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Targeting Nearby M Dwarfs

We’ve been talking for the last six years (since Centauri Dreams‘ inception) about finding a terrestrial world in the habitable zone of another star. It’s an exciting prospect, but the reality about space missions like Terrestrial Planet Finder and Darwin, each designed to make such identifications, is that the budget ax has fallen and we don’t know when they might fly. Indeed, we still face a host of technological difficulties that call for much work if the aim is not only to find a terrestrial world but also to study its atmosphere for possible biomarkers.

Alternatives are therefore welcome, and one is to look for terrestrial worlds around nearby red dwarf stars using transit methods. Usefully, an Earth-size planet orbiting such an M dwarf would be easier to spot than the same size planet orbiting a star like the Sun, and we could use ‘eclipse spectroscopy’ with the James Webb Space Telescope to study such a planet’s atmosphere. Right now we’re making Doppler surveys of nearby M dwarfs, and to good effect, with discoveries like the ‘hot Neptunes’ GJ 436b and GJ 581b, and ‘super-Earths’ like GJ 876d. We’ve also found two planets near to their star’s habitable zone in GJ 581c and d. We should be finding habitable ‘super-Earths’ in the near future with these methods and some of these, let’s hope, will be transiting.

Surveys monitoring thousands of stars can pick up transiting planets (think of Kepler), and Michaël Gillon and colleagues explain in a new paper that most known transiting planets have been detected by such dedicated photometric surveys. The MEarth Project at Mt. Hopkins, AZ monitors nearby M dwarfs with small telescopes and is sensitive to transiting worlds down to a few Earth radii. Gillon’s team is interested in a third approach, one that’s based on a helpful principle. Because planets form within disks, a planet orbiting in the habitable zone of a star will be more likely to transit as seen from Earth if that star already harbors a known transiting planet. From the paper:

Depending on the orbital inclination of the known transiting planet, on the assumed distribution of the orbital inclinations of the planetary system, on the size of the star, and on its physical distance to its HZ, significantly enhanced transit probability can be expected for habitable planets. A dedicated high-precision photometric monitoring of M dwarfs known to harbor close-in transiting planets could thus be an efficient way to detect transiting habitable planets in the near future.

The fact that planets in a system should share similar orbital inclinations is especially useful for M dwarfs because their habitable zones are close to the star. As we discover more transiting planets around M dwarfs (which are currently thought to be the most common class of star in the galaxy), we may be able to use these facts to improve the likelihood of finding habitable worlds. The researchers go on to discuss the potential of this approach for two M dwarfs known to host a transiting planet, GJ 436 and GJ 1214, using a series of simulations.

It turns out that GJ 436 is not a good target compared to GJ 1214. The transit probability of planets in the habitable zone of the latter is much larger. Moreover, GJ 1214 is smaller in radius, meaning that smaller planets could be detected around it. The latter fact also makes for a smaller habitable zone, so that any planet in that zone will be orbiting closer to the star. Ground based monitoring of GJ 1214 could theoretically find a habitable planet as small as the Earth, while space-based observatories like Spitzer could spot a transiting habitable planet down to Mars size.

The planet we already know about here, GJ 1214b, is a super-Earth about 6.6 times the mass of Earth, with a radius somewhat less than three times our planet’s, and it orbits its star every 1.6 days. Roughly 40 light years from the Sun, this system would seem to be ideal for pushing the search for a smaller companion world. The team finds that probing the habitable zone of GJ 1214 would require three weeks of constant monitoring, whereas GJ 436 would require a full two months. That three week run would allow for two transits and could lead to the detection of smaller planets than we’ve hitherto found. The paper confirms the viability of transit surveys like MEarth and offers what may be the shortest course to detecting habitable planets as small, or even smaller, than the Earth. The authors continue:

…we advocate the development of the approach used by MEarth (other facilities spread in longitude, a similar survey observing from the Southern hemisphere, larger telescopes and IR cameras to monitor cooler M dwarfs), but also an intense and high-precision photometric monitoring of GJ 1214 and of the other transiting systems that MEarth (or similar projects) will detect. This two-step approach targeting nearby M dwarfs makes possible the detection in the near-future of transiting habitable planets much smaller than our Earth that would be out of reach for existing Doppler and transit surveys.

