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Imaging Red Dwarf Planets

If you’re trying to get actual images of exoplanets, it helps to look at M-dwarfs, particularly young ones. These stars, from a class that makes up perhaps 75 percent of all the stars in the galaxy, are low in mass and much dimmer than their heavier cousins, meaning the contrast between the star’s light and that of orbiting planets is sharply reduced. Young M-dwarfs are particularly helpful, especially when they are close to Earth, because their planets will have formed recently, making them warmer and brighter than planets in older systems.

The trick, then, is to identify young M-dwarfs, and it’s not always easy. Such a star produces a higher proportion of X-rays and ultraviolet light than older stars, but even X-ray surveys have found it difficult to detect the less energetic M-dwarfs, and in any case, X-ray surveys have studied only a small portion of the sky. Astronomers at UCLA now have hopes of using a comparative approach, working with the Galaxy Evolution Explorer satellite, which has scanned a large part of the sky in ultraviolet light. These data are compared to optical and infrared observations to identify young stars that fit the bill for possible exoplanet detection.

Image: Galaxy Evolution Explorer looks at Andromeda. The wisps of blue making up the galaxy’s spiral arms are neighborhoods that harbor hot, young, massive stars. Meanwhile, the central orange-white ball reveals a congregation of cooler, old stars that formed long ago. Now scientists are using GALEX data to hunt for young, planet-bearing red dwarfs near the Sun. Credit: NASA/JPL-Caltech.

So far the results are good. Of the 24 candidates identified with these methods, 17 turn out to show signs of stellar immaturity. The stars may be too young and low in mass to show up in X-ray surveys, but the Galaxy Evolution Explorer data seem effective at finding M-dwarfs less than 100 million years old. We can hope to add, then, to the tiny number of exoplanets that have been directly imaged, a useful adjunct to existing observing methods. Direct imaging can help us with large planets in the outer reaches of their solar systems, planets that would thus far have eluded Doppler methods. That helps us flesh out our view of complete planetary systems.

And yes, WISE (Wide-field Infrared Survey Explorer) is in the hunt here as well, helping us identify candidates from the M-dwarf category that would make good imaging targets. WISE can find young, nearby stars that are still surrounded by planetary debris disks, a fertile hunting ground for new planetary imaging. Putting the tools together across the spectrum should make it possible to find close young planets whose properties should help us in our studies of solar system formation. And we can expect release of the first 105 days of WISE data later this month.

As to the Galaxy Evolution Explorer, it was launched back in 2003 with a mission to observe distant galaxies in ultraviolet light. Now operating in extended mission mode, GALEX has been conducting an ultraviolet all-sky survey intended to produce a map of galaxies in formation, helping us see how our own galaxy evolved. Turning its ultraviolet capabilities to the study of exoplanets in young solar systems gives us a new technique for finding imaging targets.

The paper is Rodriguez et al., “A New Method to Identify Nearby, Young, Low-Mass Stars,” Astrophysical Journal Vol. 727, No. 2 (2011), p. 62 (abstract).


Comments on this entry are closed.

  • Interstellar Bill April 8, 2011, 12:27

    There seem to be configurations of sea and atmosphere that would allow a tidally locked planet to keep its atmosphere and thus its habitability. Such a world, however, would have little temporal variation in weather and virtually no climate change. In spite of any animal life evolving, you could hardly expect intelligent life to arise in such a static environment.

  • andy April 8, 2011, 12:43

    On the other hand, there was a recent posting to the arXiv that suggests that A-type stars may be better bets for direct imaging than late-type stars, because they seem to have a tendency to form more massive planets on wide orbits.

    Estimates of the Planet Yield from Ground-Based High-Contrast Imaging Observations as a Function of Stellar Mass

  • Eniac April 8, 2011, 20:58

    In spite of any animal life evolving, you could hardly expect intelligent life to arise in such a static environment.

    A sweeping statement with very little justification. Lack of climate change means static environment? Hardly. There are plenty of other possibilities to make life exciting. Static environment means no intelligence? You might postulate it, but it is hardly a well-founded conclusion. Any environment favors intelligence, because intelligence is mainly about competition, not dealing with the environment.

  • Rob Henry April 8, 2011, 22:23

    Interstellar Bill, your writing off of red dwarf planets due to lack of evolutionary potential brings up a point that I have long wondered about. I have a good idea over how close planets could be before they were in some sort of tidal lock with their sun. Mercury is obviously a good marker for that, but by sheer luck it turns out to be in a complex resonance tidal lock, as if to illustrate the point. I wonder then, that given that most planets seem on more eccentric orbits than is typical of our system, how close would they have to be before we give up hope that significant libration could drive a robust weather system. Alternatively, what level of volcanic activity would it take to make up for this lack driver for its weather?

