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Unseen Planets Around a Young Star?

If you want a glimpse of how remarkable technology continues to transform the exoplanet hunt, look no further than the Subaru telescope and its SEEDS project. SEEDS (Strategic Exploration of Exoplanets and Disks with Subaru Telescope/HiCIAO) works with data from the 8.2-meter telescope that the National Observatory of Japan runs on Mauna Kea (Hawaii). In its most recent finding, Subaru was used with the adaptive optics system HiCIAO (High Contrast Instrument for the Subaru Next Generation Adaptive Optics) to image the dust ring around HR 4796 A, a relatively young star (8-10 million years old) some 240 light years from Earth.

Working with evanescent clouds of dust and debris over these distances is no easy matter, but adaptive optics can correct for atmospheric blurring to produce images that rival the Hubble Space Telescope’s in terms of clarity. Add to that an advanced image processing technique called angular differential imaging, which suppresses the glare of the central star and enhances the light from the ring, and you wind up with images like the one below. What stands out is the offset between the center of the ring and the location of the star — probably induced by one or more massive planets — as well as the presence of finer dust that extends beyond the main body of the ring.

Image: Near-infrared (1.6 micron) image of the debris ring around the star HR 4796 A. The ring consists of dust grains in a wide orbit (roughly twice the size of Pluto’s orbit) around the central star. Its edge is so precisely revealed that the researchers could confirm a previously suspected offset between the ring’s center and the star’s location. Credit: SEEDS/NAOJ.

I use the word ‘probably’ above when referring to planets because there are other factors that could explain the offset. At this point we have been able to image almost two dozen nearby debris disks, and the paper on this work notes the factors that are conceivably involved (internal citations omitted for brevity):

The morphological appearance of resolved debris disks is predicted to be influenced by interactions between dust in the disk and nearby planets… the local interstellar medium…, recent stellar flybys and binary companions… mutual grain collisions… and interaction of dust with residual gas…. Many resolved systems exhibit all morphological structures predicted by these mechanisms… The observable morphology of resolved, optically thin debris disks is also wavelength dependent, as different bandpasses sample different grain size distributions.

It’s worth noting in this context that this is a binary system, with an M-class companion to consider. So what we have here are not confirmed planets but confirmation of earlier work that suggested an offset between the debris disk’s inner edge and the star. Christian Thalmann (University of Amsterdam) and co-authors suggest in the paper on this work that if planets do lurk in this system, next-generation high-contrast imaging facilities may well be able to detect them.

Understanding how planetary systems form from the disks that surround young stars helps us make sense of the abundant harvest we’re gathering from the exoplanet hunt. In this case, we’re seeing an effect — possible unseen planets affecting a circumstellar disk — that has shown up in computer simulations, and we can also point to the eccentric dust ring around Fomalhaut as observational evidence for the same process at work. The dust ring we do observe is most likely created by the collision of remnant planetesimals that continually replenish the disk.

The paper is Thalmann et al., “Images of the Extended Outer Regions of the Debris Ring Around HR 4796 A,” Astrophysical Journal Letters, Vol. 743, No. 1, (2011). Preprint available.


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  • andy January 8, 2012, 8:19

    The HR 4796 system seems to actually be triple: according to this paper the star 2MASS J12354893-3950245 may be a distant third member of the system. If HR 4796A does indeed have planets, this would be a different setup to all previously-known cases of planets in triple stars, with the planets around one of the stars in the close binary rather than around the distant tertiary.

  • ljk January 10, 2012, 14:44

    Exoplanet explorers

    by Jeff Foust

    Monday, January 9, 2012

    For many astronomers who study extrasolar planets, or exoplanets, the ultimate goal—at least in their professional lifetimes—is to obtain images of those planets, particularly Earth-sized worlds orbiting other stars. To do this would require a large space-based telescope, likely equipped with a mechanism such as a coronagraph or starshade to block the star’s light so that the much dimmer planets can be seen. Such a mission faces many technical obstacles, but arguably even greater fiscal ones: with NASA’s science budgets likely to be constrained for the foreseeable future, there will be limited funding available for so-called “flagship” missions.

    However, exoplanet researchers are pressing ahead with laying the scientific groundwork to support future large-scale missions by proposing the development of smaller-scale ones. Emboldened by the success of Kepler, launched in 2009 as part of NASA’s Discovery program of low-cost planetary science missions, scientists have proposed other low-cost exoplanet missions. Last fall, NASA selected five proposals under its Explorer program of low-cost space science missions for initial “phase A” studies, including two exoplanet missions; a third exoplanet mission, while not selected, did win technology development funding. With NASA planning to select in early 2013 up to two missions for full-scale development, the odds are good that another exoplanet mission will join Kepler later this decade.

    Full article here: