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Planets in the Galactic Bulge

“Hot Jupiters?” read a recent e-mail. “Who cares about hot Jupiters? What I’m after are planets like Earth, rocky terrestrial worlds.” My correspondent probably feels the same way today, after NASA’s announcement of sixteen new extrasolar planet candidates, all of which fall into the hot Jupiter category and some into an even more bizarre niche — Ultra-Short-Period Planets, or USPPs. One of these star-hugging worlds, called SWEEPS-10, orbits its parent star once every ten hours.

But not so fast. Sure, getting those first terrestrial world detections, presumably through the transit method, is going to be phenomenal, but the steps leading up to that breakthrough are hugely significant. The results announced today didn’t involve nearby stars but were focused on 180,000 stars in the Milky Way’s central bulge, fully 26,000 light years from Earth. And what can be extrapolated from these sixteen planets is that planet-formation isn’t a local phenomenon limited to regions out in the spiral arms.

No, even as we move toward galactic center, we’re still seeing planets. Which is why Kailash Sahu (Space Telescope Science Institute), team leader for the project, said that the Hubble results provided “…very strong evidence that planets are as abundant in other parts of the galaxy as they are in our solar neighborhood.” The area studied with Hubble’s Advanced Camera for Surveys was a field in Sagittarius no bigger in angular size than two percent of the area of the full Moon. Push those numbers out and, for these data alone, you get six billion Jupiter-sized planets in the galaxy at large.

Hubble transit search

Image: This is an image of one-half of the Hubble Space Telescope field of view in the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS). The green circles identify 9 stars that are orbited by planets with periods of a few days. Planets so close to their stars with such short orbital periods are called “hot Jupiters.” These are considered “candidate” exoplanets because most of them are too faint to allow for spectroscopic observations that would allow for a precise measure of the planet’s mass. The bottom frame identifies one of two stars in the field where astronomers were able to spectroscopically measure the star’s back-and-forth wobble due to the pull of the planet. The planet turns out to be less than 3.8 Jupiter masses. Credit: NASA, ESA, K. Sahu (STScI) and the SWEEPS science team.

And with 24 separate microlensing projects underway in various parts of the world, we’re going to learn much more about planetary systems in the direction of the galactic bulge; the suspicion here is that the six billion figure will turn out to be conservative.

One of the limits of the transit method employed with Hubble is obtaining follow-up mass measurements for such distant objects. Yes, you can see the dimming of starlight caused by the transit, but the SWEEPS team had to rely on the European Southern Observatory’s Very Large Telescope in Chile to make mass measurements using radial velocity techniques. Only two of the sixteen stars in question are bright enough to allow such measurements.

What we do find with the VLT measurements is that one of the planetary candidates has a mass below 3.8 Jupiter masses, while the other is at 9.7 Jupiter masses. And that’s about it — we may not know much yet about these planetary systems, but simply spotting them in these precincts is a reassuring finding that the galaxy houses planets across its range. And keep in mind that we can only use the transit method when a planet crosses the face of its star as seen from Earth.

And how about the big surprise? Five of the new planets represent the hitherto unknown Ultra-Short-Period category, orbiting their stars in less than an Earth day. We see no USPPs in the local neighborhood but the Hubble data suggest they need tiny red dwarf stars, much smaller and cooler than the Sun, to exist. Such close migrations would surely disrupt gas giant planets around larger, hotter stars.

A final thought that we occasionally kick around here: The new work shows planet candidates that appear around stars abundant in elements heavier than hydrogen and helium. Thus the metallicity question, in this light, comes back to stars rich in heavy elements (i.e., of high metallicity) being the breeding ground for planetary systems. How that metallicity correlation holds up as we continue to push the extrasolar hunt deeper into the Galaxy will be fascinating to see.

The abstract of Sahu et al., “Transiting extrasolar planetary candidates in the Galactic bulge,” Nature 443 (5 October 2006), pp. 534-540 is available here.

Comments on this entry are closed.

  • Jyril October 4, 2006, 15:56

    Really cool discovery! I can’t wait to see the actual paper and the data on the planets. I’d like to know what methods they used to discount a transiting object as a planet. Only thing they mentioned about the constraints was that the maximum diameter allowed was 1.3 times Jupiter’s radius. I would also like to know if there’s more potential planets than the 16 mentioned.

    The discovery team certainly deserves respect for being cautious on their findings. We have heard way too often over-optimistic news releases (remember the infamous TMR1-C–it looked like a slingshotted planet).

    It now seems obvious that a star’s metallicity and its probability to have Jovian planets correlate strongly. It is hardly surprising given that planets need heavier elements to form.

    However, there are planets around low-metallicity stars (for example, HD 4308 ¹ and Gliese 581 ²) — and interestingly, they have low masses. So perhaps lower-mass planets, including terrestrials, are common around metal-poor stars but we have so far lacked the technology to detect them. If metal-poor stars have less massive protoplanetary disks, perhaps their planets have smaller probability to migrate which would make their detection even more difficult.

    ¹ http://exoplanet.eu/planet.php?p1=HD+4308&p2=b
    ² http://exoplanet.eu/planet.php?p1=Gl+581&p2=b

  • Administrator October 4, 2006, 16:13

    The abstract is just now up on the Nature site; I’ve added the link to the original post.

  • Jyril October 4, 2006, 16:52

    The paper is now available to subscribers.

    Here are the extremes:

    Period (d): 0.424 (SWEEPS-10) — 4.200 (#4)
    Stellar mass (Ms): 0.44 (#10) — 1.24 (#4)
    Stellar radius (Rs): 0.41 (#10) — 1.45 (#11)
    Planet radius (Rj): 0.78 (#13) — 1.40 (#16)
    Orbital radius (AU): 0.008 (#10) — 0.055 (#4)

    Three of the planets have radii over 1.3 Rj, and one less than 0.8 Rj. Of the 14 previously known transiting planets three are similarly anomalously large and one anomalously small. Coincidence?

  • Adam October 4, 2006, 17:14

    Roasting Jupiters seems rather easy to do for stars – as one notable scientist famously quipped “Who ordered that?” I’ve got to wonder what it all means for terrestrial planets, especially the metallicity relationship. The more data the better as there are too many mysteries in all this.

  • Zen Blade October 5, 2006, 11:43

    Do these most recent findings affect the “galactic census” previously mentioned here and elsewhere?

    -Zen Blade

  • Administrator October 5, 2006, 14:53

    It looks like these findings get six billion Jupiter-class worlds in the Galaxy at large while the Reid study (galactic ‘census’) comes up with 35 million planetary systems around Sun-like stars within a ring of space defined as roughly 20000 to 32000 light years from the center of the Milky Way. Note that Reid focuses on Sun-like stars and talks only of planetary ‘systems,’ while the SWEEPS findings are looking at gas giants alone. I think the sample is still way too small to consider these anything more than steep extrapolations.

  • ljk October 9, 2006, 9:20

    Astrophysics, abstract

    From: Kailash C. Sahu [view email]

    Date: Wed, 4 Oct 2006 02:35:11 GMT (603kb)

    Transiting extrasolar planetary candidates in the Galactic bulge

    Authors: Kailash C. Sahu, Stefano Casertano, Howard E. Bond, Jeff Valenti, T. Ed Smith, Dante Minniti, Manuela Zoccali, Mario Livio, Nino Panagia, Nikolai Piskunov, Thomas M. Brown, Timothy Brown, Alvio Renzini, R. Michael Rich, Will Clarkson, Stephen Lubow
    Comments: To appear in October 5, 2006 issue of Nature

    More than 200 extrasolar planets have been discovered around relatively nearby stars, primarily through the Doppler line shifts owing to the reflex motions of their host stars, and more recently through transits of some planets across the face of the host stars. The detection of planets with the shortest known periods, 1.2 to 2.5 days, has mainly resulted from transit surveys which have generally targeted stars more massive than 0.75 M_sun. Here we report the results from a planetary transit search performed in a rich stellar field towards the Galactic bulge. We discovered 16 candidates with orbital periods between 0.4 and 4.2 days, five of which orbit stars of 0.44 to 0.75 M_sun. In two cases, radial-velocity measurements support the planetary nature of the companions. Five candidates have orbital periods below 1.0 day, constituting a new class of ultra-short-period planets (USPPs), which occur only around stars of less than 0.88 M_sun. This indicates that those orbiting very close to more luminous stars might be evaporatively destroyed, or that jovian planets around lower-mass stars might migrate to smaller radii.


  • ljk October 9, 2006, 10:29

    Astrophysics, abstract

    From: Jason Steffen [view email]

    Date: Thu, 5 Oct 2006 17:33:43 GMT (239kb)

    A limit on the presence of Earth-mass planets around a Sun-like star

    Authors: Eric Agol (University of Washington) Jason H. Steffen (Fermilab)

    Comments: Accepted for publication in MNRAS. 16 pages, 9 figures

    Report-no: FERMILAB-PUB-06-279-A-CD

    We present a combined analysis of all publicly available, visible HST observations of transits of the planet HD 209458b. We derive the times of transit, planet radius, inclination, period, and ephemeris. The transit times are then used to constrain the existence of secondary planets in the system. We show that planets near an Earth mass can be ruled out in low-order mean-motion resonance, while planets less than an Earth mass are ruled out in interior, 2:1 resonance. We also present a combined analysis of the transit times and 68 high precision radial velocity measurements of the system. These results are compared to theoretical predictions for the constraints that can be placed on secondary planets.


  • ljk November 27, 2007, 0:47

    Planets in the Galactic Bulge: Results from the SWEEPS Project

    Authors: Kailash C. Sahu, Stefano Casertano, Jeff Valenti, Howard E. Bond, Thomas M. Brown, T. Ed Smith, Will Clarkson, Dante Minniti, Manuela Zoccali, Mario Livio, Alvio Renzini, R. M. Rich, Nino Panagia, Stephen Lubow, Timothy Brown, Nikolai Piskunov

    (Submitted on 26 Nov 2007)

    Abstract: The exoplanets discovered so far have been mostly around relatively nearby and bright stars. As a result, the host stars are mostly (i) in the Galactic disk, (ii) relatively massive, and (iii) relatively metal rich. The aim of the SWEEPS project is to extend our knowledge to stars which (i) are in a different part of the Galaxy, (ii) have lower masses, and (iii) have a large range of metallicities.

    To achieve this goal, we used the Hubble Space Telescope to search for transiting planets around F, G, K, and M dwarfs in the Galactic bulge. We photometrically monitored 180,000 stars in a dense bulge field continuously for 7 days.

    We discovered 16 candidate transiting extrasolar planets with periods of 0.6 to 4.2 days, including a new class of ultra-short period planets (USPPs) with P less than 1.2 days. Radial-velocity observations of the two brightest candidates support their planetary nature.

    These results suggest that planets are as abundant in the Galactic bulge as they are in the solar neighborhood, and they are equally abundant around low-mass stars (within a factor 2). The planet frequency increases with metallicity even for the stars in the Galactic bulge. All the USPP hosts are low-mass stars, suggesting either that close-in planets around higher-mass stars are irradiatively evaporated, or that the planets can migrate to close-in orbits only around such old and low-mass stars.

    Comments: To appear in “Extreme Solar Systems,” eds. D. Fischer, F. Rasio, S. Thorsett, A. Wolszczan (ASP Conf. Series). 8 pages, 5 figures

    Subjects: Astrophysics (astro-ph)

    Report number: STScI E-print #1788

    Cite as: arXiv:0711.4059v1 [astro-ph]

    Submission history

    From: Kailash C. Sahu [view email]

    [v1] Mon, 26 Nov 2007 20:28:41 GMT (589kb)


  • ljk August 6, 2009, 11:03

    Sub-Saturn Planet MOA-2008-BLG-310Lb: Likely To Be In The Galactic Bulge

    Authors: Julia Janczak (Ohio State), A. Fukui (Nagoya), Subo Dong (Ohio State), B. Monard (Bronberg Obs.), Szymon Kozlowski (Ohio State), A. Gould (Ohio State), J.P. Beaulieu (IAP), Daniel Kubas (IAP), J.B. Marquette (IAP), T. Sumi (Nagoya), I.A. Bond (Massey), D.P. Bennett (Notre Dame), the MOA collaboration, the MicroFUN collaboration, the MiNDSTEp collaboration, the PLANET collaboration

    (Submitted on 4 Aug 2009)

    Abstract: We report the detection of sub-Saturn-mass planet MOA-2008-BLG-310Lb and argue that it is the strongest candidate yet for a bulge planet. Deviations from the single-lens fit are smoothed out by finite-source effects and so are not immediately apparent from the light curve.

    Nevertheless, we find that a model in which the primary has a planetary companion is favored over the single-lens model by \Delta\chi^2 ~ 880 for an additional three degrees of freedom. Detailed analysis yields a planet/star mass ratio q=(3.3+/-0.3)x10^{-4} and an angular separation between the planet and star within 10% of the angular Einstein radius. The small angular Einstein radius, \theta_E=0.155+/-0.011 mas, constrains the distance to the lens to be D_L>6.0 kpc if it is a star (M_L>0.08 M_sun).

    This is the only microlensing exoplanet host discovered so far that must be in the bulge if it is a star. By analyzing VLT NACO adaptive optics images taken near the baseline of the event, we detect additional blended light that is aligned to within 130 mas of the lensed source.

    This light is plausibly from the lens, but could also be due to a companion to lens or source, or possibly an unassociated star. If the blended light is indeed due to the lens, we can estimate the mass of the lens, M_L=0.67+/-0.14 M_sun, planet mass m=74+/-17 M_Earth, and projected separation between the planet and host, 1.25+/-0.10 AU, putting it right on the “snow line”. If not, then the planet has lower mass, is closer to its host and is colder.

    To distinguish among these possibilities on reasonable timescales would require obtaining Hubble Space Telescope images almost immediately, before the source-lens relative motion of \mu=5 mas yr^{-1} causes them to separate substantially.

    Comments: 36 pages, 8 figures, Submitted to ApJ

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Galaxy Astrophysics (astro-ph.GA)

    Cite as: arXiv:0908.0529v1 [astro-ph.EP]

    Submission history

    From: Julia Janczak [view email]

    [v1] Tue, 4 Aug 2009 22:11:00 GMT (579kb)