The paper is Gillon et al., “Educated search for transiting habitable planets: Targeting M dwarfs with known transiting planets,” submitted to Astronomy & Astrophysics (preprint available). The betting here is what it has always been, that our first detection of a terrestrial exoplanet that is unequivocally in the habitable zone of its star will be around an M dwarf. We’re likely to spot a growing number of habitable ‘super-Earths’ in coming years, so methods that will allow us to extend our discoveries to Earth-size planets are all to the good. After all, who knows how long it will be until funds become available for the kind of terrestrial planet hunter mission we’ve long wished for?


Comments on this entry are closed.

  • James M. Essig March 4, 2010, 13:33

    Hi Folks;

    Red dwarf solar systems have some potential outstanding ramifications for the development of any ETI civilizations that might call them home as well as for future human colonists.

    Since Red Dwarfs may live as long as 10 EXP 13 to say about 10 EXP 13.33 years, the ramifications for civilizations calling such stars home is extreme. Ten trillion years is 10,000 times the age of the universe and about two billion times the time period of recorded human history. I think of the increadible advances we could make in science, technology, medicine, mathematics, and philosophy if we could last that long as a civilization.

    Studying Red Dwarfs is useful because there are so many of them thus offering so many long lived systems to call home.

    Now how to protect planets from cosmic rays as the planets lost their internal magnetic field is a different story altogether. However, I can think of several plausible ways in which such could be accomplished.

  • NS March 4, 2010, 14:34

    An extremely long-lasting habitable zone is nice to have, but there are a lot of shorter-term cultural, climate, and biological factors that limit the actual durations of civilizations/species.

  • kurt9 March 4, 2010, 14:46

    Funding is indeed an issue. The federal government debt is up to 70% of GNP and is expected to rise to nearly 100% in the next ten years. The retirement of the boomers, and resulting social security payouts, will most certainly drive the debt to this higher figure in coming years. Anything like NASA and space is going to be seen as superfluous to those in Washington DC as well as to many of the rest of us. Cuts have to be made.

    I think Obama’s push to get NASA out of the space transportation business is a good place to start. Still, scientific missions will face greater hurtles for funding justification. I think future projects like Darwin and TFP will have to be financed jointly by the U.S., E.U., and possibly some Asian countries as well. You’re not going to get all of the money you want for these things solely from the U.S. government. Scientists (including those of you here) are going to have to network and collaborate a lot more with your counterparts in Europe and Asia if you want to get some of these projects financed and developed.

    Most significantly, we need low cost space transportation. This requires the emergence of a competitive space transportation industry. Having NASA as a government run space transportation company makes no more sense than to have a government run airline.

  • tesh March 4, 2010, 15:22

    wrt to the notion that red drawves are the best candidates for ETI’s to evolve. They are the most numerous, they will last the longest, at some point in their life time they will have a about 5-10 billion yearsof the ‘right’ conditions to produce/nurthure ‘life’ and thus, most likely produce the most ETI’s.

    Does that mean that we are an early/precocious/fluky ETI? Are sol like stars likely not to produce many ETI’s?


  • drpayton March 4, 2010, 15:26

    Maybe deep space exploration needs to become a political movement… Of course a tangible reason that everyone could conceptualize would have to be evident (necessity is the mother of invention). Cost has no meaning when survival is the reason, but with our short life spans it is difficult for people to really grasp the fact that our planet has a shelf life. Tau zero and centauri dreams have started a small social movement and most that participate understand that deep space exploration and colonization are needs and not just wants (not to mention instinctual).

  • spaceman March 4, 2010, 17:09

    Hi my friends,

    Questions: What potential does Kepler have for finding planets around M dwarfs? Is Kepler targeting M dwarfs as well as G type Sun-like stars? What is the minimum size of a planet that Kepler would be capable of detecting around an M dwarf?

    Additional related question: RV for tiny planets is a thorny issue as far as being able to confirm the smallest planets found by transit photometry missions. Is there really any existing RV instrument capable of confirming the existebnce of an Earth mass planet at 1 AU around a solar type star, as finding this type of configuration is the Kepler’s primary scientific goal?


  • andy March 4, 2010, 19:08

    GJ 876e? As far as I am aware, the only M dwarf planetary system that has enough known planets to get an “e” designation is GJ 581…

  • bigdan201 March 4, 2010, 20:12

    the long lasting habitable zones of red dwarf stars could be a great haven for civilization. I believe that an earthlike planet in a red dwarfs habitable zone would be tidally locked, but that wouldnt make it unhabitable. there could be a longitudinal area between the sides facing towards and away from the red sun, and that area could be temperate and hospitable to life.

