≡ Menu

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.

Habitable Zone as Giant

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.


Comments on this entry are closed.

  • Alex Tolley May 19, 2016, 11:52

    Definitely reminds me of Stapleton’s Last and First Men with different human species living on Neptune.

    In other solar system, Neptunes will be in the HZ at various times in the star’s life, and I have to wonder about the possibility of life either evolving on such worlds, or being seeded there, accidentally or deliberately. They would make fantastic ocean worlds, offering a safe radiation environment, even from flares at depth.

    Once can imagine oceanic Earth life adapting to such an environment once the H2/He atmosphere is removed and replaced with an O2/He one. Deliberate building of structure that float in the ocean would provide anchors for reef building to maintain an rich biodiversity near the surface and food supply for the deep dwelling organisms at depth. It might offer an abode for terrestrial and aquatic “human” descendants too.

    If we assume that some civilizations are very ancient indeed, if they still retain biological form, then Neptune worlds in their star’s HZ shouldn’t be ruled out as abodes.

  • Phil May 19, 2016, 12:30

    As we have seen with hot Jupiter’s these planets have a tendency to get puffy when heated. I wonder what the effect of this extra heat on Jupiter would have and how that would affect the moon’s.

  • hiro May 19, 2016, 15:01

    This reminds me the ending of sf novel Titan, the science is all right but the other thing is…..
    Sadly, Europa and other moons are small mass body, they’ll loose lots of water in this case, I’m not sure about Titan.

    • Mark Zambelli May 20, 2016, 6:31

      30yrs agao there was often talk of Titan ‘blooming’ and becoming early-earthlike, but I think this has now been ruled out completely. Looks like it may be too small to hold its atmsphere anyways.

    • Randy McDonald May 21, 2016, 6:05

      Titan would be too light to retain its volatiles over very long periods, but then, it wouldn’t have to be.

  • ljk May 19, 2016, 15:08
  • Geoffrey Hillend May 19, 2016, 16:35

    @Alex Toley At the 12.5 billion year age of the Sun, there will be no oceans on the Earth: The oceans will be boiled away into space or it will resemble Venus since at this point in time the outer layers of the the red giant phase of our Sun might expand so large it might cross Earth and swallow it or at least be to close for the temperature for any kind of life to exist.
    According to astrophysics, a stars life is dependent on its size, and gravity which affect the rate of nuclear fusion burning in its core. Consequently our Sun’s luminosity increases by 10 percent every billion years or one percent in 100 million year so maybe 300 million years is the limit for life being able to adapt to those changes. https://en.wikipedia.org/wiki/Timeline_of_the_far_future

    • Alex Tolley May 19, 2016, 20:18

      Perhaps you misunderstood what I was saying. I was talking about Earth oceanic life translocated to Sol’s Neptune or to Neptunes-like worlds in other solar systems when the star has heated their mantles to become water oceans. Rereading my comment I don’t understand why you would think I was referring to Earth, which is likely to become uninhabitable for surface life in perhaps as little as half a billion years without heroic engineering.

  • Geoffrey Hillend May 19, 2016, 16:46

    To be more precise, the Earth will be like Venus not long after a billion years from now but but 8 billion years from now at the 12. 5 billion year age of the Sun, the Earth will have no atmosphere and its surface will be molten.

  • ljk May 19, 2016, 16:56

    It is kind of amusing watching professional astronomers announce ideas that science fiction covered decades ago. But I guess better late to the party than not at all, ay?

  • Ashley Baldwin May 19, 2016, 17:09

    Trouble with red Giants is the unstable fluctuations in their gravitationally loosely held outer layers . Apart from leading to a variable “habitable zone” this leads to the release a potent stellar wind as well as large amounts of “dust” and other such matter, particularly silicates . Very good for seeding future stars with “metals” and producing an interstellar medium with lots of organic building blocks as well as larger mass elements via the “s”process in asymptotic giant branch stars . Not so good for allowing quiescent habitability zones for the extended time periods required to produce life.

  • Andrew Palfreyman May 19, 2016, 18:17

    If we were to slowly nudge Earth further out over time, we could retain our home and remain in the HZ for the foreseeable future. Over half a billion years that would require about 50 PW of average power, which amounts to about a 50% increase in current insolation, and would be engineered from extra directed sunlight. Since we prefer to use this to push rather than to heat the atmosphere, we’d need massive mirrors above the stratosphere attached to the surface by struts. Perhaps not today’s tech, but there’s no physics forbidding it from being tomorrow’s tech.

    • Eniac May 19, 2016, 22:03

      Energy is not sufficient to nudge the Earth, you need reaction mass, too. You’d have to use up much of the Earth as propellant, most likely.

      It might be possible to arrange asteroids into repeatedly flying by Earth and Jupiter in such a way that Earth is nudged out while Jupiter is nudged in. We’d need no solar energy, then. Energy and momentum would simply be exchanged between Jupiter and Earth.

    • xcalibur May 20, 2016, 2:13

      I was going to say that with advanced engineering, we could shift the Earth’s orbit and keep our planet in the habitable zone. This could be supplemented with solar mirrors and other technology to modify the climate.

      Even in the distant future, we’re not going to abandon our homeworld. As we colonize space, Earth will continue to be our species’ capital where the majority of us reside. And even if most of us move off the Earth, we’ll maintain it as a park for biodiversity and a monument to human culture and history.

    • Moebius May 20, 2016, 4:42

      It seems more sensible to me to use the Moon’s resources and gravity to do this. There’s plenty of highly reflective material directly accessible on its surface to make giant mirrors and its lower gravity will make it easier to build space elevators (that will make for perfect struts to tether the mirrors as well as a convenient and cheap material transportation system from surface to orbit). Then we push the Moon onto a more elliptical orbit, orienting the mirrors to apply a thrust vector in a clever way so that the Earth-Moon center of mass moves a bit outwards, in a similar way to the ‘gravity tractor’ to deflect asteroids: https://en.wikipedia.org/wiki/File:NASA-Animation-ARM-opt-800-20150325.gif we’ll probably need to put the Moon on a polar orbit first, which is a long way off, but as we have a billion years or so, this can be done over eons with a small constant thrust. However by that time, if we are still around, we would have probably colonised the entire galaxy and evolved to a point where moving the Earth away won’t even be necessary or mastered ultra-futuristic techs like anti-gravity and there will be no need of giant structures to move a planet.

      • Moebius May 20, 2016, 9:23

        And a solar sail or a mirror is not very practical to push something as heavy as the Moon, it’s already very reflective and hasn’t moved so much out during 4.5 billion years.

      • Michael May 20, 2016, 12:43

        I have thought of using the moon before as well, use the light to power particle beams that use the mass of the Moon to pull the Earth further out. We will need to use the Moon and other masses though, maybe Mars or part of it or even Mercury.

    • Robert May 20, 2016, 13:26

      I would think that the technology in five billion years might allow future humans to build a gigantic shield to provide a cool zone where the earth can see similar insolation to now. But they would only do that because earth has special significance, not because they need to live here.

  • Michael May 20, 2016, 5:39

    Advanced aliens around at the time may be in for good times around these aging stars, not only is there plenty of energy, a nice wide HZ, gas and dust are emitted perfect for building materials and nuclear fuels (He3). And when the star has vented its fury they can move in to hug the white dwarf.

  • Mark Zambelli May 20, 2016, 11:14

    Assuming we make it through the next ten thousand years and continue to thrive, in whatever form for the next few million, I doubt the earth will still hold the same importance in the year 350 million CE… even if it survives at all… we would’ve moved onto countless new ventures in the interim. Our species may return periodically a lá Wells’ Time Traveller, to inspect our early O’Neill and Dyson structures for archeologic interest maybe, before rejoining its brethren out amongst the spiral arms. With the sheer stupenditude in number of thoses intervening years it’s rather a quaint thought that people think we would still be around by the time sol’s HZ starts to shift perceptively.

    But, way before any of that ‘long game’ musing has relevance, looking for life around stars nearing their old age is one more way we can improve our chances of finding life out there and it’s important to our success to include as many avenues as possible. Besides, finding a thriving civ splashing around in its version of a Neptune (as Alex suggests) would give us hope for our longterm prospects. Maybe they’d be visitors there (not indigenous) if the rewards outweighed the costs and trouble of leapfrogging from habitable zone to habitable zone.

  • andy May 20, 2016, 13:38

    One question that comes to mind is whether the planets around red giant stars will still be able to support much in the way of geological/tectonic activity, particularly at lower stellar masses where there is a longer main-sequence lifetime for the planet’s interior to cool down. I suppose you might be able to keep a world active via tidal heating in certain situations (e.g. resonant moons around giant planets, with the caveat that moon systems may evolve into and out of resonances as the system ages) but the general case doesn’t seem quite as promising.

    • Michael May 20, 2016, 16:59

      You have a point at some point without internal heating the carbon cycle will end, only tidal heating and the occasional asteroid impact will release carbon back into the biosphere. But there is the possibility of carbon been added to worlds via red giant stellar winds but I am not sure if it would be enough for a long term biosphere. Or perhaps we are part of the universal life cycle in which like countless organisms before us gives creation to a new form of life, that based on silicon -A silicon genesis.

  • Geoffrey Hillend May 20, 2016, 17:09

    @Alex Tolley I didn’t read your post carefully enough. I apologize for my ignorant assumption. Uranus and Neptune are still out side of the life belt though in the 12.5 billion year red giant phase example. The life belt is a zone where only liquid water can exist. A planet outside the lifebelt must remain frozen so it is not habitable.
    I like the idea of migration to a habitable zone which moves further out due to the red giant phase but I am critical of finding any life there for these reasons:
    1) Most likely due to technological advancement most civilizations including ours will have interstellar travel and simply move to another planet. We might visit a red giant afterwards but not stay there for any length of time.
    2) Moons and small planets lack gravity for long term habitability (Humans born on tiny worlds would have bones which would become too weak and thin under lower gravity) and an Earth like world usually ends up becoming Gas giant at the distance of Jupiter’s orbit and beyond it because the Sun’s light percentage on it’s surface result in a temperature too low to propel a gas molecules like H, and He to escape velocity.
    For example. Titan, Saturn’s largest moon has a low escape velocity but it’s so distant and cold it could retain a thick atmosphere. During the red giant phase it will loose a lot of it’s atmosphere when the temperature increases and it has no magnetic field to protect it from cosmic rays or protons and electrons from the Sun’s solar wind and ultra violet radiation. There is always the potential for the rare event of planetary migration of an Earth like world into the Gas giant region.

  • Geoffrey Hillend May 20, 2016, 17:28

    @Alex Tolley if Uranus and Neptune were moved into the life belt, their escape velocity is 13 miles a second due to their mass so the molecular hydrogen and helium would not be removed. The wind speed might increase and maybe a small amount of atmosphere might escape compared to the total volume but they would remain gas giants without any solid surface. One would also need a lot of oxygen.

    • Alex Tolley May 20, 2016, 21:07

      Unfortunately Sol’s Neptune is just a little too massive to allow the H2 to escape on warming.


      Which doesn’t preclude smaller exoplanet Neptune-like worlds from working in this way. Photosynthesis of water will create the free O2 just as on Earth. Even photolysis will work to some extent as long as the H2 escapes fast enough.

      In the vastness of the universe, one might expect a Neptune to form outside the HZ, then enter the HZ at some point in the evolution of its star. How such a world might be populated with forms is open to a number of possibilities. Just fanciful speculation of what might be possible and is outside the usual box of astrobiological discussion.

  • Geoffrey Hillend May 20, 2016, 17:36

    My point is that it would be too difficult to remove H2 and He and replace it with O2 and H.

  • Charlie May 20, 2016, 19:39

    I love the fact that you referenced the near ending of the ‘Time Machine’ in the opening paragraphs of your story here; I love that story tremendously and I must’ve read it now about 50 times in my life (yes I know that’s a lot, but it’s a great story). Along with Far Centauris and the story Tau Zero, I’d say those are probably the among the best science fiction stories that is currently out there .

    With regards to the topic at hand. I can only add this to the discussion the fact that habitable zones won’t really matter whether you’re moving planets in and out of habitable zones on a solar system type scale or not. Ultimately, the sun is meant to exhaust its nuclear fuel and its fate is to become a white dwarf, which will be almost nothing but a cold cinder which will eventually become nothing but a compact but not heat generating body. So no matter what the people of the future do the solar system is as good as dead in about 5,000,000,000 years. But I don’t believe I have to worry about that…

  • Navin Weeraratne May 23, 2016, 23:05

    @Alex Tolley, thanks for the link. Pity our ice giants are too massive to lose their H/He envelopes. By that time though, a scifi writer could say they’ve been cut down to size by gas mining!

    Except for Class Ms in the RG phase (9 billion years! What more do you want?), these seem less environments for life to evolve in, than to retreat to, or bloom in if already present.

    Life-bearing inner worlds could contaminate these outer worlds over time (asteroid impacts, sloppy planetary protection, mad scientists). As the HZ moves, they would become “kickstarters” for ecosystems.

  • Michael May 24, 2016, 10:36

    @Navin Weeraratne May 23, 2016 at 23:05

    ‘Pity our ice giants are too massive to lose their H/He envelopes. By that time though, a scifi writer could say they’ve been cut down to size by gas mining! ‘

    Planetary nebulae show a large amount of oxygen, if the oxygen is favoured and funneled via the magnetic field into the ice giants atmospheres they could react with the hydrogen there to form water leaving a water world with helium atmosphere.

    • Alex Tolley May 24, 2016, 14:01

      Or you could create extreme hot spots with mirrors to make teh water turn into a plasma. The H2 is energetic enough to escape. This will apply to the initially free H2 and the hydrogen in the plasma.This will result in a thinner, He and O2 atmosphere, as well as a water clouds. It could, however, take a long time. :(

  • Michael May 24, 2016, 10:37