How about a planet around Alpha Centauri?

Earth’s older brother: If there is a planet in the nearby Alpha Centauri binary star system, it could be Earth-like in many ways, as models based on spectroscopic measurement data reveal. The exoplanet would therefore have a slightly larger iron core than Earth, an Earth-like rock mantle with a slightly higher proportion of graphite/diamond and a gas shell similar to the earth’s original atmosphere.

The red dwarf Proxima Centauri and the double star Alpha Centauri are our closest neighbors in space – and thus potential targets for future interstellar space missions. However, while three planets have already been discovered around Proxima Centauri, the search for Alpha Centauri is still ongoing. Because its stars are sun-like and not very active, an exoplanet in this system would be a more promising candidate for a “second earth” or even life in space.

Proxima Centauri
Other than the red dwarf Proxima Centauri (picture) are the stars of Alpha Centauri sun-like and calmer. ©ESO/L. Calçada

In fact, there are first indications of an exoplanet around Alpha Centauri, possibly even orbiting in the habitable zone of Alpha Centauri A. However, there is still no confirmation as to whether the “suspicious” point of light really comes from a planet.

How could an Alpha Centauri planet look like?

All the more exciting is the question of how earth-like and life-friendly such a planet in the Alpha Centauri system could be. This is exactly what Haiyang Wang from ETH Zurich and his colleagues have now investigated in detail for the first time. They used spectroscopic data on the chemical composition of the two stars, as well as models of planet formation and elemental composition of planetary systems, to determine the possible features of an Earth-sized rocky planet in the habitable zone of Alpha Centauri A or B.

The result of this modeling is a detailed picture of this hypothetical “Alpha Centauri Earth”, including its internal structure, mineralogy and atmospheric composition. The latter in particular is an important advance, because the distribution of volatile elements in planetary systems is far more difficult to predict than that of rocks or metals. Even in the case of the earth, it has not yet been fully clarified where it once got its water and atmosphere from.

Basic structure very similar

So what does a potential Earth Twin around Alpha Centauri look like? According to the models, the basic structure of this planet corresponds to that of Earth: it has an iron core surrounded by a rocky mantle and an atmosphere. In the details, however, there are some differences: The metallic core of Alpha Centauri Earth would be slightly larger than Earth’s – it’s doing well 38 Mass Percent instead of Earth Percent off like that Team reports.

Like Earth, the rocky mantle of the Alpha Centauri planet consists primarily of silicates. The mineral olivine dominates in the upper mantle, as in the terrestrial one, while perovskite minerals predominate in the lower mantle. However, because the two stars in the Alpha Centauri system have a higher carbon content than our sun, the proportion of this element could also be higher in the planet’s mantle: “Native types of carbon such as graphite and diamond could be enriched there,” report Wang and his team .

Volcanism and plate tectonics probably missing

These interior features also reveal some of what the planet’s surface might look like. As the team determined, the heat exchange between the deep layers and the surface was probably not sufficient to initiate active mantle convection. As a result, there is probably no real plate tectonics on Alpha Centauri Earth.

“This in turn means that there is less volcanism and outgassing on our model planet,” explain Wang and his team . The processes that shape the distribution of land masses, the formation of mountains and grabens on Earth may therefore be absent or weak on this nearby exoplanet.

Mantle rock could be water rich

However, there are major parallels in the transition zone from the upper to the lower mantle. On Earth, below about 410 kilometers depth, the olivine transforms into the more compact minerals ringwoodite and wadsleyite . The exciting thing about it: ringwoodite and wadsleyite can bind plenty of water into their crystal structure. As a result, the transition layer in the mantle could contain more water than all the world’s oceans combined.

For the Alpha Centauri planet this means: “The very similar amount of wadsleyite and ringwoodite and the comparable proportion of olivine to pyroxene suggests that this planet should have the same water storage capacity as Earth,” the scientists report. Although this does not say anything about how much water there is on the planet or how available it is, it does provide initial indications of a possible friendliness.

Atmosphere like on the primordial earth

Also exciting is the question of what atmosphere an earth-like planet around Alpha Centauri has would have. The team determined that its gas envelope could be strikingly similar to that of early Earth. Accordingly, carbon dioxide, methane and water vapor dominate in the atmosphere of Alpha Centauri Earth – just as originally on our planet. Only with the appearance of the first forms of life did the original atmosphere of the earth change and enrich itself with nitrogen and oxygen.

“Of course we have no possibility of a similar change for our Alpha -Centauri Earth,” emphasize Wang and his colleagues. Because that would depend on whether life has also developed on this close neighbor. If this is not the case, then the atmosphere of the Alpha Centauri planet could be rather dry and rich in CO2 today.

Earth’s older brother

All in all, the astronomers draw the picture of a planet that is very similar to Earth in many basic features. Because the stars of Alpha Centauri are one and a half to two billion years older than the sun, their planet would be a kind of older brother of our earthly home. But it remains to be seen whether he exists.

Nevertheless: The chances of a successful search are particularly good in the next few years. Because from 1538 to 410 the two stars of this double system move at maximum distance from each other. This makes it easier to pick up a potential planet’s faint signal despite the stellar clutter. (Astrophysical Journal, 1538; doi: .1538/1538-4357/ac4e8c)

Source: ETH Zurich

. March 2022

– Nadia Podbregar

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