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A Planetary Detection in Andromeda?

Gravitational microlensing has been actively employed in the search for MACHOs (Massive Astrophysical Compact Halo Objects) in the galactic halo, although with ambiguous results. The idea here is to find large, dark objects by detecting the microlensing effects they produce on stars behind them. While these dark matter studies have looked toward the Large Magellanic Cloud, we are using the same technique elsewhere in the planet hunt, finding that exoplanets can magnify the light of stars behind them in the galactic bulge, producing a clear detection.

Remember, for this kind of work, you want a dense background field of stars because the alignment needed for microlensing is obviously rare. The Magellanics are ideal, as is the galactic bulge, and so, for that matter, is M31, the Andromeda galaxy.

And if our early exoplanet work, relying on radial velocity and transit methods, has naturally produced large planets in the Jupiter class, microlensing can be quite effective at smaller scales. Now a new paper examines yet another benefit of the technique, that it works better at large distances from the source star, giving us the chance to detect planetary systems not only here in the Milky Way but in other galaxies as well.

The paper, by Gabriele Ingrosso (INFN, Italy) and colleagues, notes that at these distances, only giant stars with large radii can produce detectable microlensing events. The work is tricky because the source stars cannot be resolved by ground-based telescopes. ‘Pixel lensing’ is the name used for gravitational microlensing in such situations, and applied successfully, it can tell us much about the distribution of matter in a galaxy like M31, showing us its own dark halo objects as they cause microlensing of starlight in the background disk.


Image: The M31 galaxy may be offering up evidence of planets in its halo, if pixel lensing data can be correctly interpreted. Credit: Space Telescope Science Institute.

In fact, there have been a small number of microlensing events already detected towards M31 by two different collaborations, and planets may well be in the mix. Note this from the paper, which explains how exoplanet discoveries tie in with ongoing work on compact dark matter objects in the galactic halo (I’ve deleted internal references for brevity):

…new observational campaigns towards M31 have been undertaken… and hopefully a few planets might be detected in the future, providing a better statistics on the masses and orbital radii of extrasolar planets. It is in fact expected, and supported by observations and numerical simulations, that almost any star has at least a planet orbiting around it… In other words… the rate of single lens events towards M31 may suffer of a strong contamination of binary lensing events, most of which are expected to be due to extrasolar planets.

Planets in Andromeda? The mind boggles at the thought of detecting such, not that we don’t assume they’re present, but who would have believed our technology capable of such a reach? One anomalous pixel-lensing event has already turned up, possibly indicating a planet some six times as massive as Jupiter. It’s too early to claim a planet in Andromeda’s halo, but the candidate event labeled PA-99-N2 looks suspiciously like one, and pixel lensing itself seems destined to flag more.

The paper is Ingrosso et al., “Pixel-lensing as a way to detect extrasolar planets in M31,” accepted for publication in Monthly Notices of the Royal Astronomical Society and available online.

Comments on this entry are closed.

  • Athena Andreadis June 11, 2009, 12:05

    My bags are packed! (*smiles*)

  • James M. Essig June 11, 2009, 16:32

    This finding of excess of lensing events is truely outstanding. In order for such lensing events to significantly contaminate the observations of M-31, the number of planets must be huge. We might indeed be able to infer the ratio of planets at the threshold of microlensing detection relative to M-31 stars and compare that value with other similar phenomenon observed in simmilar galaxies to obtain a meta-type statistical study estimate of the number of planets in the observable universe. What a potentially huge number of future homes for our species, as well as a potentially huge number of distinct ETI species such large numbers of planets might imply.

  • andy June 11, 2009, 18:00

    Well the existence of the planet is not particularly surprising (after all there is quite a lot of evidence for the existence of planets in large spiral galaxies), but the detection is impressive. This paper suggests that JWST (formerly NGST) would be able to detect planets in the giant elliptical galaxy M87. Be interesting to know what the planetary systems in other types of galaxies are like.

  • Ronald June 12, 2009, 5:07

    @James: would this method also work down to the level of small, terrestrial planets, so that we can get a really good idea not only of planet abundance but also of mass distribution?

    BTW, once we manage to utilize the sun’s gravitational focal point at some 550 AU, we will be able to do some tremendous direct imaging of planets inside Andromeda (and maybe even other large galaxies).

  • James M. Essig June 12, 2009, 10:47

    Hi Ronald;

    I definately feel that microlensing can be made to work for small terrestrial planets once the sensitivity of the systems and methods of microlensing techniques are improved.

