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An Icy ‘Super-Earth’

9000 light years away, a planet thirteen times as massive as the Earth orbits a star half the size of the Sun. At -330 degrees Fahrenheit, the newly discovered planet is one of the coldest worlds ever discovered. And its placement within its solar system is interesting indeed, for the icy object occupies an area where, in our system, the asteroid belt holds sway. “We’ve never seen a system like this before,” said Andrew Gould (Ohio State University, and leader of the MicroFUN collaboration, “because we’ve never had the means to find them.”

MicroFUN (MicroLensing Follow-Up Network) is exoplanetary hunting via gravitational microlensing. A star crosses in front of a far more distant one as seen from Earth. The gravity of the intervening object bends light rays from the more distant star and magnifies the image, operating much like a lens. From our observational standpoint, the image of the star brightens as the ‘lensing’ star crosses in front of it, then fades as the lens moves further away. If there are any planets around the foreground star, they can create additional distortions to the light, thus making their presence known to astronomers.

It was the OGLE (Optical Gravitational Lensing Experiment) collaboration that first found the microlensed star last April while looking toward the galaxy’s center. After gathering extensive data, Gould and OGLE leader Andrzej Udalski (Warsaw University Observatory) worked through computer models that confirmed the existence of the planet. They believe it is a ‘super-Earth’, a mixture of rock and ice with a diameter several times that of Earth. Gould’s team argues that one-third of all main sequence stars are likely to have icy super-Earths like this one.

An icy super-Earth discovered

Image: The discovery of a “super-Earth” orbiting a red dwarf star 9,000 light-years away suggests that such worlds are three times more common than Jupiter-sized planets. The 13-Earth-mass planet (shown in this artist’s conception with a hypothetical moon) was detected by a search for microlensing events, in which the gravity of a foreground star distorts the light of a more distant background star. Microlensing is the only way to detect Earth-mass planets from the ground with current technology. Credit: David A. Aguilar (CfA).

This could mean super-Earths are more common than the Jupiter-class worlds we have grown used to finding through our exoplanet hunts. “Our discovery suggests that different types of solar systems form around different types of stars,” explains Scott Gaudi (Harvard-Smithsonian Center for Astrophysics). “Sun-like stars form Jupiters, while red dwarf stars only form super-Earths. Larger A-type stars may even form brown dwarfs in their disks.”

Microlensing is remarkably promising in terms of detecting small worlds. Listen to Gaudi again:

“Microlensing is the only way to detect Earth-mass planets from the ground with current technology. If there had been an Earth-mass planet in the same region as this super-Earth, and if the alignment had been just right, we could have detected it. By adding one more two-meter telescope to our arsenal, we may be able to find up to a dozen Earth-mass planets every year.”

The name of the new world? OGLE-2005-BLG-169Lb. The paper is Gould, Udalski, Bennett, et al., “Microlens OGLE-2005-BLG-169 Implies Cool Neptune-Like Planets are Common,” now available at the arXiv site, with abstract here.

Comments on this entry are closed.

  • qraal March 16, 2006, 7:57

    Hi Paul

    How did they work out the mass and the radius of this putative object. The journal preprint wasn’t too clear or did I miss something?

    Pretty impressive work even so. And so many super-Earths are starting to come to light around other stars. Microlensing surveys will tell us a lot about exoplanet statistics, but what’ll really make Joe Public sit up and take notice is finding Earths around nearby stars. That’ll be news. A big planet 9,000 ly away is just too distant.


  • Administrator March 16, 2006, 9:03

    My understanding is that the mass and radius of the newly discovered planet depend upon the figures plugged in for the parent star, and that there is a fair degree of confidence in these. If the star is half as big as the Sun, the team’s 1000 measurements, made at the MDM Observatory in Arizona, allow enough precision to pin down the approximate mass. Impressive indeed, and as to the details, I’ll see if I can get some deeper information from the astronomers involved, and will post it here.

  • ljk June 19, 2007, 11:20

    An anomaly detector with immediate feedback to hunt for planets of Earth mass and below by microlensing

    Authors: M. Dominik (SUPA, University of St Andrews), N. J. Rattenbury, A. Allan, S. Mao, D. M. Bramich, M. J. Burgdorf, E. Kerins, Y. Tsapras, L. Wyrzykowski

    (Submitted on 18 Jun 2007)

    Abstract: (abridged) The discovery of OGLE 2005-BLG-390Lb, the first cool rocky/icy exoplanet, impressively demonstrated the sensitivity of the microlensing technique to extra-solar planets below 10 M_earth. A planet of 1 M_earth in the same spot would have provided a detectable deviation with an amplitude of ~ 3 % and a duration of ~ 12 h. An early detection of a deviation could trigger higher-cadence sampling which would have allowed the discovery of an Earth-mass planet in this case. Here, we describe the implementation of an automated anomaly detector, embedded into the eSTAR system, that profits from immediate feedback provided by the robotic telescopes that form the RoboNet-1.0 network. It went into operation for the 2007 microlensing observing season. As part of our discussion about an optimal strategy for planet detection, we shed some new light on whether concentrating on highly-magnified events is promising and planets in the ‘resonant’ angular separation equal to the angular Einstein radius are revealed most easily.

