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On Planets in the Galactic Bulge

One thing we’d like to know about exoplanets is where they are likely to be found. We’ve located more than 250 of them, but most are confined within about 650 light years. That’s very much in the local neighborhood by galactic standards — our methods have led us to nearby, bright stars. We do have a small number of planets detected through microlensing, some as far away as 6000 parsecs (about 19,500 light years), but our radial velocity detections, which form the great bulk of the current catalog, tend to be confined to relatively close higher mass stars.

Other similarities? The exoplanet host stars we know about are generally metal rich. And because they’re nearby, they’re located in the galactic disk. This leaves us with some key questions, among them whether planets are equally abundant elsewhere in the galaxy. Other issues:

  • Do planets occur with the same frequency around lower mass stars?
  • Does the presence of heavy elements favor particular parts of the galaxy for planet formation?
  • How do ‘hot Jupiters’ fit into the galactic map of planet formation?

SWEEPS field

The SWEEPS Project (Sagittarius Window Eclipsing Extrasolar Planet Search) sets out to study these questions. Using the Hubble Space Telescope and the Wide Field Camera of the Advanced Camera for Surveys, this effort has looked at approximately 180,000 F, G, K and M dwarfs in a dense stellar field in the galactic bulge. The goal is to find transits of Jupiter-size planets in an area over 27,000 light years from Earth. SWEEPS monitored this field over a seven day period in 2004. Remarkably, Hubble can work with an M dwarf having an apparent visual magnitude of 25.5 at this range and still detect planetary transits.

Image (click to enlarge): Part of the stellar field observed in the SWEEPS survey. The green circles indicate the position of 11 of the 16 host stars that have been found. Copyright : NASA/HST, ESA, K. Sahu (STScI) and the SWEEPS Science Team.

The results: SWEEPS found sixteen candidate transiting exoplanets. From the paper:

After correcting for geometric transit probability and our detection efficiency, our detections suggest that the frequency of planets in the SWEEPS field is similar to that in the local neighborhood.

The frequency of planets around low-mass stars is also similar to the frequency of planets around higher-mass stars, but given the small number statistics, the uncertainty is large which can easily be a factor of 2 or 3.

Interestingly, the project identified a new class of ultra-short period planets with orbital periods shorter than one day. The host stars for this category are all low mass, suggesting “…that planets orbiting very close to more massive stars might be evaporatively destroyed, or that planets can migrate to close-in orbits and survive there only around such old and low-mass stars.” A final SWEEPS finding: Planets seem to occur more frequently with higher metallicity even in the galactic bulge.

The paper is Sahu et al., “Planets in the Galactic Bulge: Results from the SWEEPS Project,” scheduled to appear in Extreme Solar Systems, eds. D. Fischer, F. Rasio, S. Thorsett, A. Wolszczan (ASP Conf. Series). The paper is available online.

Comments on this entry are closed.

  • andy November 28, 2007, 20:57

    As far as I can tell, this is a conference summary of a Nature paper published last year, see also an article on space.com from October 2006.

  • ljk November 30, 2007, 12:32

    Microlensing search for extrasolar planets: observational strategy, discoveries and implications

    Authors: Arnaud Cassan, Takahiro Sumi, Daniel Kubas (1. ARI Heidelberg University Germany, 2. Nagoya University Japan, 3. ESO Chile)

    (Submitted on 29 Nov 2007)

    Abstract: Microlensing has proven to be a valuable tool to search for extrasolar planets of Jovian- to Super-Earth-mass planets at orbits of a few AU. Since planetary signals are of very short duration, an intense and continuous monitoring is required. This is achieved by ground-based networks of telescopes (PLANET/RoboNET, microFUN) following up targets, which are identified as microlensing events by single dedicated telescopes (OGLE, MOA).

    Microlensing has led to four already published detections of extrasolar planets, one of them being OGLE-2005-BLG-390Lb, a planet of only ~5.5 M_earth orbiting its M-dwarf host star at ~2.6 AU. Very recent observations (May–September 2007) provided more planetary candidates, still under study, that will double the number of detections.

