Imagine an earthly neighbor

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We don’t yet know if the solar-type stars closest to us, the α Centauri A/B binary, harbor an Earth-like planet. However, thanks to new modeling work, we now have a good idea of ​​what such a planet would look like, if it existed, and how it might have evolved.

We live in exciting times for exoplanet research, moving from demography to detailed characterization. The James Webb Space Telescope (JWST), successfully launched in December 2021, is expected to detect the atmospheres of rocky exoplanets transiting in front of M dwarfs – stars fainter than the Sun – orbiting in the habitable zone. The Extremely Large Telescope (ELT), currently under construction in Chile, will be set up to directly image rocky exoplanets around stars near the sun by the end of the decade. Looking even further afield, ambitious future concepts for space missions are being explored, including the Large Exoplanet Interferometer (LIFE), which targets rocky exoplanets in the habitable zone and their atmospheres.

ETH Zurich is leading or heavily involved in these and other observation infrastructures. The complementary research of the Institute of Particle Physics and Astrophysics of the Department of Physics focuses on numerical modeling, which is essential for understanding rocky exoplanets in the habitable zone and guiding future observations and instrumentation developments. Today, an international team led by ETH academics presents the results of such a study, in which they directed their attention to the sun-like stars closest to Earth, α Centauri A and α Centauri B. The Astrophysical Journalthey provide a benchmark prediction of what an Earth-sized planet would look like, if it existed in this system.

A hypothetical α-Cen-Terre

The team, which includes ETH astrophysicists Haiyang Wang, Sascha Quanz and Fabian Seidler as well as Paolo Sossi from the Department of Earth Sciences, set out to estimate the elemental composition of a hypothetical rocky planet in the habitable zone of the α Centauri A/B system. Their modeling is based on the chemical compositions measured by spectroscopy of α Centauri A and α Centauri B, for which a wide range of information is available both for rock elements (such as iron, magnesium and silicon) and volatile elements (including hydrogen, carbon and oxygen).

From this data, they were able to project possible compositions of a hypothetical planetary body orbiting either star. In this way, the researchers arrived at detailed predictions regarding the properties of their model planet, which they dubbed “α-Cen-Earth”, including its internal structure, mineralogy and atmospheric composition. These characteristics, in turn, are of central importance in understanding its long-term evolution and potential habitability.

With this work, Wang and his colleagues began to paint a captivating picture of an exoplanet orbiting α Centauri A/B. If it exists, α-Cen-Earth is likely to be geochemically similar to our Earth, they predict, with a mantle dominated by silicates, but enriched in carbonaceous species such as graphite and diamond. The water storage capacity in its rocky interior should be equivalent to that of our home planet. According to the study, α-Cen-Earth would also differ in interesting ways from Earth, with a slightly larger iron core, lower geological activity and a possible lack of plate tectonics. The biggest surprise, however, was that the early atmosphere of the hypothetical planet could have been dominated by carbon dioxide, methane and water – similar to that of Earth in the Archean eon 4 years ago. 2.5 billion years ago, when the first life appeared on our planet. planet.

The chemical star-planet connection

The study is unique in that it includes predictions about volatile elements on a rocky exoplanet. While it is well established that the chemical composition of “terrestrial” planets (mainly made up of rock and metal) generally reflects that of their host star, this is only true for so-called refractory elements; that is, the main constituents of rock and metal. The match breaks down for volatile elements, those that vaporize easily. This class includes hydrogen, carbon, and nitrogen, which are essential to understanding whether a planet is potentially habitable.

During his doctoral research at the Australian National University of Canberra (supervised by Charley Lineweaver and Trevor Ireland, who are co-authors of the new paper), Wang developed the first quantitative model that relates the chemical compositions of sun-like stars and any rocky element. planets that surround them, both for volatile and refractory elements. Wang joined the Quanz group at ETH Zurich in 2019, where he has since developed the applications of this model further away. More sophisticated models of the chemical relationship between stars and planets are also developed in the group, thanks to collaborations within the framework of the national research center PlanetS.

window of opportunity

The probability of finding an older brother to our Earth – the α Centauri A/B system is 1.5 to 2 billion years older than the sun – could hardly be more favourable. From 2022 to 2035, α Centauri A and α Centauri B will be separated enough to benefit the search for planets around each of the stars through reduced light contamination from the other. With the new observational power that can be expected in the coming years, there is legitimate hope that one or more exoplanets orbiting α Centauri A/B will join the nearly 5,000 exoplanets discovered since 1995, when University of Geneva astrophysicists Michel Le Maire and Didier Queloz (who joined the faculty of ETH Zurich last year) announced the discovery of the first planet outside our solar system in orbit around a sun-like star – for which they were awarded the 2019 Nobel Prize in Physics, shared with Canadian-American cosmologist Jim Peebles.

The work of Wang et al. provides a landmark study for the field of exoplanet research, in terms of a detailed theoretical characterization of rocky (hypothetical) habitable zone exoplanets around solar-type stars in the solar neighborhood. This is important to guide future observations of these planets and therefore to maximize the scientific return of the unprecedented astronomical infrastructure, on the ground and in space, being developed. With all of these capabilities in place, we can expect a new chapter in the discovery of planets and life in the cosmos.

A new planet detected around the star closest to the sun

More information:
Haiyang S. Wang et al, An Earth-sized model planet in the habitable zone of α Centauri A/B, The Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac4e8c

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