    Given that sub-Neptune mass planets can be considered being common around the host stars probed by microlensing (preferentially M- and K-dwarfs), the higher number of events that can be monitored with a network of 2m telescopes and the increased detection efficiency for planets below 5 M_earth arising from an optimized strategy gives a common effort of current microlensing campaigns a fair chance to detect an Earth-mass planet (from the ground) ahead of the COROT or Kepler missions. The detection limit of gravitational microlensing extends even below 0.1 M_earth, but such planets are not very likely to be detected from current campaigns. However, these will be within the reach of high-cadence monitoring with a network of wide-field telescopes or a space-based telescope.

    Comments: 13 pages, 4 figures and 1 table. Accepted for publication in MNRAS

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0706.2566v1 [astro-ph]

    Submission history

    From: Martin Dominik [view email]

    [v1] Mon, 18 Jun 2007 10:14:45 GMT (176kb)


  • ljk September 10, 2007, 0:57

    Looking for Super-Earths in the HD 189733 System: A Search for Transits in Most Space-Based Photometry

    Authors: Bryce Croll, Jaymie M. Matthews, Jason F. Rowe, Brett Gladman, Eliza Miller-Ricci, Dimitar Sasselov, Gordon A.H. Walker, Rainer Kuschnig, Douglas N.C. Lin, David B. Guenther, Anthony F.J. Moffat, Slavek M. Rucinski, Werner W. Weiss

    (Submitted on 7 Sep 2007)

    Abstract: We have made a comprehensive transit search for exoplanets down to ~1.5 – 2 Earth radii in the HD 189733 system, based on 21-days of nearly uninterrupted broadband optical photometry obtained with the MOST (Microvariability & Oscillations of STars) satellite in 2006. We have searched these data for realistic limb-darkened transits from exoplanets other than the known hot Jupiter, HD 189733b, with periods ranging from about 0.4 days to one week. Monte Carlo statistical tests of the data with synthetic transits inserted into the data-set allow us to rule out additional close-in exoplanets with sizes ranging from about 0.15 – 0.31 RJ (Jupiter radii), or 1.7 – 3.5 RE (Earth radii) on orbits whose planes are near that of HD 189733b. These null results constrain theories that invoke lower-mass hot Super-Earth and hot Neptune planets in orbits similar to HD 189733b due to the inward migration of this hot Jupiter. This work also illustrates the feasibility of discovering smaller transiting planets around chromospherically active stars.

    Comments: Accepted for publication in ApJ: 10 pages, 5 figures

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0709.1137v1 [astro-ph]

    Submission history

    From: Bryce Croll [view email]

    [v1] Fri, 7 Sep 2007 19:08:26 GMT (353kb)


  • ljk October 4, 2007, 21:43

    Inevitability of Plate Tectonics on Super-Earths

    Authors: Diana Valencia (1), Richard J. O’Connell (1), Dimitar D. Sasselov (2) ((1) Earth and Planetary Sciences, Harvard University, (2) Harvard-Smithsonian Center for Astrophysics)

    (Submitted on 3 Oct 2007)

    Abstract: The recent discovery of super-Earths (masses less or equal to 10 earth-masses) has initiated a discussion about conditions for habitable worlds. Among these is the mode of convection, which influences a planet’s thermal evolution and surface conditions. On Earth, plate tectonics has been proposed as a necessary condition for life. Here we show, that super-Earths will also have plate tectonics. We demonstrate that as planetary mass increases, the shear stress available to overcome resistance to plate motion increases while the plate thickness decreases, thereby enhancing plate weakness. These effects contribute favorably to the subduction of the lithosphere, an essential component of plate tectonics. Moreover, uncertainties in achieving plate tectonics in the one earth-mass regime disappear as mass increases: super-Earths, even if dry, will exhibit plate tectonic behaviour.

    Comments: 13 pages, 2 figures and 1 table; in press in ApJL

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0710.0699v1 [astro-ph]

    Submission history

    From: Diana Valencia [view email]

    [v1] Wed, 3 Oct 2007 03:01:01 GMT (850kb)