    A microlensing by a small terrestrial planet of a red dwarf star might work because of the larger signal to noise ratio afforded using a red dwarf as opposed to a red giant or even a sun like G-2 class star. Space-based telescopes with large collection areas such as might be built on the lunar surface could definately help here. Since President Obama still wants the U.S. to go back to the Moon by 2020, perhaps even by 2019, for extended stays, the impetous to develop lunar science facilities which can include large telescopes with exist.

  • spaceman June 12, 2009, 15:30

    Wow, an extragalactic planet! Underscores how microlensing is an underrated though enormously important exoplanet detection technique. I think it is a travesty that MPF has not yet been funded given this mission’s unique potential to gather statistics on how many planets down to Pluto size exist in our galaxy.

    In the meantime, here is a link to a microlensing presentation given by astronomer Joachim Wambsganss kipac-prod.stanford.edu/collab/seminars/acks/talks_spring_2009/090226/at_download/file

    in which the actual detection of an EXOMOON via microlensing is being hinted at?

    Apparently, as mentioned on page 46 of the pdf, an exomoon has been detected and they are just waiting for some reason to publish the paper describing it!

    Has anyone else heard of this? Big news if it turns out to be true….

  • andy June 13, 2009, 17:17

    spaceman writes:

    Apparently, as mentioned on page 46 of the pdf, an exomoon has been detected and they are just waiting for some reason to publish the paper describing it!

    It’s not entirely clear from that paper whether those represent actual detections, or theoretical work to determine the light curves of the various lens systems. Not holding my breath for that one.

  • ljk June 17, 2009, 9:30

    Palomar Transient Factory Captures the Andromeda Galaxy

    Using Palomar Observatory’s 48-inch Samuel Oschin Telescope, the Palomar Transient Factory captured M31, the Andromeda Galaxy:


  • ljk September 17, 2009, 12:55

    Ultraviolet Andromeda

    Credit: UV – NASA/Swift/Stefan Immler (GSFC) and Erin Grand (UMCP)
    Optical – Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF

    Explanation: Taken by a telescope onboard NASA’s Swift satellite, this stunning vista represents the highest resolution image ever made of the Andromeda Galaxy (aka M31) – at ultraviolet wavelengths.

    The mosaic is composed of 330 individual images covering a region 200,000 light-years wide. It shows about 20,000 sources, dominated by hot, young stars and dense star clusters that radiate strongly in energetic ultraviolet light.

    Of course, the Andromeda Galaxy is the closest large spiral galaxy to our own Milky Way, at a distance of some 2.5 million light-years. To compare this gorgeous island universe’s appearance in optical light with its ultraviolet portrait, just slide your cursor over the image.

    Full article here:


  • ljk November 3, 2009, 0:09


    Signs of alien worlds from long ago and far, far away

    02 November 2009

    Magazine issue 2732. Subscribe and get 4 free issues.

    WITH the discovery of planets around distant stars in the Milky Way now almost routine, it takes evidence of planets beyond our own galaxy to turn heads – and that’s what Erin Mentuch at the University of Toronto in Canada has produced.

    Mentuch analysed 88 remote galaxies whose light was emitted when the universe was between a quarter and half its current age – making them far too remote for their stars to be seen individually. The galaxies’ light output peaks at two distinct wavelengths. One represents the combined light of a galaxy’s stars; the other, at longer wavelengths, comes from the glow of its interstellar dust.

    In each case, Mentuch noticed a faint third component between the two peaks. Whatever produces this light is too cold to be stars and too warm to be dust. The most likely source is circumstellar discs – embryonic solar systems around young stars. “It’s the most surprising result I’ve ever worked on,” says Roberto Abraham, who collaborated with Mentuch.

    The opportunity to study discs that existed so long ago could help reveal how the rate of planet formation across the universe has changed over time, says Mentuch. The work will appear in The Astrophysical Journal.

    A near-infrared excess in the continuum of high-redshift galaxies: a tracer of star formation and circumstellar disks?

    Authors: E. Mentuch, R.G. Abraham, K. Glazebrook, P.J. McCarthy, H. Yan, D.V. O’Donnell, D. Le Borgne, S. Savaglio, D. Crampton, R. Murowinski, S. Juneau, R. G. Carlberg, I. Jorgensen, K. Roth, H. Chen, R.O. Marzke

    (Submitted on 7 May 2009 (v1), last revised 13 Oct 2009 (this version, v2))