≡ Menu

SETI: The Red Giant Factor

The ‘slow boat’ to Centauri concept we’ve discussed before in these pages envisions generation ships, vessels that take thousands of years to cross to their destination. And based on current thinking, that’s about the best we could manage with the propulsion systems currently in our inventory. Specifically, a solar sail making a close solar pass (a ‘sundiver’ maneuver) could get us up to 500 or 600 kilometers per second (0.002c), making a 2000-year journey to the nearest star possible. It’s hard to imagine under what circumstances such a mission might be launched.

But let’s think long-term, as Greg Matloff (New York City College of Technology) did in a session that just concluded at the International Astronautical Congress in Prague. Matloff, a solar sail expert and well known figure in the interstellar community, notes that when the Sun leaves the main sequence and becomes a red giant, its luminosity may have increased by a factor of a thousand. Imagine using that kind of star as the source of the photons for your solar sail and you’ve dropped the time for an interstellar transit down to centuries or even decades.

So let’s think about a civilization leaving the vicinity of a giant star, looking for a new home as their system is destroyed. They’ve got fast propulsion options by virtue of their star. The catch is, how do they slow down when they arrive? Traveling at the kind of velocities that would allow an interstellar crossing in decades, their photon sail would be all but useless for deceleration upon arrival at a main sequence star, meaning they’d have to supplement it with something like a magnetic sail. Or perhaps there’s another way, and one that has SETI implications.

Image: Greg Matloff begins his presentation this morning in Prague. Credit: Pat Galea.

After all, most stars are members of binary or multiple systems. Now we have an improved scenario: An advanced civilization targets a binary system composed of a giant and a widely separated red dwarf star. It uses the red giant’s intense luminosity for deceleration, then crosses to the planetary system of the red dwarf. We know that red dwarfs remain stable on the main sequence for as long as a trillion years, making building a new home there an attractive and long-term proposition.

We may find such thinking remote because we’re not all that concerned about our own Sun’s far future, with five billion years to go before it enters its giant phase. But if intelligent life is widespread in the universe, then a species evolving on planets circling stars older than our own or far more massive than our Sun may have already taken such a path. Matloff thus suggests that SETI take a look at red giant/red dwarf binaries within 100 light years from the Sun.

He then proceeds to go through a list of candidates, finding four possibilities: Alschain (Beta Aquilae), Gienah (Epsilon Cygni), Aldebaran (Alpha Tauri) and Theta Ursae Majoris [see note in the comments section]. Alschain’s red dwarf companion is roughly 150 AU from the star, while Gienah’s is at some 1700 AU. Aldebaran is just over 600 AU from its red dwarf companion, and Theta Ursae Majoris has an M6 red dwarf within 94 AU. All of these stars are northern hemisphere objects, so that for SETI purposes, the Allen Telescope Array should be able to conduct observations on them.

And our own future? Matloff’s figures show that at intervals of about 100,000 years, random motions cause a star to approach the Sun within two light years. In the paper his presentation is based on, he writes:

If the Sunʼs luminosity will be at least twice its current level for 2 billion years between its late-main-sequence and planetary-nebulae phases, at least two stellar systems of the type considered here should closely approach the Sun. If we consider trips to destinations as far as 4 light years, we expect close approaches by 16 of these systems.

And actually, the number is higher, for Matloff goes on to show that most stars in the Sun’s vicinity are of roughly the same age. Five billion years from now, when and if a civilization remains in our system, many nearby stars will also have entered their red giant phase. Our remote posterity will, then, have plenty of red giant/red dwarf pairs to choose from, and thus planets to terraform and colonize in their habitable zones. “Terrestrial-derived life,” Matloff adds, “could have one or more planetary homes that will remain habitable for perhaps a trillion years.”

But back to SETI. Just as we have four red giant/red dwarf binaries to look at for SETI purposes, we might also consider widely separated white dwarf/red dwarf binaries. After all, a star’s red giant phase is finite, after which it becomes a white dwarf. Any civilization that used these methods to enter a planetary system around a red dwarf would survive to find itself in close proximity to the white dwarf remnant. And in fact, we do have one system like this close to the Sun. It’s Omicron-2 Eridani, 16.4 light years away, a K1 orange dwarf with a white dwarf that has an M4 dwarf companion of its own at 35 AU. The system is also called 40 Eridani.

Image: The triple star system 40 Eridani. The fainter stars are B and C. 40 Eri B is a white dwarf star, 40 Eri C is a red dwarf. Credit: RDP, Tulane University.

Ring any bells? 40 Eridani is considered by many fans to be the location of the planet Vulcan in Star Trek. Now wouldn’t it be a kick if a place like that yielded our first confirmed signal from an extraterrestrial civilization?


Comments on this entry are closed.

  • philw1776 September 29, 2010, 12:38

    I don’t think that this scenario is very well thought out. One problem being that the ‘suddenly’ luminous Red Giant may have toasted the putative Earthlike planets surrounding its Red Dwarf companion. Also, as the big stars age they puff out lots of gas and dust that would seriously impinge on the neighboring solar system’s ecosystems. You don’t want to be ‘nearby’ e.g. 100s or even thousands (dust & gas) of AUs.

  • Ronald September 29, 2010, 13:00

    Speculative to the extreme; I am sorry, but I just do not see the point why a civilization that is capable of interstellar travel so easily and quickly would bother whether their destination star will last for 10, 100 or 1000 gy.
    Rather go for a comfy and stable G star, something like G5/G6, or an orange K0/K1. And then hop over to another one, when needs be. The remaining main-sequence lifespan of your star (provided it is still in the habitable main-sequence phase) would be the least of your problems.

  • NS September 29, 2010, 14:19

    Some scientists think the Earth could become uninhabitable in a lot less than 5 billion years. Long before the Sun enters the red-giant phase it will brighten to the point where rock weathering removes too much carbon dioxide from Earth’s atmosphere. Probably in about a billion years Earth’s oceans will evaporate.


  • andy September 29, 2010, 14:45

    One thing to consider is that you could end up with “second-generation” planet formation around the companion star, fuelled by matter expelled by the red giant. Something along these lines is apparently occurring at Mira B.

    Also, the genitive cases are Aquilae, Cygni, Tauri and Ursae Majoris…

  • Paul Gilster September 29, 2010, 14:57

    Good point re the genitives, which I’ll correct in the original post.

  • kurt9 September 29, 2010, 15:12

    Generation ships are a poor joke. Who wants to live and condemn the next x number of generations to live within the confines of a generation ship, even if its the size of an O’neill tin can? This idea really sucks.

    Surely we can do better if we want to go to the stars. You stay home and forget about star travel until you have a propulsion technology that can get you there in, say, 40-50 years. This requires a propulsion technology that can get you to 10% of c.

    If this is not possible, the other approach analogous to the generation ship is to “grow” your way to the stars. Human settlement in the solar system will be mostly space habitats in the asteroid and, later, Kuiper belts. These will slowly migrate out into the Ort cloud. Since the Ort cloud takes you half-way to Alpha-Centauri, its likely that Alpha-Centauri’s Ort cloud will pick up where our’s leave off and take you the rest of the way in. Other stars will have their own Ort Clouds as well.

    Humanity slowly migrates to the stars, a comet at a time, over a period of thousands of years.

    Since there will be a zillion of these habitats and they last for a long, long time, there will be lots of funky cultures to emerge out in space.

  • LarryD September 29, 2010, 16:26

    IF the article I read has it right, the brightening of the sun as it ages will move the habitable zone out past Earth in a mere half-billion years.

    Ah, that’s the point of the page NS has linked to. Anyway, we’re either going to have to move off of Earth, permanently, or move the planet long before the Sun gets to the end of the main sequence.

    Another thing, a half-billion years is somewhat over two orbits of the solar system around the galactic core, it’s going to be a whole different set of stars nearby when that time comes.

  • Mang September 29, 2010, 17:22

    I wonder if it occurred to anyone that setting up a new home near a red giant could still leave you with a big problem. Your red dwarf will be very close to a star that could blow up. Isn’t being within 50 or so LY a problem then?

    Choose carefully.

  • Robotbeat September 29, 2010, 17:46

    Speaking of colonizing a red dwarf star system… yet another planet was found around Gliese 581, this time only 3 Earth masses and right smack in the middle of the habitable zone:

  • Ronald September 29, 2010, 18:53

    Mang: I think you are confusing a red giant with a supernova. Only superheavy stars (> about 8 solar masses) will turn supernova, being dangerous indeed up to a few tens of ly. An ordinary red giant will devour its own (inner) planets, but not a neighboring planetary system in a wide binary.

    LarryD: yes, the earth is already uncomfortably close to the inner edge of our HZ (estimated at about 0.95 AU) and we’ll leave it on the inside in about 0.5 or 0.6 gy, too hot for higher life. This gives us about as much (evolutionary) time though as we have had since the start of the Cambrium, the ‘explosion’ of life.

    Robotbeat: I just got this news about Gl 581g as well. But tidal locking remains a problem for such a planet.

  • Adam September 29, 2010, 20:23

    Re-engineering one’s star might be preferable to letting it go red-giant. Presumably, if anyone still cares by then, our successors to Intelligence in this system might start to do something about that ~0.5-1.0 Gyr from now. Migration when other stars are nearby makes sense even without your home star going off the Main Sequence. But stellar re-engineering might allow stellar navigation. At maybe 1 AU/year we’d be able to arrange close encounters within a few billennia.

  • NS September 29, 2010, 21:48

    This article suggests that after the sun’s expansion there will still be habitable bodies (at least in terms of temperature) in the solar system:


    Seems like colonizing Mars, the asteroid belt, the moons of the outer planets, and eventually even the Kuiper Belt would be more practical than trying to transfer huge numbers of people to other solar systems…

  • NS September 29, 2010, 21:55

    The main article (linked from the link above) even proposes ways of moving the Earth into a larger orbit, as LarryD suggested. Amazing…

  • amphiox September 29, 2010, 22:39

    Amother problem with generation ships was touched upon I think in Greg Bear’s Eon series (as a minor side-point to the main plot). The asteroid Thistledown was originally launched as generation ship, but after several generations of the original crew’s descendents developing along their own unique political and social trajectory, they decided there were more interesting things to do than continue on a mission planned by an extinct culture on a distant world centuries ago, at which point they, among other things, turn the ship around and head back to earth. . . .