    For non-planetary microlensing events observed from 1995 to 2006 we compute detection efficiency diagrams, which can then be used to derive an estimate of the Galactic abundance of cool planets in the mass regime from Jupiters to Sub-Neptunes.

    Comments: 4 pages, 2 figures. To appear in the proceedings of “IAU conference 249: Exoplanets: Detection, Formation and Dynamics”, held in Suzhou, China, 22-26 Oct. 2007

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0711.4750v1 [astro-ph]

    Submission history

    From: Cassan Arnaud [view email]

    [v1] Thu, 29 Nov 2007 15:59:42 GMT (70kb,D)


  • ljk January 16, 2008, 1:42

    A High-Resolution Spectrum of the Highly Magnified Bulge G-Dwarf MOA-2006-BLG-099S

    Authors: Jennifer A. Johnson (1), B. Scott Gaudi (1), Takahiro Sumi (2), Ian A. Bond (3), Andrew Gould (1) ((1) Ohio State University, (2) Nagoya University, (3) Massey University)

    (Submitted on 14 Jan 2008)

    Abstract: We analyze a high-resolution spectrum of a microlensed G-dwarf in the Galactic bulge, acquired when the star was magnified by a factor of 110.

    We measure a spectroscopic temperature, derived from the wings of the Balmer lines, that is the same as the photometric temperature, derived using the color determined by standard microlensing techniques. We measure [Fe/H]=0.36 +/-0.18, which places this star at the upper end of the Bulge giant metallicity distribution. In particular, this star is more metal-rich than any bulge M giant with high-resolution abundances. We find that the abundance ratios of alpha and iron-peak elements are similar to those of Bulge giants with the same metallicity. For the first time, we measure the abundances of K and Zn for a star in the Bulge. The [K/Mg] ratio is similar to the value measured in the halo and the disk, suggesting that K production closely tracks alpha production. The [Cu/Fe] and [Zn/Fe] ratios support the theory that those elements are produced in Type II SNe, rather than Type Ia SNe. We also measured the first C and N abundances in the Bulge that have not been affected by first dredge-up. The [C/Fe] and [N/Fe] ratios are close to solar, in agreement with the hypothesis that giants experience only canonical mixing.

    Comments: 42 pages, 14 figures, submitted to ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0801.2159v1 [astro-ph]

    Submission history

    From: Jennifer A. Johnson [view email]

    [v1] Mon, 14 Jan 2008 21:05:27 GMT (159kb)


  • ljk January 21, 2008, 10:57

    Primordial black holes and the observed Galactic 511 keV line

    Authors: Cosimo Bambi, Alexander D. Dolgov, Alexey A. Petrov

    (Submitted on 17 Jan 2008)

    Abstract: The observed 511 keV line from the Galactic Bulge is a real challenge for theoretical astrophysics: despite a lot of suggested mechanisms, there is still no convincing explanation and the origin of the annihilated positrons remains unknown. Here we discuss the possibility that a population of slowly evaporating primordial black holes with the mass around $10^{16}$ g ejects (among other particles) low–energy positrons into the Galaxy. In addition to positrons, we have also calculated the spectrum and number density of photons and neutrinos produced by such black holes and found that the photons are potentially observable in the near future, while the neutrino flux is too weak and below the terrestrial and extra–terrestrial backgrounds. Depending on their mass distribution, such black holes could make a small fraction or the whole cosmological dark matter.

    Comments: 5 pages, 7 figures

    Subjects: Astrophysics (astro-ph); High Energy Physics – Phenomenology (hep-ph)

    Report number: WSU-HEP-0801

    Cite as: arXiv:0801.2786v1 [astro-ph]

    Submission history

    From: Cosimo Bambi [view email]

    [v1] Thu, 17 Jan 2008 22:29:16 GMT (513kb)