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More on Red Dwarf Planet-Hunting

If you’re thinking about detecting Earth-like planets around other stars, here’s an item that may set the pulse racing a bit faster. Michael Endl, who is an expert at the planet hunt around red dwarf stars (he’s searched for planets around 100 of them already), notes that the diminutive objects are prime targets for exoplanet hunters. And listen to this: “For the red dwarfs with the lowest masses, like Proxima Centauri, we are sensitive to planets down to two Earth masses using the standard radial velocity technique.”

Endl works at the University of Texas, out of which a study led by graduate student Jacob Bean has focused on planet formation around red dwarfs — we looked at this work not long ago. Few gas giants have been detected around red dwarfs. The study examined the dwarfs known to have planets: Gliese 876, Gliese 436, and Gliese 581. Of the three, Gliese 876 is perhaps the most intriguing, as it’s known to have two Jupiter mass planets and a likely third, lower-mass world orbiting around it.

Bean wanted to find out whether dwarfs like these known planet-bearers show high metallicity values (elements heavier than hydrogen and helium). Using computer modeling coupled with telescope observations, he was able to determine that the three dwarfs under study actually have significantly fewer metals than the 200 or so Sun-like stars known to harbor planets. Thus the possibility that the low number of high-mass planets found in our red dwarf surveys thus far may have something to do with low metallicity values in the stars selected for study.

And that makes sense, because a proto-stellar cloud with higher metallicity would be more likely to ‘grow’ a planet. Here’s co-author Fritz Benedict (University of Texas) on the matter:

“Just as rain drops need a speck of dust in the air around which to form, the formation of planets is thought to be assisted by a similar successful first step. More dust in the protoplanetary disk might increase the chances for planet formation.”

But it’s clear that these findings — on just three planetary systems — are the tip of the iceberg, so much work on red dwarf metallicity lies ahead. It’s significant because we need to know why there are so few gas giants around these stars, or whether there are gas giants in wider orbits that have simply not been discovered yet. We also need to know how best to target our planet searches, and if metallicity values for red dwarfs can be established in that regard, we can focus in on the most likely candidates.

I come back to Endl’s comment that radial velocity techniques are sensitive down to planets of just two Earth masses for some red dwarf studies. Given enough time to gather the RV data, we should begin to learn how often such worlds form around these stars. And Bean’s work on metallicity may help us estimate how often they’re joined by Jupiter-class objects in wider orbits. That’s exciting in its own right, but it also works toward filling in much broader patterns of planet formation that will help us characterize solar systems around a variety of stellar types.

The paper is Bean et al., “Metallicities of M Dwarf Planet Hosts from Spectral Synthesis,” Astrophysical Journal Letters 653 (December 10, 2006), L65-L68. And as referenced in our earlier article on this work, it’s also available at the arXiv site.

Comments on this entry are closed.

  • andy December 14, 2006, 10:00

    Just to throw a bit of a dampener on things, I notice that in the bibliography listing on the Extrasolar Planets Encyclopaedia there is currently a reference with the rather ominous title of “Planets Formed in Habitable Zones of M Dwarf Stars Probably Lack Volatiles”. Unfortunately no link to an abstract or a paper is given.

  • Administrator December 14, 2006, 10:15

    That’s one we need to look at here. Let me know if you get an abstract.

  • Edg Duveyoung December 14, 2006, 11:01

    If I point my radio telescope at a star with a technological civilization equal to humanity’s present abilities, how would we expect that experience to differ from the same radio telescope pointing at a similar star that did not have any radiation-from-a-civilization? With our present instrumentation, how close do two “identical” stars have to be to us to determine which one is the living one?

    Seems to me that the “living” world would be putting out something quite measurably different than the “dead” world. I know that with our present instrumentation, if we saw our “I Love Lucy” radiation from even a light year’s distance, our resolving power would be too weak to differentiate from Lucy’s laughter from the normal background “noise of the universe,” but still, discovering how to define a “not normal background” seems doable, seems to be measurable property” that would be detectable. Are red dwarfs of equal size-metallicity-distance from us pretty much as indistinguishable from each other as peas in a pod? Can’t some math-giant devise an analysis that can tell the differences between the “randomness” of emissions from a dead star from the “randomness” of “white-out, blurred” living systems? What our telescopes can’t resolve, maybe our math can “smell something hinky?” Wouldn’t all dead systems have a uniformity of sorts that differs from the kinds of emissions that living systems would generate?

    And as we move up to more advanced civilizations, at some point they become “brighter” in one of our measurements, right? So, a radiologically brighter object might still be a “signal without a Rosetta Stone,” but we’d know it wasn’t “like what the dead stuff is like.” If we see a star that has the same mass, same metal content, as our sun, but it is radiologically brighter, would that be “almost certainly” a sign of life, or does our own sun have such a wide array of radiological outputs that a “life indicating blip” would be hidden in the spectra?

    These must be very amateurishly posed questions to some folks here, but I’m finding myself thinking that the DRAGNs’ noise can’t be so loud that we can’t see at least the very nearby stars “whispering” enough to indicate life. What’s wrong with my theory that “most stars are pretty quiet so a “nearby noisy-radio” should be detectable if it’s, say, in between two DRAGNs?”

    Obviously my questions have been asked by every astronomer already, and we just can’t tell a living system from a dead one yet, so let me ask how much better do our instruments have to be to begin to hear life out there? When can we expect that technological level to be achieved? Do we need radio telescopes that are are on opposite sides of the Kuiper Belt to get “enough parallax,” or what?


  • andy December 14, 2006, 13:18

    For a human-level civilisation and our present level of detection technology, they’d have to be within about 10 light years. Within that distance there are two sunlike stars (excluding our Sun itself), both of which are in the Alpha Centauri system.

    In addition, there are many other things which give out radio, so you can’t say that a radio-bright system contains an alien civilisation trying to say hello. It is often said that the best contrast between a star and orbiting planets is at infrared wavelengths. For habitable terrestrial planets this is true, but for gas giants the best contrast is obtained at radio wavelengths. There are several planned programmes to detect radio emission from extrasolar planets, since this will give us information not easily obtainable otherwise, e.g. rotation periods, possibly information on orbiting satellites (Jupiter’s radio bursts are correlated with the orbital position of Io).

  • ljk January 12, 2007, 10:45

    Astrophysics, abstract

    From: Andrea Buccino [view email]

    Date: Thu, 11 Jan 2007 13:48:13 GMT (18kb)

    UV habitable zones around M stars

    Authors: Andrea P. Buccino, Guillermo A. Lemarchand, Pablo J. D. Mauas

    Comments: 4 pages, 2 figures, letter submited to A&A

    During the last decade, there was a paradigm-shift in order to consider terrestrial planets within liquid-water habitable zones (LW-HZ) around M stars, as suitable places for the emergence and evolution of life. Here we analyze the influence of UV boundary conditions to three planetary systems around dM (HIP 74995, HIP 109388 and HIP 113020). We apply our model of UV habitable zone (UV-HZ) (Buccino et al. 2006) to these cases and show that during the quiescent UV output there would not be enough UV radiation within the LW-HZ in order to trigger biogenic processes.

    We also analyze the cases of two other M flare stars and show that the flares of moderate intensity could provide the necessary energy to trigger those biogenic processes, while the strong flares not necessary rule-out the possibility of life-bearing planets.


  • andy January 12, 2007, 18:14

    ljk: I saw that one. I don’t know how they did the LWHZ calculations but for all the cases there it seems rather close to the star… my own back-of-the-envelope calculations suggest a planet at those distances would be very toasty! Some other papers put the LWHZ of a dM star further out, in which case you’d need a bigger flare to push the UVHZ out to the LWHZ.

    Aargh too many acronyms…

  • ljk February 26, 2007, 1:08

    Astrophysics, abstract

    From: John Grenfell [view email]

    Date: Fri, 23 Feb 2007 10:37:09 GMT (1248kb)

    Biomarker Response to Galactic Cosmic Ray-Induced NOx and the Methane Greenhouse Effect in the Atmosphere of an Earthlike Planet Orbiting an M-Dwarf Star

    Authors: John Lee Grenfell, Jean-Mathias Griessmeier, Beate Patzer, Heike Rauer, Antigona Segura, Anja Stadelmann, Barbara Stracke, Ruth Titz, Philip von Paris

    Planets orbiting in the habitable zone (HZ) of M-Dwarf stars are subject to high levels of galactic cosmic rays (GCRs) which produce nitrogen oxides in earthlike atmospheres. We investigate to what extent this NOx may modify biomarker compounds such as ozone (O3) and nitrous oxide (N2O), as well as related compounds such as water (H2O) (essential for life) and methane (CH4) (which has both abiotic and biotic sources) . Our model results suggest that such signals are robust, changing in the M-star world atmospheric column by up to 20% due to the GCR NOx effects compared to an M-star run without GCR effects and can therefore survive at least the effects of galactic cosmic rays. We have not however investigated stellar cosmic rays here. CH4 levels are about 10 times higher than on the Earth related to a lowering in hydroxyl (OH) in response to changes in UV. The increase is less than reported in previous studies. This difference arose partly because we used different biogenic input. For example, we employed 23% lower CH4 fluxes compared to those studies. Unlike on the Earth, relatively modest changes in these fluxes can lead to larger changes in the concentrations of biomarker and related species on the M-star world. We calculate a CH4 greenhouse heating effect of up to 4K. O3 photochemistry in terms of the smog mechanism and the catalytic loss cycles on the M-star world differs considerably compared with the Earth.


  • andy March 23, 2007, 9:03

    The paper I mentioned earlier has just shown up on arXiv: Planets Formed in Habitable Zones of M Dwarf Stars Probably are Deficient in Volatiles

    Apparently, while Earth-mass planets should be able to form in the habitable zone of M-dwarfs, a combination of factors conspire to keep them from gaining many volatiles: first off, the temperature of a red dwarf during its pre-main-sequence phase is much more luminous than during its main sequence phase (the same is true for G-dwarfs, but the factor for M-dwarfs is much higher), so the sources of volatiles are relatively speaking much further out. Also, so close to the star, the velocity of collisions is high, making it more likely that a planet will lose volatiles rather than accrete them.

    There may be ways around this (perhaps an inward-migrating outer giant forcing volatile-rich material into the inner system), but it may be that habitable red dwarf planets are very rare.

  • Administrator March 23, 2007, 12:47

    Andy, I see you’re referring to Jack Lissauer’s paper. It’s an important one, and I’ll give it a closer look next week. We may indeed find those Mars-size planets around such stars but this paper’s implications for life are significant.