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Reflections on Visible Exoplanets

The images of planets around Fomalhaut and HR8799 carried more clout than I expected, with traffic to the site quadrupling when the story ran, and substantial coverage from major media outlets as well. I ran the exciting images of both stars and their companions, but because I enjoy astronomical artwork, I now want to include the visualization below, showing Fomalhaut b surrounded by a large ring of autumnal russet and gold. Note, too, the extensive debris disk surrounding the distant star. Orbiting every 872 years, Fomalhaut b lies some 2.9 billion kilometers inside that disk’s inner edge.

Credit: ESA, NASA, and L. Calcada (ESO for STScI).

Greg Laughlin (University of California, Santa Cruz) was surprised at the even-handed media treatment of HR8799, considering the brightness of Fomalhaut (‘A star with a name like a rocket’), not to mention the acknowledged skills of the Hubble Space Telescope’s media office. But while HR8799 isn’t exactly a household word, the faint object trumps Fomalhaut in sheer numbers. Imagine being able to announce that you’ve imaged not one but three planets at the same time! We’re getting better at this, and as more eyes focus on young systems bright in the infrared, we’ll doubtless be snaring more examples of multiple worlds forming around their stars.

Laughlin is an ace exoplanet hunter whose systemic site should be on your priority list, and I was glad to see that he turned his attention to the question of planetary formation around these two stars, for the new planets are gas giants that make us take another look at planet formation. A news release from the Hubble site noted this point in relation to Fomalhaut b:

The planet may have formed at its location in a primordial circumstellar disk by gravitationally sweeping up remaining gas. Or, it may have migrated outward through a game of gravitational billiards where it exchanged momentum with smaller planetary bodies. It is commonly believed that the planets Uranus and Neptune migrated out to their present orbits after forming closer to the sun and then gravitationally interacted with smaller bodies.

The conventional model (insofar as anything can be considered ‘conventional’ in this fast-moving field) of gas giant formation is that it occurs through core accretion, meaning that a massive solid core eventually builds up out of much smaller pieces. Early dust and debris gradually become rocky planetesimals which, in turn, collide and coalesce. But does this have time to happen in the distant outer reaches of a nascent solar system, a place where the disk is thinner and intense stellar radiation can cause it to dissipate quickly? Gravitational instability offers the counter-model: Instabilities in the debris disk cause quick collapse of matter into planets that can form in a matter of a few millennia.

Laughlin’s take on this relates the new planets to an interesting exoplanet detection by Bunei Sato:

Sato’s detection in early 2007 of a 7.6 Jupiter-mass planet orbiting Epsilon Tauri (2.7 solar masses) in the Hyades is probably a good example of the type of planet that’s showing up in these new images, and Eps Tau b provides good support for the case that this category of objects arose from gravitational instability. The Hyades were a tough environment for planet formation via core accretion, due to the intense UV radiation that caused the disks to lose gas quickly…

And there are ways we might firm up this model:

Remnant debris disks would be expected around young stars that had massive enough disks to trigger gravitational instability. Also, in general, the more massive the star, the more massive the disk. And finally, if the planets formed via gravitational instability, one wouldn’t expect a bias toward high metallicity. If this idea is correct, as more of these planets are imaged, there shouldn’t be a metallicity correlation with the parent star.

Both models of planet formation seem robust in particular environments. Laughlin notes that, if the planets that circle HR8799 really were formed by gravitational instability, then our own Solar System’s gas giants, which formed through core accretion, would have less in common with them than with ‘hot Jupiter’ systems like 51 Peg, where the relevant gas giant was also the result of core accretion. Odd to think we have something in common with such bizarre systems… For more, check this helpful page of links to HR8799 materials, including images and movies, that the discovery team has made available. But look fast — I have no idea how long this temporary page will be up.

Comments on this entry are closed.

  • Christopher L. Bennett November 17, 2008, 11:25

    Could the gravitational-instability model be a way around those simulations suggesting that planet accretion around Alpha Centauri A & B was unlikely? Something that would happen that quickly would seem to be less susceptible to disruption by a companion star’s gravity. And it seems to me that the simulations by Thebault et al. are based on the gradual-accretion model.

  • andy November 17, 2008, 12:14

    There are hints that Fomalhaut may have additional planets further in: the planet’s orbital velocity does not seem to match up with what would be expected if the orbit is apsidally-aligned with the dust ring. Such alignment is expected if the planet is mainly responsible for the observed ring structure. There’s also a detection of an anomalous acceleration of the star by the Hipparcos satellite, which is consistent with there being a 30 Jupiter-mass object orbiting about 5 AU from Fomalhaut (this puts it in the liquid water zone, whether that counts as “habitable” is another matter). See Chiang et al. (2008) “Fomalhaut’s Debris Disk and Planet: Constraining the Mass of Fomalhaut b From Disk Morphology”

    As for HR 8799, it seems to be a very interesting star even without the added excitement of a planetary system: it is a Lambda Bootis star, which means its metal abundances are weird (low levels of iron, but more normal levels of lighter elements such as C, N, O, S), possibly due to contamination of the stellar surface with metal-poor gas. It is also a member of the fairly recently-discovered class of Gamma Doradus variables, as far as I am able to tell it is the only known member of both categories. Add the Vega-like nature of the infrared spectrum (because of the dust disk) and the planets and you’ve got a very interesting environment.

  • ljk November 17, 2008, 13:08

    HR 8799: Discovery of a Multi-planet Star System

    Credit: C. Marois et al., NRC Canada

    Explanation: How common are planetary systems like our own Solar System? In the twelve years previous to 2008, over 300 candidate planetary systems have been found orbiting nearby stars. None, however, were directly imaged, few showed evidence for multiple planets, and many had a Jupiter-sized planet orbiting inside the orbit of Mercury.

    Last week, however, together with recent images of Fomalhaut b, the above picture was released showing one of first confirmed images of planets orbiting a distant Sun-like star. HR 8799 has a mass about 1.5 times that of our own Sun, and lies about 130 light years from the Sun — a distance similar to many stars easily visible in the night sky. Pictured above, a 10-meter Keck telescope in Hawaii captured in infrared light three planets orbiting an artificially obscured central star. The 8-meter Gemini North telescope captured a similar image.

    Each planet likely contains several times the mass of Jupiter, but even the innermost planet, labelled d, orbits out near the orbit of Neptune. Although the HR 8799 planetary system has significant differences with our Solar System, it is a clear demonstration that complex planetary systems exists, systems that could conceivable contain an Earth-like planet.

    http://antwrp.gsfc.nasa.gov/apod/ap081117.html

  • ljk November 18, 2008, 0:44

    arXiv:0811.2443Date: Mon, 17 Nov 2008 18:35:58 GMT (136kb)

    Title: MMT/AO 5 micron Imaging Constraints on the Existence of Giant PlanetsOrbiting Fomalhaut at ~13-40 AU

    Authors: Matthew A. Kenworthy, Eric E. Mamajek, Philip M. Hinz, Michael R.Meyer, Aren N. Heinze, Douglas L. Miller, Suresh Sivanandam, Melanie

    FreedCategories: astro-phComments: 4 pages, 3 figures, submitted to ApJL, uses emulateapj.sty

    A candidate < 3 Jupiter mass, extrasolar planet was recently imaged by Kalaset al. (2008) using HST/ACS at 12.7″ (96 AU) separation from the nearby (d =7.7 pc) young (~200 Myr) A2V star Fomalhaut. Here we report results from M-band(4.8 micron) imaging of Fomalhaut on 5 Dec 2006 using the Clio IR imager on the6.5-m MMT with the adaptive secondary mirror.

    Our images are sensitive to giantplanets at orbital radii comparable to the outer solar system (~5-40 AU).Comparing our 5-sigma M-band photometric limits to theoretical evolutionarytracks for substellar objects, our results rule out the existence of planetswith masses greater than 2 Jupiter masses, from ~13-40 AU and objects greaterthan 13 Jupiter masses from ~8-40 AU.

    http://arxiv.org/abs/0811.2443 , 136kb

  • ljk November 18, 2008, 0:48

    arXiv:0811.2496Date: Sat, 15 Nov 2008 14:53:09 GMT (500kb)

    Title: Diversity among other worlds: characterization of exoplanets by direct detection

    Authors: J. Schneider (LUTH), A. Boccaletti (LESIA), A. Aylward (UCL), P.Baudoz (LESIA), J. L. Beuzit (LAOG), R. Brown (STScI), J. Cho (QMUL), K.Dohlen (LAM), M. Ferrari (LAM), R. Galicher (LESIA), O. Grasset (U. Nantes),L. Grenfell (Tech. U. Berlin), O. Guyon (Subaru), J. Hough (U. HertfordS.),M. Kasper (ESO), Ch. Keller (U. Utrecht), A. Longmore (ROE), B. Lopez (OCA),E. Martin (IAC), D. Mawet (JPL/ULg), F. Menard (LAOG), B. Merin (ESAC), E.Palle (IAC), G. Perrin (LESIA), D. Pinfield (U. HertfordS.), E. Sein(Astrium), P. Shore (U. Cranfield), Ch. Sotin (JPL/U. Nantes), A. Sozzetti(INAF-OATo), D. Stam (SRON), J. Surdej (ULg), F. Tamburini (U. Padova), G.Tinetti (UCL), S. Udry (Obs. Geneve), C. Verinaud (LAOG), D. Walker(UCL/Zeeko Ltd)

    Categories: astro-ph

    Comments: Update of a White Paper submitted to the ESA ExoPlanet Roadmap Advisory Team

    The physical characterization of exoplanets will require to take spectra atseveral orbital positions. For that purpose, a direct imaging capability isnecessary. Direct imaging requires an efficient stellar suppression mechanism, associated with an ultrasmooth telescope.

    We show that before future large space missions (interferometer, 4-8 m class coronograph, external occulter orFresnel imager), direct imaging of giant planets and close-by super-Earth areat the cross-road of a high scientific interest and a reasonable feasibility.

    The scientific interest lies in the fact that super-Earths share common geophysical attributes with Earths. They already begin to be detected by radialvelocity (RV) and, together with giant planets, they have a larger area than Earths, making them detectable with a 1.5-2 m class telescope in reflected light. We propose such a (space) telescope be a first step before large directimaging missions.

    http://arxiv.org/abs/0811.2496 , 500kb

  • ljk November 18, 2008, 1:03

    arXiv:0811.2606

    Date: Sun, 16 Nov 2008 23:53:52 GMT (1190kb)

    Title: Direct Imaging of Multiple Planets Orbiting the Star HR 8799

    Authors: C. Marois, B. Macintosh, T. Barman, B. Zuckerman, I. Song, J. Patience, D. Lafreniere and R. Doyon

    Categories: astro-ph

    Comments: 30 pages, 5 figures, Research Article published online in ScienceExpress Nov 13th, 2008

    DOI: 10.1126/science.1166585

    Direct imaging of exoplanetary systems is a powerful technique that canreveal Jupiter-like planets in wide orbits, can enable detailed characterization of planetary atmospheres, and is a key step towards imaging Earth-like planets.

    Imaging detections are challenging due to the combined effect of small angular separation and large luminosity contrast between a planet and its host star. High-contrast observations with the Keck and Gemini telescopes have revealed three planets orbiting the star HR 8799, with projected separations of 24, 38, and 68 astronomical units. Multi-epoch datashow counter-clockwise orbital motion for all three imaged planets.

    The lowluminosity of the companions and the estimated age of the system imply planetary masses between 5 and 13 times that of Jupiter. This system resembles a scaled-up version of the outer portion of our Solar System.

    http://arxiv.org/abs/0811.2606 , 1190kb

  • Adam November 18, 2008, 2:19

    Several papers have appeared on the arXiv in the last few months which simulate how such planets might form. Dimitris Stamatellos and Anthony Whitworth simulated a heavy circumstellar disk which broke up through gravitational instability into a bunch of heavy planets and low mass dwarfs/stars. An earlier study by Whitworth also found that forming planets closer than 40 AU is incredibly difficult for gravitational instability as any over-densities get broken up by shockwaves before they can collapse sufficiently to form planets.

    Another paper, by a different team, suggests that “inner planets” (inside 40 AU) form via instability in the outer disk, then they perturb each other into highly eccentric orbits that pass through the dust-disk closer to the star. This slows the planets and circularises their orbits via viscous interactions and could also boost the metallicity levels of the planets. That might mean that observational tests of exoplanets won’t be able to tell which planets formed by instability or core accretion very easily.

    Also the drag of the disk would cause precession and criss-crossing interactions between the planets – even collisions, thus making the raw materials needed for forming terrestrial planets like Earth and Venus… which sounded so much like Woolfson’s Capture Theory, but it’s a totally independent finding. They don’t even cite Woolfson as a source!

    Amazing days we live in!

  • Ronald November 18, 2008, 5:48

    Christopher L. Bennett Says:
    November 17th, 2008 at 11:25
    “Could the gravitational-instability model be a way around those simulations suggesting that planet accretion around Alpha Centauri A & B was unlikely?”

    Unfortunately, I don’t think this will be a viable escape route for Alpha Centauri A & B (but I am open to any expert correction): from what I have understood, gravitational instability as a planetary formation process is relatively rare and more typical of large stars with (very) massive dust disks, typically resulting in gas giants in outer regions.

  • Ronald November 18, 2008, 9:39

    I repeat my question of 15 Nov. under thread 4266 (Exoplanet images: Two Observational Coups):

    “Further to James’s encouraging comment of November 14th at 9:48, I wonder (again) whether such direct imaging of *earthlike* planets near relatively nearby *sunlike* stars would also be theoretically possible by means of ground-based systems, rendering the more expensive and risky space-based platforms unnecessary.
    For instance something like the planned Extremely Large Telescope (ELT) combined with advanced adaptive optics.”

    Anyone?

  • Administrator November 18, 2008, 15:47

    Ronald, I think Larry has already posted a link to this, but I do draw your attention to it in hopes it may offer some useful material — I haven’t yet had the chance to get into it, so I’m not yet sure whether it does or not:

    http://arxiv.org/abs/0811.2496

    This is Schneider et al., “Diversity among other worlds: characterization of exoplanets by direct detection.” We do see remarkable things happening with Earth-based telescopes and adaptive optics, although space seems optimum for the kind of infrared work that would be required for an early Earth-like planet. If you’re talking about a visible light detection in an older system, I haven’t run into anything suggesting this would be possible from something like the ELT, but maybe others here have seen reference to such.

  • Lee Billings November 18, 2008, 17:34

    >>I repeat my question of 15 Nov. under thread 4266 (Exoplanet images: Two >>Observational Coups):
    >>
    >>“Further to James’s encouraging comment of November 14th at 9:48, I wonder >>(again) whether such direct imaging of *earthlike* planets near relatively nearby >>*sunlike* stars would also be theoretically possible by means of ground-based >>systems, rendering the more expensive and risky space-based platforms >>unnecessary.
    >>For instance something like the planned Extremely Large Telescope (ELT) >>combined with advanced adaptive optics.”
    >>
    >>Anyone?

    From what I understand, it’s theoretically possible, in the same way that it’s theoretically possible for all the molecules of gas in a room to cluster in one corner and suffocate the room’s occupants. In terms of praxis, though, the short answer seems to be, “no, we need space-based observatories to directly image Earth-like planets around Sun-like stars.”

  • ljk November 23, 2008, 23:54

    arXiv:0811.3420

    Date: Thu, 20 Nov 2008 21:13:17 GMT (161kb)

    Title: On the origin of eccentricities among extrasolar planets

    Authors: Daniel Malmberg, Melvyn B. Davies

    Categories: astro-ph

    Comments: Accepted for publication in MNRAS Letters

    Most observed extrasolar planets have masses similar to, but orbits very different from, the gas giants of our solar system. Many are much closer to their parent stars than would have been expected and their orbits are often rather eccentric.

    We show that some of these planets might have formed in
    systems much like our solar system, i.e. in systems where the gas giants were originally on orbits with a semi-major axis of several au, but where the massesof the gas giants were all rather similar.

    If such a system is perturbed byanother star, strong planet-planet interactions follow, causing the ejection ofseveral planets while leaving those remaining on much tighter and moreeccentric orbits. The eccentricity distribution of these perturbed systems isvery similar to that of the observed extrasolar planets with semi-major axisbetween 1 and 6 au.

    http://arxiv.org/abs/0811.3420 , 161kb

  • ljk February 20, 2009, 14:46

    HST/NICMOS detection of HR 8799 b in 1998

    Authors: David Lafrenière, Christian Marois, René Doyon, Travis Barman

    (Submitted on 18 Feb 2009)

    Abstract: Three planets have been directly imaged around the young star HR 8799. The planets are 5-13 Mjup and orbit the star at projected separations of 24-68 AU. While the initial detection occurred in 2007, two of the planets were recovered in a re-analysis of data obtained in 2004.

    Here we present a detection of the furthest planet of that system, HR 8799 b, in archival HST/NICMOS data from 1998. The detection was made using the locally-optimized combination of images algorithm to construct, from a large set of HST/NICMOS images of different stars taken from the archive, an optimized reference point-spread function image used to subtract the light of the primary star from the images of HR 8799.

    This new approach improves the sensitivity to planets at small separations by a factor of ~10 compared to traditional roll deconvolution. The new detection provides an astrometry point 10 years before the most recent observations, and is consistent with a Keplerian circular orbit with a~70 AU and low orbital inclination.

    The new photometry point, in the F160W filter, is in good agreement with an atmosphere model with intermediate clouds and vertical stratification, and thus suggests the presence of significant water absorption in the planet’s atmosphere.

    The success of the new approach used here highlights a path for the search and characterization of exoplanets with future space telescopes, such as the James Webb Space Telescope or a Terrestrial Planet Finder.

    Comments: ApJL, in press

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)

    Cite as: arXiv:0902.3247v1 [astro-ph.EP]

    Submission history

    From: David Lafrenière [view email]

    [v1] Wed, 18 Feb 2009 21:14:22 GMT (94kb)

    http://arxiv.org/abs/0902.3247

  • ljk March 2, 2009, 11:45

    February 28, 2009

    New Technique Allows Astronomers to Discover Exoplanets in Old Hubble Images

    Written by Ian O’Neill

    Using a new imaging technique on an 11 year old Hubble observation, an exoplanet has been discovered orbiting the young star HR 8799 (NASA/HST)

    The Hubble Space Telescope has recently provided us with some astonishing images of exoplanets orbiting distant stars. This is a departure from the indirect detection of exoplanets by measuring the “wobble” of stars (revealing the gravitational presence of a massive planetary body) or the transit of exoplanets through the line of sight of the parent star (causing its brightness to dim).

    Scientists have refined Hubble’s exoplanet hunting abilities to directly image these alien worlds in visible light. However, astronomers now have another trick to find these mysterious worlds. A new imaging technique is allowing us to see exoplanets already hiding in archival Hubble data…

    It has been estimated that another 100 previously unknown exoplanets could be discovered in old Hubble data. The technique being tested by astronomers at the University of Toronto could be a very powerful new way to reveal the existence of a huge number of buried jewels buried by the glare of star light.

    Full article here:

    http://www.universetoday.com/2009/02/28/new-technique-allows-astronomers-to-discover-exoplanets-in-old-hubble-images/