Looking for Life Around Red Giant Stars

I suppose the most famous fictional depiction of the Sun as it swells to red giant stage is in H. G. Wells’ The Time Machine, in a passage where the time traveler takes his device by greater and greater jumps into the remote future. This is heady stuff:

I moved on a hundred years, and there was the same red sun–a little larger, a little duller–the same dying sea, the same chill air, and the same crowd of earthy crustacea creeping in and out among the green weed and the red rocks. And in the westward sky, I saw a curved pale line like a vast new moon. ‘So I travelled, stopping ever and again, in great strides of a thousand years or more, drawn on by the mystery of the earth’s fate, watching with a strange fascination the sun grow larger and duller in the westward sky, and the life of the old earth ebb away. At last, more than thirty million years hence, the huge red-hot dome of the sun had come to obscure nearly a tenth part of the darkling heavens.

Wells would have had no real idea of the chronology here, but we now know that in several billion years, the Sun will become a red giant, with effects upon our own planet’s habitability showing up long before that. Because the inner planets will be consumed when this happens, and Earth itself rendered a rocky hellscape, it’s easy to assume that life in our Solar System will come to an end. But Ramses Ramirez and Lisa Kaltenegger (Carl Sagan Institute at Cornell University) beg to disagree. Their new paper paints the possibilities of post red-giant habitable zones.

Just where is the habitable zone located in the later stages of a star’s evolution? To find out, the researchers have computed the luminosities of stars as they move off the main sequence, expanding through the Red Giant Branch and Asymptotic Giant Branch. They work with a grid of stars ranging all the way from A5 down to M1, with calculations starting at the beginning of the red giant phase. Here the stellar luminosities increase in stars of the Sun’s mass and greater, decreasing after the helium flash before again increasing along the Asymptotic Giant Branch.

For stars that are less massive, stellar winds during the Red Giant Branch phase reduce their masses sharply, enough to prevent them undergoing the Asymptotic Giant Branch phase. The fascination here — and this paper should provide scenarios for more than a few science fiction writers — is watching what happens to the habitable zone given high stellar winds, atmospheric erosion and an expanding central star. We might take our own Solar System as a starting point, since while life on Earth would be devastated, prospects further out begin to open up.

Image: Normal yellow stars, like our Sun, become red giants after several billion years. When they do, the planetary habitable zone changes, as analyzed in a new paper by Lisa Kaltenegger and Ramses Ramirez. Credit: Wendy Kenigsburg / Carl Sagan Institute.

Consider this: Over 99.9 percent of the water in the Solar System is found beyond the so-called ‘snowline,’ meaning that the outer system could offer the potential of biological evolution. As Ramirez and Kaltenegger calculate post-main sequence habitable zone distances for the stars on their grid, they show that a planet at Jupiter’s distance could remain in the newly warmed, much more distant habitable zone of a G-class star for hundreds of millions of years. We don’t know how long it takes life to evolve, but this may be long enough for the process to start, given that life on Earth is now thought to have begun about 3.8 billion years ago, and perhaps even earlier.

But here is a key point: Evolution in a post-main sequence phase may not be necessary. One possibility is that life could have started in an early habitable environment, perhaps before the star ever reached the main sequence. Moving below the surface as conditions changed, it could emerge once again after the star goes into its red giant phase. Another possibility: Life could evolve below the surface on a world outside the traditional habitable zone, only emerging once the post-main sequence phase begins. Let’s look more closely at this issue, as it has implications for the detection of life in other solar systems:

In our own Solar System, if life exists in the subsurface ocean of icy exo-moons like Europa or Enceladus, this life may be exposed during our Sun’s red giant branch phase (RGB), during which the post-MS HZ will move outward to Jupiter’s orbit, allowing atmospheric biosignatures to potentially become remotely detectable at those orbital distances. For planets or moons as small as Europa, such atmospheric signatures would be short-lived due to the low gravity. But for super-Europa analogues or other habitable former icy planets such atmospheric signatures could build up. Higher disk densities around massive stars may translate into more massive objects than in our Kuiper belt region (~ 3 times the terrestrial planets; Gladman et al., 2001). Such planets may be present around current post-main-sequence stars.

In a 2014 paper, Ramirez and Kaltenegger looked at habitable zone boundaries in stars that had not yet reached the main sequence, considering the possibilities for the detection of biomarkers, which obviously affect how we choose the stars we target for observation. Now we’re moving into the later stages of a star’s evolution, finding that habitable zone limits evolve throughout this period thanks to changing luminosity and stellar energy distribution. The notion of a habitable zone expands to include multiple periods and places in a star’s long development.

We learn that the orbital distance of the post-main sequence habitable zone changes over time for all the stellar types studied. Our Sun, for example, shows initial post-MS habitable zone limits of 1.3 and 3.3 AU respectively, but these expand outward to 46 and 123 AU by the end of the Red Giant Branch phase, covering a timespan of about 850 million years. During the Asymptotic Giant Branch phase, the habitable zone edges move from 5 and 13 AU to 39 and 110 AU, during a timespan of some 160 million years.

The coolest stellar type the authors consider is an M1 dwarf, which can sustain a planet in a post-main sequence habitable zone for about 9 billion years (assuming metallicity levels like our Sun). A planet orbiting an A5 star, the hottest the researchers consider, can only remain in the post-MS habitable zone for tens of millions of years. A planet around a post-MS Sun may have up to 500 million years. These numbers assume an unchanging orbital radius, though the authors note that as the star loses mass, orbits move outward, thus increasing time in the HZ.

Image: The distance of the habitable zone as a small red star ages. Credit: Ramses Ramirez.

It’s interesting to consider that cool K stars and the even cooler M1 stars under discussion here would not yet have had time to reach the post-MS phase, but they’re a useful part of the model as we try to expand our notions of astrobiological detectability. In our own system, icy moons that might currently have life beneath their surfaces are not massive enough to maintain a dense atmosphere once they are heated. But more massive moons or Earth-mass planets could well be found at equivalent distances in other solar systems. The paper thus calculates how long Earth-mass bodies would retain their atmospheres at the location of Mars, Jupiter, Saturn and the Kuiper Belt in our own Solar System. Let me turn to the paper on this point:

Planetary atmospheric erosion during the post-main-sequence is mainly due to high stellar winds produced by the stellar mass loss, which can erode planetary atmospheres. Super-Moons to super-Earths’ atmospheres can survive the RGB and AGB phase of their host star – except for planets on close-in orbits. Even super-moons survive at a Kuiper-belt equivalent distance for all grid stars to the end of the AGB phase.

So while it’s natural enough to look for life around stars comparable to the Sun in type and age, this work argues that we should widen our parameters, considering that we have a ‘red giant habitable zone’ that can last for long enough to allow life to emerge. In terms of life’s future in the universe, it’s remarkable that a small red star of M1 class could sustain a habitable zone for up to 9 billion years in a phase of its life when we would expect life to be destroyed. In this Cornell University news release, Ramirez refers to a planetary system’s ‘second wind,’ a fascinating metaphor indeed as we model the future evolution of living worlds.

The paper is Ramirez and Kaltenegger, “Habitable Zones of Post-Main Sequence Stars,” Astrophysical Journal Vol. 823, No. 1 (16 May 2016). Abstract / preprint.