    With the economic troubles we have, it only follows that we cant divert the same level of funding to space projects. However, letting the commercial sector compete should be helpful – not only in saving money, but also in trying new methods and building new infrastructure.

    I also think we shouldnt rely too much on the transit method – while it is effective, theres going to be a large contingent of exoplanets that do not transit, so we need to use all the tools at our disposal.

    If you look at the chart I linked, you can get a sense of the type of exoplanets we’ve been finding with certain methods. I believe that it’s only a matter of time before we fill in many of the blank areas – including earthlike planets.

    Hopefully the detection methods we’re using now will suffice.

  • philw1776 March 4, 2010, 21:44

    Kepler ‘should’ be able to detect Earth sized planets around early M stars, hoter than M5 or so.

  • Administrator March 4, 2010, 21:45

    andy, just checked — the paper does refer to GJ 876e, but they must mean GJ 876d, right? It’s the super-Earth in that system. I changed the text above to reflect this.

  • Mike March 4, 2010, 21:50

    To spaceman, Kepler can certainly detect earth-size transits around G class to M-class stars. While the majority of target stars are G and K class there are
    a few thousand M-dwarf target stars.
    At this time the best that the RV follow up techniques can do regarding identifying earth mass planets is constrain the high end of the possible mass of the detected transit event.
    To put it another way if Kepler is detecting multiple repeated transits over a length of time (say a year) around a M2 dwarf with time between transits of say,20 days,that may indicate a planet or a grazing eclipse by a stellar companion.
    Then what if a very good RV instrument,like HIRES at Keck1 doesn’t show any Doppler shift with the same period as the transit?
    Then we won’t know exactly what the mass of the planet is but we will know it is a planet with a mass below the RV detectability for this example. That allows any higher mass planet or companion star to be ruled out.
    As I understand it this is the way the ground based RV follow up for all of Kepler’s earth size transits will have to proceed for now until better instrumentation is available.Like FINDS Exo-Earths if it works up to it’s expectations.
    Keep in mind that the smaller the host star the larger the Doppler shift produced by an earth-size planet in the star’s habitable zone. That may mean that the first RV confirmation of an earth-size planet from Kepler’s transits detections may be orbiting a M dwarf.

  • Mike March 4, 2010, 22:18

    The concept of follow-up dedicated searches of M-Dwarfs with identified transiting planets sounds like a great idea. There are a fair number of mid-size under utilized telescopes that could be used if funding can be found to install the kind of CCDs needed for this task and to pay for the observing runs.
    I’m thinking of older observatories located in light polluted areas.
    With a large enough aperture and advanced CCDs and filters it might prove feasible. Ofcourse there are better ways but I’m trying to think cheap.
    Regarding GJ 1214 with a 6.6 mass planet orbiting 1.6 days I wonder if it would still retain smaller planets in the HZ or if they would have been ejected by possible planetary migration?
    Still worth a look I think.

  • Mike March 4, 2010, 23:20

    After reading the article in depth and considering the difficulties I still think
    that it’s a good idea but there is no way it can be done as cheaply as I previously suggested.
    The Transiting Exoplanet Survey Satellite (TESS) may be capable of detecting numerous M dwarf HZ planets down to earth-size.
    If it ever gets funded.

  • T_U_T March 5, 2010, 13:53

    @drpayton :

    Maybe deep space exploration needs to become a political movement…

    An interesting idea. How do you think should such a political movement look like ?

  • george scaglione March 7, 2010, 13:02

    jim,yes as you put it above having a look at some red dwarfs can only be a good idea! we may find exactly what we are looking for ! naturally too i find that afew of the postings that filled in below yours made alot of sense too.for me one of the great benefits of keeping up with tau zero. as always,thank you one and all.your friend george

  • Ronald March 8, 2010, 6:57

    With regard to some above posts about the promising prospects of earthlike planets around M dwarfs, I had a kind of déjà vu experience.
    We have had several threads and discussions about this before, I couldn’t find the threads so quickly (Paul, my impression is that your search engine doesn’t search in the comments, or am I wrong here?), but summarizing:

    Yes, M dwarfs are very abundant and long-lived, however:
    – M dwarfs have, as a logical result of their very low luminosity, a very narrow habitable zone (not to mention the occurrence of M flare stars).
    – All M dwarf planets in the HZ would quickly become tidally locked.
    – Further to the previous two points: the total ‘habitable territory’ becomes even less, when we take into consideration that, as bigdan is mentioning, planets in such a HZ would mainly be habitable (for higher life) in a narrow zone between day and night side, if even there.
    – M dwarfs are now seriously suspected to be generally planet poor, as a result of their low mass often in combination with (too) low metallicity. This paucity mainly pertains to giant and subgiant planets, particularly those in close orbits (because these can be detected the easiest, i.e. observational bias), but it is not unlikely that this relative planet paucity extends into the earthlike planet realm, because of the strong correllation of planet occurrence (or rather: total planetary mass) with stellar mass and stellar metallicity.

    A very interesting post by leading planet hunter Gres Laughlin at his systemic site is: http://oklo.org/2010/01/17/red-dwarf-metallicities/

    Anyway, as I wrote before, when we compare total habitable territory of all M dwarfs to that of all solar type stars (roughly F9/G0 through K1/K2), the solar type category actually (just) wins.

    I don’t think we are such a fluke, at least as far as our star is concerned, on the contrary: I think it is quite significant, that, despite its relative numeric scarcity, we are orbiting a G star. Not that our particular star itself would be the optimal one, but its stellar type (spectral class, etc.) is telling.

    With regard to longevity of M dwarfs: that is nice, however, the geological lifespan of the planet is probably at least as important for the development and persistence of higher life. If most M dwarfs appear to be orbited by Mars sized planets and dwarf planets, these will become geologically and atmospherically lethargic within a few gigayears.
    Besides, later G stars (G5 and up), and early K stars have pretty long stable main sequence lifespans too. A large (2 – 3 Me) earthlike planet in the HZ of such a star could have a habitable lifespan (for higher life) of a few to several gigayears longer than earth.

  • Administrator March 8, 2010, 9:40

    Ronald writes:

    Paul, my impression is that your search engine doesn’t search in the comments, or am I wrong here?

    Right — the standard engine doesn’t go into the comments. I may experiment again with one that does if you think it would be useful — the problem in the past is that when the comments are also flagged, it can make it difficult to drill down to a specific post because of the numerous comment hits.

  • drpayton March 8, 2010, 20:34

    It seems it would have to be an intellectual party comprised of experts from all fields. Physicists to economists, electrical engineers to dieticians with the common goal of achieving unity with the idea that exploiting resources in our local space could relieve the earth of its energy burden. I have had this idea running around in my head for some time ( not that it is that original ). Also, I think it is just as important to consider this as a social movement. One might consider a type of symbiotic community to start. Meaning that farmers ranchers electricians etc. Work together in a small community that’s sole purpose is to get off the planet. ( Again no, original , this type of. community is already gaining some popularity on the oceans “seacommunitynetwork.ning.com”) If nothing else, there are thousands of orginizations that lobby for certain things they find important, and reserve their votes for the candidates that they believe will further their agendas… I don’t see why a political movement like the green party or libertarians etc wouldn’t also be viable for a group of people lobbying to become a space. fairing species. The idea is to become a true presence and for lack of a better way to describe it, invent something that is “cool” to be apart of. I don’t know, what do u think? It could become the foundation of true space travel… I just think we have to start somewhere.

  • drpayton March 9, 2010, 1:13

    I have a question for a physicist (forgive me for my ignorance) would time dialation work if the speed of light were not the constant in e=mc3? What is the opposite of light, and what is it made of?

  • Eniac March 9, 2010, 10:02

    Dr. Payton,

    Time dilation and the equivalence of mass and energy are both facets of the same theory, special relativity, and by extension general relativity also. According to this very well established theory, the speed of light IS the constant in E=mc^2, period. If you want it to be different, you have to go find a different theory.

    “The opposite of light” is not a phrase that makes much sense to me. Shadow comes to mind, and it is made by the absence of light.

  • drpayton March 9, 2010, 10:59

    Ok… I guess I shouldn’t delve into subjects I really don’t understand well… But with light I guess I meant is there an anti-photon and does the opposite stand true when using it in the equation?

  • Ron S March 9, 2010, 15:52

    A photon is its own anti-particle.

  • drpayton March 11, 2010, 1:01

    Ahhhh… So it doesn’t have a charge?