  • Tulse September 30, 2010, 0:06

    How big of a problem is tidal locking if a planet has an atmosphere and liquid water oceans? Could these provide enough of a heat sink to moderate the temperature differences? Is there any modeling of this? Ultimately it’s not necessary that the whole planet have a habitable temperature, just that the atmosphere and surface water doesn’t either completely boil away or freeze out. As long as that’s the case there should be a habitable zone on a tidally lock planet.

  • John Hunt September 30, 2010, 2:26

    > making a 2000-year journey to the nearest star possible. It’s hard to imagine under what circumstances such a mission might be launched.

    Not at all. If it HAS to be a scientific mission then yes. If it HAS to have living astronauts then yes. But if it contains frozen embryos for the purpose of establishing a distant human civilization in case we are wiped out (likely by our own self-replicating technology) then a 2,000 year mission would be about right. It might be possible for us to achieve those speeds before we develop dangerous self-replicating technology and, with cellular reproduction and a protective, long-duration, superconducting magnetic field then frozen cells might be able to survive for that long.

  • John Hunt September 30, 2010, 2:26

    Boy…Can’t wait for tomorrow’s Centauri Dreams. Wonder what it might be about !!!

  • Ronald September 30, 2010, 7:35

    And speaking of news, off-topic: a solar twin was just discovered in the open star cluster M67, known for its great similarity in age and composition to our sun (i.e. rich in sunlike stars). It is designated M67-1194.

    See my additional comment under the thread about ‘Hot Jupiters and Short Lifetimes’, https://centauri-dreams.org/?p=14297#comments.

    Reference: http://arxiv.org/abs/1009.4579.

    Sorry Paul, for being so off-topic and maybe ‘mowing the grass away in front of your feet’ (Dutch expression, I think you get it, is it in English ‘cutting the grass under someone’s feet’?).

  • Paul Gilster September 30, 2010, 10:02

    Ronald, yes, the phrase makes perfect sense in English, though I’ve never seen it used before. Perfectly clear, though. In any case, have no fear about mowing the grass for me — I get so many of my ideas from reader comments like these.

  • Ron S September 30, 2010, 11:04

    Tulse: “How big of a problem is tidal locking if a planet has an atmosphere and liquid water oceans? Could these provide enough of a heat sink to moderate the temperature differences? Is there any modeling of this?”

    I don’t know the answer but we do have one partial example here on Earth, namely the arctic and antarctic where for part of the year there is either full (though low angle) sunlight and night. Those winters get pretty darn cold, with the temperature decline moderated only somewhat by the heat stored in the sea during summer and some atmospheric circulation. Life adapts to the seasonal cycles but what if it’s always cold? This doesn’t augur well for at least the night side of a tidally-locked planet.

  • philw1776 October 1, 2010, 12:00

    Forget just the cold on the perpetual nightside. No photosynthethis. No plants. Nothing for animals to eat. A Dead Zone, briefly accessible at the edges by motile animals although I cannot conceive for what purpose other than evading predators.

    Another problem with a cold side vs a warm side is perpetual storms as the Second Law of Thermodynamics wreaks havoc.

  • Eniac October 2, 2010, 1:15

    Phil: Those same storms could carry food onto the dark side, similar to how gravity brings food into the depth of the oceans, where there is also no photosynthesis, yet plenty of life.

    As I understand, the only dead zones on Earth are places where there is no water. Neither hot nor cold nor the absence of light seem to be much of a problem.

    The real question is what conditions are needed or detrimental for abiogenesis, and we know next to nothing about that. That said, a tidally locked planet would provide a great variety of conditions, which ought to increase the chances.

  • Tim Whitworth October 4, 2010, 5:35

    What is the decceleration mechanism? If we’re relying on a supernova to reach velocities far higher than available by ordinary solar sails, then using a solar sail to deccelerate isn’t going to work.

  • Paul Gilster October 4, 2010, 7:40

    Tim, the idea is to use the red giant’s luminosity for deceleration to interplanetary velocities, then move to the system around the nearby red dwarf.

  • ljk October 27, 2011, 10:01

    Three new planets and a mystery object discovered outside our solar system

    October 27, 2011

    (PhysOrg.com) — Three planets — each orbiting its own giant, dying star — have been discovered by an international research team led by a Penn State University astronomer.

    Using the Hobby-Eberly Telescope, astronomers observed the planets’ parent stars — called HD 240237, BD +48 738, and HD 96127 — tens of light years away from our solar system.

    One of the massive, dying stars has an additional mystery object orbiting it, according to team leader Alex Wolszczan, an Evan Pugh Professor of Astronomy and Astrophysics at Penn State, who, in 1992, became the first astronomer ever to discover planets outside our solar system.

    Full article here: