Direct Imaging of Planets

HR8799 direct imaging planet detections Credit: Marois et al (2010)

Today we have a guest blog from Sasha Hinkley talking about a different way of detecting exoplanets than the transit method we use at Planet Hunters. Sasha is a Sagan Postdoctoral Fellow at the California Institute of Technology in Pasadena, CA. Sasha received his PhD from Columbia University in New York City and has been involved in the direct imaging of exoplanets for several years.

In recent years, astronomers have identified hundreds exoplanets (as well as over 2000 new candidates from the Kepler mission), launching the new and thriving field of exoplanetary science.  The vast majority of these objects have been discovered indirectly by observing the variations induced in their host star’s light.  The Doppler surveys detect stellar “wobbles” induced by the planets, and provide valuable information about the orbital separations, eccentricities, as well as lower limits on the masses of companion planets. At the same time, observations of planets that transit their host star, creating a brief dimming of the stellar light, can provide fundamental data on planet radii and even some coarse information about the compositions and atmospheres of these extremely hot planets.  However, studying those objects out of reach to the Doppler and transit methods will reveal completely new aspects of exoplanetary science in great detail.

The direct imaging of exoplanets, i.e. actually obtaining an image of exoplanets, is a technique that is sensitive to massive planets at much larger orbital distances—larger than even the orbital distance of our Neptune. This technique is already providing a completeley new and complementary set of parameters such as luminosity, as well as detailed spectroscopic information.  This spectroscopic information will provide clues to the planets’ atmospheric chemistry, compositions, and perhaps may even shed light on non-equilibrium chemistry associated with these objects.   Moreover, the direct imaging of these exoplanets will allow astronomers to more fully characterize the architecture of planetary systems, especially at young ages where the radial velocity methods are hampered by the instrinsic stellar “jitter” of the stars. Observing the placement of these planetary mass companions at very young ages serves as a “birth snapshot”, lending support to various planet formation models.

The major obstacle to the direct detection of planetary companions to nearby stars is the overwhelming brightness of the host star.  For example, if our solar system were viewed from 70 light years (average for a nearby star), Jupiter would appear roughly a billion  times fainter than our Sun with a separation on the sky comparable to the size of a dime viewed from 5 miles away. As such, these planets are completely lost in the glare of their host star. The key requirement is the suppression of the star’s overwhelming brightness through precise starlight control.

Astronomers are currently overcoming this incredibly challening task through precise starlight control, using sophisticated instruments and observing strategies at the largest ground-based telescopes.  So far, astronomers have successfully obtained direct images of a handful of exoplanets, including around the stars HR 8799, Fomalhaut,and Beta Pictoris.  These studies have demonstrated that direct imaging of exoplanets is now a mature technique and may become routine using ground-based observatories. More so, we will soon see a new fleet of instruments dedicated to detailed spectroscopic characterization of planetary mass companions making these kinds of discoveries routine, initiating an era of comparative exoplanetary science.

One such instrument, the Gemini Planet Imager (GPI), has been built by a consortium of American and Canadian institutions and will be deployed to the 8 meter Gemini South telescope in 2013.  This instrument will survey several hundred, nearby young stars achieving sensitivities that will allow it to image planets with masses a few times that of Jupiter, and  gather information on the detected exoplanets’ spectrum and any polarized light they may emit.  The Europoean counterpart to GPI is the SPHERE project  at the Very Large Telescope.  This project, also in the Southern Hemisphere, will be a similar dedicated exoplanet imaging instrument with similar science goals as GPI. A pre-cursor project called Project 1640, on the Palomar 5m telescope is currently testing some of the techniques to be used by these projects and hopes to image exoplanets in the Northern Hemisphere.  These instruments will likely obtain images of dozens of exoplanets in the next several years, and reveal completely new aspects of planetary science that we could not yet have imagined.

Image Credit: Marois et al (2010)