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Red Dwarfs and Planetary Anomalies

The challenge of working with a small sample of exoplanetary systems — and one tilted toward those detectible through radial-velocity methods — is that building up solid models of planet formation is tricky. I’m thinking about this in terms of the recent planetary conference at Santorini, and also recalling work performed at the University of Texas, where Michael Endl and team have looked into the relationship between planets around red dwarfs and the metallicity of their stars.

It’s an intriguing question and one that only continuing observations can nail down. Metallicity refers to the presence of elements higher than hydrogen and helium in a star’s composition, something we can determine through spectroscopic analysis. Endl and co-author Fritz Benedict, as originally noted in this post, worked with graduate student Jacob bean on a study of three dwarfs known to have planets: Gliese 876, Gliese 436 and Gliese 581, noting their lower values of metallicity compared to stars of spectral types F, G and K (our Sun is a G-type star).

Most red dwarfs studied in our surveys thus far show low metallicity, and the number of high-mass planets found around them is small. Are higher levels of metallicity necessary for gas giants to form? It sounds perfectly logical: More dust in the protoplanetary disk should encourage planetary formation, so we should expect few gas giants around red dwarfs, as seems to fit current observation. This backs the core accretion model of planet formation, in which planets build up rocky cores as they make their way through crowded protoplanetary disks, eventually becoming massive enough to begin the process of accumulating dense atmospheres.

But every exception to the rule helps us understand the rule better, and draws the comparison between it and alternatives like the disk instability model. We already have two gas giants around Gliese 876. And now the California & Carnegie team have come up with two more gas giants, both around the star GJ 317. Steinn Sigurðsson (Pennsylvania State) noted this find at the Santorini conference. GJ 317 is an M3 red dwarf some 10 parsecs from here with about a fourth the Sun’s mass. The planetary masses are 1.2 and 0.8 Jupiter masses respectively, with orbital periods of 673 and 2700 days. “Bit of a theory buster,” muses Sigurðsson, “high mass planets in wide orbits around a low mass metal poor star…”

Maybe not so much a theory buster as a theory tweaker. But that’s how we learn things, finding the anomalies and figuring out how to account for them. The result is a sounder theory that can encompass oddball worlds where we do run into them. We need a larger sample of M dwarfs to develop broader patterns for mass and metallicity, and we need time — perhaps we’re going to begin finding more gas giants in wide orbits around M dwarfs as our data accumulate. Ultimately, we’ll use such observational data to help tune up our target list for planet-finder missions looking for terrestrial worlds around such stars.

The Extrasolar Planets Encyclopedia offers what is known about GJ 317b and GJ 317c. Note that the latter is still a work in progress as radial-velocity data accumulate. The Texas work is Bean et al., “Metallicities of M Dwarf Planet Hosts from Spectral Synthesis,” Astrophysical Journal Letters 653 (December 10, 2006), L65-L68 (available online).

Comments on this entry are closed.

  • Angela July 9, 2007, 15:45

    Amazing how rapidly science is discovering new planets and extra-solar bodies out there. I follow the news eagerly.
    -Angela

  • philw July 9, 2007, 15:46

    I always read that habitable zone planets of Red Dwarfs must be tidally locked. But what prevents a 3:2 resonance rotation like Mercury has? A typical M dwarf planet with a two week ‘year’ could have a rotational peood of just several Earth days.

  • andy July 9, 2007, 16:17

    There’s also a jovian-mass planet around the red dwarf star Gliese 849, though the star in that case is metal-rich.

    What’s interesting is that, apart from the Gliese 876 system, the jovians around M-dwarfs seem to be long-period objects: short-period jovians around M-dwarfs still seem very rare.

  • andy July 9, 2007, 19:08

    We already have a gas giant around Gliese 876.

    Make that two gas giants around Gliese 876. :-)

  • Administrator July 9, 2007, 20:38

    Andy’s right, two gas giants around GL 876. I’ve just corrected the original post.

  • ljk July 10, 2007, 12:09

    Millimagnitude Photometry for Transiting Extrasolar Planetary Candidates IV: The Puzzle of the Extremely Red OGLE-TR-82 Primary Solved

    Authors: Sergio Hoyer, Sebastian Ramirez Alegria, Valentin D. Ivanov, Dante Minniti, Grzegorz Pietrzynski, Maria Teresa Ruiz, Wolfgang Gieren, Andrzej Udalski, Manuela Zoccali, Rodrigo Carrasco, Rodrigo F. Diaz, Jose Miguel Fernandez, Jose Gallardo, Marina Rejkuba, Felipe Perez

    (Submitted on 9 Jul 2007)

    Abstract: We present precise new V, I, and K-band photometry for the planetary transit candidate star OGLE-TR-82. Good seeing V-band images acquired with VIMOS instrument at ESO VLT allowed us to measure V=20.6+-0.03 mag star in spite of the presence of a brighter neighbour about 1″ away. This faint magnitude answers the question why it has not been possible to measure radial velocities for this object. One transit of this star has been observed with GMOS-S instrument of GEMINI-South telescope in i and g-bands. The measurement of the transit allows us to verify that this is not a false positive, to confirm the transit amplitude measured by OGLE, and to improve the ephemeris. The transit is well defined in i-band light curve, with a depth of A_i=0.034 mag. It is however, less well defined, but deeper (A_g=0.1 mag) in the g-band, in which the star is significantly fainter. The near-infrared photometry obtained with SofI array at the ESO-NTT yields K=12.2+-0.1 and V-K=8.4+-0.1, so red that it is unlike any other transit candidate studied before.

    Due to the extreme nature of this object, we have not yet been able to measure velocities for this star, but based on the new data we consider two different possible configurations:(1) a nearby M7V star, or (2) a blend with a very reddened distant red giant. The nearby M7V dwarf hypothesis would give a radius for the companion of R_p=0.3+-0.1 R_J, i.e. the size of Neptune.

    Quantitative analysis of near-IR spectroscopy finally shows that OGLE-TR-82 is a distant, reddened metal poor early K giant. This result is confirmed by direct comparison with stellar templates that gives the best match for a K3III star. Therefore, we discard the planetary nature of the companion. Based on all the new data, we conclude that this system is a main-sequence binary blended with a background red giant.

    Comments: 26 pages, 9 figures, ApJ accepted

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0707.1248v1 [astro-ph]

    Submission history

    From: Sergio Hoyer [view email]

    [v1] Mon, 9 Jul 2007 13:24:24 GMT (1011kb)

    http://arxiv.org/abs/0707.1248

  • george scaglione July 10, 2007, 13:04

    angela,thank you.it is amazing is it not!? i recall when the only planets we knew of where those of our own solar system! more and more it appears that the time is dwindling (i hope!) before we find intelligent alien life! i really look forward to it and i think that most people today half expect it! thank you very much your friend george

  • ljk July 18, 2007, 8:35

    A New Planet Around an M Dwarf: Revealing a Correlation Between Exoplanets and Stellar Mass

    Authors: John A. Johnson, R. Paul Butler, Geoffrey W. Marcy, Debra A. Fischer, Steven S. Vogt, Jason T. Wright, Kathryn M. G. Peek

    (Submitted on 16 Jul 2007)

    Abstract: We report precise Doppler measurements of GJ317 (M3.5V) that reveal the presence of a planet with a minimum mass Msini = 1.2 Mjup in an eccentric, 692.9 day orbit. GJ317 is only the third M dwarf with a Doppler-detected Jovian planet. The residuals to a single-Keplerian fit show evidence of a possible second orbital companion. The inclusion of an additional Jupiter-mass planet (P = 2700 days, Msini = 0.83 Mjup) improves the quality of fit significantly, reducing the rms from 12.5 m/s to 6.32 m/s. A false-alarm test yields a 1.1% probability that the curvature in the residuals of the single-planet fit is due to random fluctuations, lending additional credibility to the two-planet model. However, our data only marginally constrain a two-planet fit and further monitoring is necessary to fully characterize the properties of the second planet. To study the effect of stellar mass on Jovian planet occurrence we combine our samples of M stars, Solar-mass dwarfs and intermediate-mass subgiants. We find a positive correlation between stellar mass and the occurrence rate of Jovian planets within 2.5 AU; the former A-type stars in our sample are nearly 5 times more likely than the M dwarfs to harbor a giant planet. Our analysis shows that the correlation between Jovian planet occurrence and stellar mass remains even after accounting for the effects of stellar metallicity.

