a lag between the orbital phase curve and the thermal phase curve.
NASA, ESA, CSA, Dani Player (STScI)
Another illustration showing how the visibility of the heated face of the exoplanet varies around its orbit, producing a variation in the thermal emission we can see, i.e., a thermal phase curve. The temperature scale is shown for the hot gas exoplanet WASP-43 b.
NASA, ESA, CSA, Ralf Crawford (STScI)
The thermal phase curve of WASP-43 b observed by the James Webb Space Telescope is the shallow variation on which the primary (front) and secondary (back) transits are superimposed.
NASA, ESA, CSA, Ralf Crawford (STScI)
Evidence for Lack of Atmosphere
This thermal phase curve of an Earth-like exoplanet is symmetric and has a large amplitude that implies a dayside temperature of over 700C and a nightside temperature consistent with -273C (zero kelvin). The data are well fitted by a bare-rock model with no atmosphere (red line).
Kreidberg, L., et al. (2019), Nature 573, 87
Evidence for the Presence of an Atmosphere
Thermal phase curves from observations of the hot-Jupiter exoplanet WASP-121 b by the Hubble Space Telescope. The differences between the phase curves in different years are thought to be caused by variability in the planet’s atmosphere.
NASA, ESA, Changeat, Q., et al. (2024) https://doi.org/10.48550/arXiv.2401.01465
Asymmetrical phase curve showing an offset in the thermal emission peak from the orbital phase (0.5 on the x axis) when the planet is on the other side of the star and showing its full hot side to us. As Seager says in the podcast, this could be explained by the presence of atmospheric winds that transport heat across the surface.
Credit https://wasp-planets.net/tag/phase-curve/
Starshot
In the podcast, Seager mentions the idea, very much only in concept phase now, of sending a fleet of space ships to our nearest neighbor star, Alpha Centauri. They would be propelled by Earth-based laser beams to 20 percent of the speed of light and take 20 years to reach the star. As they shoot past, they would capture closeup images of its planets and transmit them back to Earth.
Breakthrough Initiatives
Rocket Lab Mission to Venus
Artist’s concept of the Rocket Lab probe approaching the dense atmosphere of Venus. The probe, which is only 40 cm in diameter, will search for organic chemicals in the cloud particles and explore the habitability of the clouds. It does this by shining a laser into the clouds and looking for polarization of light scattered by particles and fluorescence from organic molecules.
French, R., et al. (2022), Aerospace 9(8), 445 https://doi.org/10.3390/aerospace9080445
During a period of only about six minutes as it descends through the atmosphere, the probe collects data from the cloud layer between 60 and 45 km above the surface.