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2024 kavli prize in Astrophysics

2024 Kavli
Prize in

Exoplanets, MEarth-South at night. Photo: Jonathan Irwin
This shows an artist's impression of the exoplanet that David Charbonneau and colleagues first observed with the transit method. Credit: NASA/European Space Agency/Alfred Vidal-Madjar (Institut d'Astrophysique de Paris, CNRS)
This shows the star shade concept that Sara Seager has worked on, which would be used to search for Earth-like planets around Sun-like stars. Credit: NASA/JPL-Caltech

The Norwegian Academy of Science and Letters has decided to award the 2024 Kavli Prize in Astrophysics to

"for their ground-breaking work on the discovery and characterization of extra-solar planets and their atmospheres."

Committee Members

  • Viggo Hansteen (Chair), University of Oslo, Norway
  • Francoise Combes, Observatoire de Paris, France
  • Max Pettini, University of Cambridge, UK
  • Martha Haynes, Cornell University, USA
  • Thomas Henning, Max Planck Institute for Astronomy, Germany

Citation from the Committee

Exoplanet science has moved from the detection of the first giant planet orbiting a solar-type star outside of our planetary system to the revelation of the diversity of exoplanets, the demographics of their global properties and the physical characterization of their atmospheres. The last decades have seen amazing developments of the field. A new category of planets – the Super-Earths – were discovered and it was realized that low-mass planets are remarkably frequent. In fact, the Milky Way galaxy may contain more than 100 billion planets, with low-mass planets by far the most common. Measurements of mass and radius have allowed the determination of the mean density of planets, and thereby the discrimination between gas giants and rocky planets. Furthermore, locating rocky planets in the habitable zone, where liquid water may exist, is possible.

Today, the characterization of exoplanet atmospheres, particularly via transit spectroscopy, is an emerging field. Seager and Charbonneau have pioneered methods for the detection of atomic species in planetary atmospheres and the measurement of their thermal infrared emission, thus setting the stage for finding the molecular fingerprints of atmospheres around both giant and rocky planets. The next big step lies in detecting molecular biosignatures – an endeavor involving significant technical challenges. Seager and Charbonneau have led the field also in this respect, developing innovative instrumentation concepts, the basis of progress in current and future exoplanet science.

David Charbonneau is the Fred Kavli Professor of Astrophysics at Harvard University. He led the discovery team of the first transit of the giant exoplanet HD 209458b which allowed the precise determination of the planetary surface gravity and its mean density, proving that this planet is a gas giant. Charbonneau pioneered the application of space-based observatories to perform the first studies of the atmospheres of giant extrasolar planets. In 2002 he used the Hubble Space Telescope (HST) to directly determine the chemical composition of the atmosphere around a giant planet and found evidence for sodium in its atmosphere. In 2005, he used the Spitzer Space Telescope (SST) to make the first direct detection of the thermal infrared emission from an exoplanet. He was a member of the Kepler mission and determined the occurrence rates of exoplanets. Presently he is actively involved in the Transiting Exoplanet Survey Satellite (TESS) mission and James Webb Space Telescope (JWST) observations. Charbonneau currently leads the innovative MEarth Project aiming to detect rocky planets such as GJ 1214b.

Sara Seager is the Class of 1941 Professor of Planetary Science, a Professor of Aeronautics and Astronautics and of Physics at the Massachusetts Institute for Technology. She pioneered the theoretical study of planetary atmospheres and predicted the presence of atomic and molecular species detectable by transit spectroscopy, most notably the alkali gases. Seager made outstanding contributions to our understanding of planets with masses below that of Neptune, finding that higher-mass variants are dominated by hydrogen and helium while smaller-mass ones share the properties of rocky bodies. Seager provided new concepts for our understanding of the habitable zone, where liquid water can exist, and thus established the importance of a variety of biomarkers. She was among the first to recognize the importance of a starshade as a free-flying external occulter to block the light of the parental star and to allow the detection and characterization of a “second” Earth around a solar analog.

Exoplanets, MEarth-South at night. Photo: Jonathan Irwin

Looking for signs of life on other worlds

Having discovered over 5,000 extra-solar planets to date, scientists are now busy measuring the very varied properties of these objects. The two winners of this year's Kavli prize in Astrophysics, David Charbonneau and Sara Seager, have pioneered the measurement of exoplanet atmospheres and are heading the charge to find signatures of life there.

By Edwin Cartlidge, science writer

This shows an artist's impression of the exoplanet that David Charbonneau and colleagues first observed with the transit method. Credit: NASA/European Space Agency/Alfred Vidal-Madjar (Institut d'Astrophysique de Paris, CNRS)

This shows an artist's impression of the exoplanet that David Charbonneau and colleagues first observed with the transit method. Credit: NASA/European Space Agency/Alfred Vidal-Madjar (Institut d'Astrophysique de Paris, CNRS)

David Charbonneau led the team that first observed an extra-solar planet, known as HD 209458b, using what is today called the transit method. This involves measuring the dip in light emitted by the exoplanet's parent star as the planet passes between the star and Earth. By using this measurement to work out the planet's radius and then combining that value with its mass – previously obtained using an existing technique that measures a wobble in the star's motion caused by the planet – the researchers could establish the planet's density. Their conclusion: HD 209458b is a "gas giant", like Jupiter and Saturn.

With that measurement in the bag, Charbonneau then used the transit technique to study smaller exoplanets, such as "super-Earths" – which are up to twice as big as our planet – or indeed Earth-like planets. Such objects are thought to make up the bulk of the perhaps 100 billion or more exoplanets in our Milky Way galaxy, but generally produce tiny dips in star light. Charbonneau has got round this problem by using a fleet of robotic telescopes to probe very small, cool "M dwarf" stars – against which even Earth-like planets can leave their mark.

Beyond measuring exoplanets' bulk properties, Charbonneau has also exploited the transit method to carry out spectroscopy of exoplanetary atmospheres. One such approach involves measuring the starlight that filters through a planet's atmosphere. The planet's body and atmosphere together block starlight but the blocking power of the atmosphere depends on wavelength (given the specific molecular absorption spectra). The upshot is that the size of the planet (body plus atmosphere) appears to vary according to the wavelength of light being measured, so revealing the atmosphere's content.

This shows NASA's Spitzer Space Telescope, which Charbonneau used to obtain an exoplanet's absorption spectrum from its own infrared emission. Credit: NASA

This shows an artist's impression of one of the planets discovered as part of David Charbonneau's MEarth project, which targets very small, cool "M dwarf" stars.

Charbonneau exploited this detection technique in 2002 to work out the chemical composition of a giant exoplanet's atmosphere using data from NASA's Hubble Space Telescope. Then three years later he showed how to obtain atmospheric absorption spectra by instead isolating an exoplanet's own infrared emission. He did this by using the Spitzer Space Telescope to record data at two points in the planet's orbit around its star – once when in front of the star and the other when eclipsed by it. Subtracting the latter data from the former, he was left with the planetary emission.

Despite all the progress, however, much work remains to be done before scientists can realistically start to detect arguably the most interesting of atmospheric constituents – those indicative of life. These "biomarkers" – such as oxygen, ozone and carbon dioxide – must be continually replenished by living organisms if they are not to be depleted by non-living chemical processes, meaning they would not be present on planets devoid of life. But these molecules generate quite small spectral signals, and are also not reckoned to be present in the atmospheres of accessible exoplanets such as gas giants or smaller bodies orbiting M dwarf stars (these stars bombarding their satellites with huge amounts of radiation that is harmful to life).

Sara Seager has advanced the search for bio-signatures by investigating the theoretical applicability of transit spectroscopy to exoplanet atmospheres generally, predicting that a range of atoms and molecules, particularly alkali metals (in gas form), should be present. More specifically she has screened many molecular biomarkers, identifying the best such molecules for flagging up biological activity on a planet.

This shows the star shade concept that Sara Seager has worked on, which would be used to search for Earth-like planets around Sun-like stars. Credit: NASA/JPL-Caltech

Seager has also done extensive work analysing what is known as the habitable zone, the range of radii from a given star in which planets would be neither too hot nor too cold to contain liquid water on their surfaces. She calculated how big this zone could be, taking into account atmospheric influences on a planet's temperature – such as the greenhouse effect.

Sara Seager (in the middle) with a Starshade petal prototype used to demonstrate manufacturing tolerances.

Sara Seager (in the middle) with a star shade petal prototype used to demonstrate manufacturing tolerances. Photo from her autobiography.

In addition to her more theoretical research, however, Seager has also worked on a concept that might enable space observatories to detect biomarkers in practice – the star shade. The aim is to observe Earth-like planets around Sun-like stars, which, unlike M dwarfs, emit relatively little high-energy radiation and so are good candidates for life-hosting exoplanets. But the planets in question would be too far from their respective stars to be observable using the transit method. Star shades – which would be separated in space from the observatory that sits in their shadow – should instead allow such planets to be imaged directly, by screening out the vast majority of the 100 billion photons that reach us from the star for each single photon emitted by the exoplanet.

It remains to be seen whether a full-scale mission launched in pursuit of biomarkers will rely on a star shade or a more conventional radiation mask known as a coronagraph (which would be located onboard the space observatory). Such a mission will also not be taking off any time soon – NASA being unlikely to launch the 6-metre diameter multi-wavelength telescope selected for this purpose in 2021 by the National Academy of Sciences before the 2040s.

Indeed, in announcing its prize winners this year, the Norwegian Academy of Science and Letters stresses the difficulty of such research – describing this as an endeavour "involving significant technical challenges". At the same time, however, it has no doubt about the influence of Seager and Charbonneau on the field. Their innovative instrumentation concepts, it says, form the basis of "progress in current and future exoplanet science".