Could life exist around ancient red giant stars?

When the Sun transitions into a red giant, the habitable zone is expected to shift to a more distant orbit(Credit: Cornell University)

According to a study carried out by researchers from Cornell University, aged red giant stars could harbor exoplanets suited to the evolution of extraterrestrial life. The team used advanced stellar evolution models to estimate the boundaries of the habitable zones (HZ) of post main sequence (MS) ancient red giant stars, taking into account a wide range of stellar ages and properties.

Ordinarily, astronomers engaged in the hunt for potentially habitable exoplanets focus their attention on MS middle-aged stars such as our Sun. This is due to the fact that Earth is the only world on which we have confirmed the existence of life, and so planetary scientists naturally prioritize solar systems with characteristics similar to our own that may share the vital ingredients. Because of this, stellar bodies in the later stages of the evolutionary process are much lower down the pecking order in the search for life.

The habitable zone of a star is the region in which a rocky planet can orbit and maintain liquid water on its surface. As an MS star evolves into a red giant, the star grows significantly in size. When our Sun finally makes the transition into a red giant in a few billion years time, it will engulf the planets Mercury and Venus, and scour Earth and Mars into barren rocky worlds.

As a star transitions into a red giant, its HZ shifts further out from its core, bringing new worlds into the sweet spot for life. When our Sun becomes a red giant, Jupiter, Saturn, Neptune and their moons are expected to orbit within a newly established habitable zone.

Whilst these are incredibly inhospitable worlds, the sheer diversity of exoplanets already discovered by telescopes such as Kepler and Hubble in the future post-MS HZ of middle-aged stars suggests that there is significant scope for the presence of life around a red giant.

The team state that planets in the post-MS HZ could remain habitable for anywhere between 200 million to 9 billion years, and that this period could be increased if the star in question boasts a high metal content.

Furthermore, the signature of lifeforms that had been undetectable during the MS period of a star could potentially be identified in the atmospheres of worlds in the HZ of a red giant, as the star's relative luminosity to the planet increases. For example, if life existed below the surface of an icy world such as Europa or Enceladus, the dramatic alteration in the body's atmosphere due to warming, paired with increased starlight striking its atmosphere could combine to make it remotely detectable.

Of course, bodies lacking sufficient gravity would quickly lose their atmosphere to space. However, the researchers believe that the atmospheres of super-Earths and super-moons with sufficient mass would be able to withstand the erosion inflicted by powerful stellar winds triggered by solar mass loss for a significant period of time.

"When a star ages and brightens, the habitable zone moves outward and you're basically giving a second wind to a planetary system," comments Ramses M. Ramirez, research associate at Cornell's Carl Sagan Institute. "Currently objects in these outer regions are frozen in our own solar system, and Europa and Enceladus – moons orbiting Jupiter and Saturn – are icy for now."

A paper detailing the research has been published in The Astrophysical Journal.

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