Could life exist around ancient red giant stars?
According to a studycarried out by researchers from Cornell University, aged red giantstars could harbor exoplanets suited to the evolution ofextraterrestrial life. The team used advanced stellar evolutionmodels to estimate the boundaries of the habitable zones (HZ) of postmain sequence (MS) ancient red giant stars, taking into account awide range of stellar ages and properties.
Ordinarily, astronomersengaged in the hunt for potentially habitable exoplanets focus theirattention on MS middle-aged stars such as our Sun. This is due to thefact that Earth is the only world on which we have confirmed theexistence 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 evolutionaryprocess are much lower down the pecking order in the search for life.
The habitable zone of astar is the region in which a rocky planet can orbit and maintainliquid water on its surface. As an MS star evolves into a red giant,the star grows significantly in size. When our Sun finally makes thetransition into a red giant in a few billion years time, it willengulf the planets Mercury and Venus, and scour Earth and Mars intobarren rocky worlds.
As a star transitionsinto a red giant, its HZ shifts further out from its core, bringingnew worlds into the sweet spot for life. When our Sun becomes a redgiant, Jupiter, Saturn, Neptune and their moons are expected to orbitwithin a newly established habitable zone.
Whilst these areincredibly inhospitable worlds, the sheer diversity of exoplanetsalready discovered by telescopes such as Kepler and Hubble in thefuture post-MS HZ of middle-aged stars suggests that there issignificant scope for the presence of life around a red giant.
The team state thatplanets in the post-MS HZ could remain habitable for anywhere between200 million to 9 billionyears, and that this periodcould be increased if the star in question boasts a high metalcontent.
Furthermore, thesignature of lifeforms that had been undetectable during the MS period of a star could potentially be identified in the atmospheres ofworlds in the HZ of a red giant, as the star'srelative luminosity to the planet increases. For example, if life existed below thesurface of an icy world such as Europa or Enceladus, the dramaticalteration in the body's atmosphere due to warming, paired withincreased starlight striking its atmosphere could combine to make itremotely detectable.
Of course, bodieslacking sufficient gravity would quickly lose their atmosphere tospace. However, the researchers believe that the atmospheres ofsuper-Earths and super-moons with sufficient mass would be able towithstand the erosion inflicted by powerful stellar winds triggeredby 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.
Source: Cornell University