    Comments: ApJ accepted, 27 pages, 6 figures, 3 tables

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0707.2409v1 [astro-ph]

    Submission history

    From: John Johnson [view email]

    [v1] Mon, 16 Jul 2007 23:26:25 GMT (81kb)

    http://arxiv.org/abs/0707.2409

  • ljk August 16, 2007, 0:13

    UV habitable zones around M stars

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

    (Submitted on 11 Jan 2007 (v1), last revised 14 Aug 2007 (this version, v4))

    Abstract: 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.

    Comments: 17 pages, 3 figures, accepted for publication in Icarus

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:astro-ph/0701330v4

    Submission history

    From: Andrea Buccino [view email]

    [v1] Thu, 11 Jan 2007 13:48:13 GMT (18kb)
    [v2] Thu, 22 Mar 2007 20:44:35 GMT (32kb)
    [v3] Thu, 19 Jul 2007 21:24:24 GMT (27kb)
    [v4] Tue, 14 Aug 2007 20:37:09 GMT (29kb)

    http://arxiv.org/abs/astro-ph/0701330

  • ljk January 14, 2008, 10:33

    Red Dwarfs Have Teeny Tiny Habitable Zones

    Written by Fraser Cain

    As space telescopes get larger and more sensitive,
    the search for Earth-sized worlds surrounding other
    stars is about to get rolling. But astronomers are going
    to need to know where to look.

    A team of researchers are working on a survey of nearby
    stars, calculating the habitable zones around them. When
    the search begins, astronomers are going to want to study
    these regions.

    The Research Consortium on Nearby Stars (RECONS) is
    a survey using relatively small telescopes to study the
    habitable zones in the nearby stars. The team uses
    measurements of various stars brightnesses at optical
    and infrared wavelengths matched with their distances
    to get a sense of the stars’ habitability.

    After gathering together a big list of potential candidate
    stars, the researchers can then categorize stars by size
    and temperature to find ones that might harbour life.

    Full article here:

    http://www.universetoday.com/2008/01/11/red-dwarfs-have-teeny-tiny-habitable-zones/

  • ljk January 30, 2008, 13:38

    2MASS J01542930+0053266 : A New Eclipsing M-dwarf Binary System

    Authors: A.C.Becker, E.Agol, N.M.Silvestri, J.J.Bochanski, C.Laws, A.A.West, G.Basri, V.Belokurov, D.M.Bramich, J.M.Carpenter, P.Challis, K.R.Covey, R.M.Cutri, N.W.Evans, M.Fellhauer, A.Garg, G.Gilmore, P.Hewett, P.Plavchan, D.P.Schneider, C.L.Slesnick, S.Vidrih, L.M.Walkowicz, D.B.Zucker

    (Submitted on 29 Jan 2008)

    Abstract: We report on 2MASS J01542930+0053266, a faint eclipsing system composed of two M dwarfs. The variability of this system was originally discovered during a pilot study of the 2MASS Calibration Point Source Working Database. Additional photometry from the Sloan Digital Sky Survey yields an 8-passband lightcurve, from which we derive an orbital period of 2.6390157 +/- 0.0000016 days. Spectroscopic followup confirms our photometric classification of the system, which is likely composed of M0 and M1 dwarfs. Radial velocity measurements allow us to derive the masses (M_1 = 0.66 +/- 0.03 M_sun; M_2 = 0.62 +/- 0.03 M_sun) and radii (R_1 = 0.64 +/- 0.08 R_sun; R_2 = 0.61 +/- 0.09 R_sun) of the components, which are consistent with empirical mass-radius relationships for low-mass stars in binary systems. We perform Monte Carlo simulations of the lightcurves which allow us to uncover complicated degeneracies between the system parameters.

    Both stars show evidence of H-alpha emission, something not common in early-type M dwarfs. This suggests that binarity may influence the magnetic activity properties of low-mass stars; activity in the binary may persist long after the dynamos in their isolated counterparts have decayed, yielding a new potential foreground of flaring activity for next generation variability surveys.

    Comments: 8 figures and 3 tables, accepted for publication in MNRAS

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0801.4474v1 [astro-ph]

    Submission history

    From: Andrew Becker [view email]

    [v1] Tue, 29 Jan 2008 18:45:16 GMT (123kb)

    http://arxiv.org/abs/0801.4474