    If volcanic activity is a more typical driver of the sort of environmental diversity that drives the evolution of higher life, this could explain the Fermi paradox. We have then been left alone because our system lacks super Earths or large Lagrange resonance bodies within our habitable zone.

  • Ronald April 10, 2011, 5:39

    Interstellar Bill: you certainly have a valid point, but not entirely: selective pressure, adaptation and evolution are not only driven by changing (abiotic) environment, but also by interspecific interactions (predation, herbivory, competition) and intraspecific competition.

    Rob Henry: I may not have entirely understood your point about tidal locking, but *all* planets (of reasonable age) in the HZ around a red dwarf star will be tidally locked, since tidal locking rate is inversely related to the 6th (!) power of distance between star and planet. So any planet orbitting such a red dwarf close enough to be in its HZ will be tidally locked very quickly (in atsronomical terms).

  • andy April 10, 2011, 12:52

    I’m not sure that the environment will necessarily be that static. Flare events or starspots would presumably be more of an influence, that could help to drive short-term “weather”. And presumably a habitable planet would have some kind of tectonics, the shifting continents would then be able to drive climate change on geological timescales.

    Furthermore it is not entirely clear what are good conditions to evolve intelligence, I don’t see that there is a strong case for it being a particularly strong attractor in the evolutionary landscape: the requirements to support high-level intelligence in terms of providing energy to the brain and other such factors may tend to limit the evolution of intelligence.

  • Mike April 10, 2011, 16:15

    So many questionable assumptions jumped at due to our astrobiological sample of one hobbling our perspectives. How I yearn for more data. Hopefully in my lifetime. Funding, funding.

  • Rob Henry April 10, 2011, 18:14

    Ronald, I am very uneasy about the modern interpretation that other life constitute the vast majority of the environmental adaptive pressures on them. Has this idea ever been proven, or does it just seem so much like commonsense that everyone adopted it. Actually, Interstellar Bill was only looking at just one evolutionary aspect: the drive to higher life. Looking at that one aspect in isolation, if complex life could really evolve in ideal environments that were stable ‘paradises’ why is it that if I tip the most ideal growth factor for higher life into a river, lower forms win out so convincingly.

    Also note that in my earlier comment the word Laplace should replace Lagrange

  • Ronald April 11, 2011, 14:49

    Rob Henry: I did not say that species inter- and intra-actions constitute “the vast majority of the environmental adaptive pressures on them”, but just that these interactions can also play important roles in evolution.

    Just think of tropical rainforests, among the most climatically stable environments on our planet and yet also the most biodiverse (species richest).
    This may be primarily due to a high speciation rate or a low extinction rate, or a combination of both, but the result is the same: great diversity in a relatively stable abiotic environment.

  • Rob Henry April 11, 2011, 17:40

    Tropical biodiversity is an interesting counter example Ronald, though, despite many theories no one has come up for an explanation for it that has a definitive ring to it. I note that there is even recent evidence that much of the diversity evident in temporal zones is also generated in the more stable tropics! To continue my speculations, its possible that this biodiversity mechanism only works once complex life is entrenched, and does not encourage the generation of higher life. It is also possible, as many others have postulated, that some difficult conditions, such as Snowball Earth, were a spur to higher life.

    I should stop there but I can’t help myself making an outrageous suggestion. I am wondering how important the length of the diurnal cycle is to the generation of higher life. Our moon ensures that there is a steady increase in it, its length and depth of temperature variation, and that this also might have crossed a threshold some time before the Cambrian.

  • ljk April 18, 2011, 9:02

    Kepler Discovers a Rare Triple Gem

    by Jason Major on April 18, 2011

    It may be visible to the naked eye, but it took the unblinking gaze of NASA’s Kepler space telescope to reveal the true triple nature of this star system.

    Unofficially dubbed “Trinity”, object HD 181068 is a multiple star system comprised of three stars: a red giant over twelve times the diameter of the Sun and two red dwarf stars each slightly smaller than the Sun. The red dwarfs orbit each other in tight rotation around a central point, which in turn orbits the red giant. The smaller stars complete a full orbit around the giant every 45.5 days and, from our point of view, pass directly in front of and behind the huge star.

    The orbital eclipse events of HD 181068 last about 2 days. What’s surprising is that during these eclipses the brightness of the system is not affected very much. This is because the surface brightnesses of the three stars are very similar. The current metaphor is a “white rabbit in a snowfall”, wherein the two red dwarfs effectively become invisible when they pass in front of the red giant. It wasn’t until the Kepler mission that we had an observational tool precise enough to detect the structure of this intriguing star system, located 800 light-years away from our own.

    Full article here: