A billion years ago, our ancestors were amoeba-like creatures fond of engulfing paramecium-like creatures.
“Exoplanet statistics tell us all stars are orbited by some kind of planetary systems. The planets in the Alpha Centauri system are about a billion years older than the Sun and the Earth. Thus, if life has emerged on an Earth-like planet in the Alpha Centauri system, that life has had about a billion years longer to evolve than life on Earth,” wrote Australia National University astrobiologist and cosmologist, Charley Lineweaver, co-author of a new study about the sun-like stars closest to us, the α Centauri A/B binary, in an email to The Daily Galaxy. “To put that in perspective, about a billion years ago, our ancestors were amoeba-like creatures fond of engulfing paramecium-like creatures. However, we have very few reliable ideas about how life evolves in general.”
“In fact I don’t even think we know what life is,” Lineweaver notes in his reply. “Thus, our modeling of the Earth-like planets in the Alpha Centauri system can tell much about the elemental composition of those planets and how that composition is different from Earth’s. But, making predictions about life there and its evolution will require much more data”
The Search for Planets is the Search for Life
It has been said that the search for planets is the search for life. In the search for habitable worlds beyond our solar system, the best opportunities may be found around the smallest, coolest M (or red-dwarf) hydrogen-burning stars. M dwarfs have a mere fraction of the sun’s mass and luminosity, but are more than 10 times as numerous in our Milky Way Galaxy, which could number tens of billions of worlds. Life, or at least complex / multicellular life, may not start right after the formation of the planet. So an older planet does not necessarily suggest that life would also be older.
“It is difficult to make any concrete predictions on the state of any potential life on another planet,” co-author Paolo Sossi at ETH Zurich told The Daily Galaxy: “However, what emerges from our study is that the composition of a rocky planet around α Centauri A/B, both on its surface and in its interior, would not be grossly different from that of the Earth. Therefore, the primary ingredients that rendered the emergence of life possible on the Earth may be rather more common on exoplanets than first thought.”
Based on early Earth history, worlds with low oxygen may be teeming with life. Proxima-b, only 4.24 light years away, receives 250 times more X-ray radiation than Earth and could experience deadly levels of ultraviolet radiation on its surface. How could life survive such a bombardment? Astronomers say that life already has survived this kind of fierce radiation, and they have proof: Homo sapiens. We do not yet know whether the sun-like stars closest to us, the α Centauri A/B binary, harbor an Earth-like planet, but thanks to new modeling work at EHT Zurich, we now have a good sense of what such a planet, should it exist, would look like and how it might have evolved.
The James Webb Space Telescope (JWST), successfully launched in December 2021, is projected to detect the atmospheres of rocky exoplanets transiting in front of M dwarfs orbiting within the habitable zone. The Extremely Large Telescope (ELT), currently under construction in Chile, will be set up to directly image rocky exoplanets around nearby sun-like stars by the end of the decade. Looking even further ahead, ambitious future space mission concepts are currently being explored, including the Large Interferometer for Exoplanets (LIFE), which targets habitable-zone rocky exoplanets and their atmospheres.
Now an international team led by ETH researchers present the results of such a study, based on the sun-like stars nearest to Earth, α Centauri A and α Centauri B. Reporting in The Astrophysical Journal, they provide a benchmark prediction of what an Earth-sized planet, should it exist in this system, would look like.
Hypothetical Rocky Planet in the α Centauri A/B system
The team, which includes ETH astrophysicists Haiyang Wang, Sascha Quanz and Fabian Seidler as well as Paolo Sossi at 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 spectroscopically measured chemical compositions of α Centauri A and α Centauri B, for which a breadth of information is available for both rock-forming elements (such as iron, magnesium and silicon) and volatile elements (including hydrogen, carbon and oxygen).
From these data they were able to project possible compositions of a hypothetical planetary body orbiting either of the stars. 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 features, in turn, are of central importance to understanding its long-term evolution and potential habitability
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 carbon-bearing species such as graphite and diamond. The capacity for water storage 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. Researchers at the University of Oxford recently uncovered the importance of iron for the development of complex life on Earth – which also may hint at the likelihood of complex life on other planets.
Similar to the Archean eon, 4 to 2.5 Billion Years Ago
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 to 2.5 billion years ago, when first life emerged on our planet.
The study stands out in that it includes predictions about volatile elements on a rocky exoplanet. While it is well established that the chemical composition of “terrestrial” planets (which are made up predominantly of rock and metal) generally reflects that of their host stars, this is true only for so-called refractory elements; that is, the main constituents of rock and metal. The correspondence breaks down for volatile elements—those that readily vaporize. This class includes hydrogen, carbon and nitrogen, which are key to understanding whether a planet is potentially habitable.
Model Connects the Chemical compositions of Sun-like Stars and Their Rocky Planets
During his doctoral research at the Australian National University in Canberra (supervised by Charley Lineweaver and Trevor Ireland, who are co-authors of the new paper), Wang developed the first quantitative model that connects the chemical compositions of sun-like stars and any rocky planets that surround them, for both 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. More sophisticated models of the chemical relationship between star and planet are being developed in the group as well, through collaborations within the framework of the National Center of Competence in Research PlanetS.
Window of Opportunity from 2022 to 2036
The probability of actually finding an older sibling of our Earth—the α Centauri A/B system is estimated to have an age of ~6-7 billion years—could hardly be more favorable. From 2022 to 2035, α Centauri A and α Centauri B will be sufficiently separated to benefit the search for planets around each of the stars thanks to reduced light contamination from the other. Together with the new observational power that can be expected in the years to come, there is legitimate hope that one or several exoplanets orbiting α Centauri A/B will join the nearly 5,000 exoplanets that have been discovered since 1995. University of Geneva astrophysicists Michel Mayor and Didier Queloz (who joined the faculty at ETH Zurich last year) announced the discovery of the first planet outside our Solar System orbiting a sun-like star in 1995 — for which they were awarded the 2019 Nobel Prize in Physics, shared with the Princeton University cosmologist Jim Peebles.
The work of Wang et al. provides a benchmark study for the field of exoplanet research, in terms of a detailed theoretical characterization of (hypothetical) habitable-zone rocky exoplanets around sun-like stars in the solar neighborhood. This is important in guiding future observations of such planets and in therefore maximizing the scientific return from the unprecedented, ground- and space-based astronomical infrastructures being developed. With all of this capability in place, we can look forward to a new chapter in discovery of planets and life in the cosmos.
The Last Word
“In 2001,” Charley Lineweaver told The Daily Galaxy, “I published a paper that estimated the age distribution of Earth-like planets in the universe. Here are the last two sentences from the abstract: The analysis done here indicates that three-quarters of the Earth-like planets in the Universe are older than the Earth and that their average age is 1.8 ± 0.9 billion years older than the Earth. If life forms readily on Earth-like planets—as suggested by the rapid appearance of life on Earth—this analysis gives us an age distribution for life on such planets and a rare clue about how we compare to other life which may inhabit the Universe. I think that the phrase ‘stage of evolution’ has very little meaning. Most people think it does”
“Our modeling doesn’t allude to ‘life’ on the hypothetical planet (alpha-Cen-Earth), while we indeed discuss in detail about the possible chemistry, interior structure, and geodynamics of the planet based on our data and model,” lead author Haiyang Wang wrote in an email to The Daily Galaxy. “With these aspects as discussed, you may reasonably draw some implications for the habitability of the planet. For example, from the geological perspective, we do see some unfavorable conditions for the planet to be habitable even if it is located in the habitable zone. More specifically, you may draw the fact that the system is 1.5-2.5 billion years older than our Solar System to the further reduced radiogenic heating from long-lived radionuclides in the model planet due to the decay of these radionuclides. By giving a number, it was ~ a quarter less in the model alpha-Cen-Earth compared to the Earth upon their formation, but it should be ~a half less at the present day.”
“A significantly reduced radiogenic heating, plus the evolutionary cooling of the planet itself, can imply that there is no sufficient energy to drive the mantle convection – or geological activity in a plain term, which is critical to the carbon-silicate cycle between interior, surface and atmosphere, and thus to habitability,” Wang explained. “Also, although our modelled ‘early’ atmosphere resembles that of the Archean Earth, such an atmosphere may also evolve to a Venus-like atmosphere (if the planet appears to the inner edge of the habitable zone) or even a Mars-like atmosphere (if the planet appears to the outer edge of the habitable zone), since there may be no sufficient replenishment of volatiles from interiors due to the sluggish carbon-silicate cycling.”
Source: Haiyang S. Wang et al, A Model Earth-sized Planet in the Habitable Zone of α Centauri A/B, The Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac4e8c
The image at the top of the page is an ESO artist’s impression of the Proxima d, an exoplanet candidate identified earlier this year orbiting the faint red dwarf star α Centauri C. The work models a hypothetical exoplanet orbiting the much brighter Sun-like stars α Centauri A and α Centauri B. (ESO/L. Calçada)