Fireflies Next to Spotlights: The Direct Imaging Method
Direct imaging of exoplanets is extremely difficult and, in most cases, impossible. Being small and dim, planets are easily lost in the brilliant glare of the stars they orbit. Nevertheless, even with existing telescope technolog,y there are special circumstances in which a planet can be directly observed.
Some methods almost sound like science fiction: Using gravity as a magnifying glass, watching stars wobble at turtle-like speeds, and searching for tiny dips in starlight.
In November 2008, a group of astronomers using the Hubble space telescope announced that it had imaged a planet orbiting the star Fomalhaut. The discovery was made possible by the fact that Fomalhaut is surrounded by thick disk of gas and dust. The sharp inner edge of the disk suggested to astronomers that a planet had cleared out debris from its path, and pointed out where its orbit would be. Following up on these clues the astronomers were able to locate the planet in the Hubble images of the disk. Even so the planet, estimated at no more than twice the mass of Jupiter, might well have remained invisible were it not for the fact that it is extraordinarily bright. This led scientists to believe that it is surrounded by a ring system many times thicker and more luminous than that of Saturn.
NASA, ESA, and P. Kalas
Fomalhaut b, an eccentric directly-imaged exoplanet
This false-color composite image, taken with the Hubble Space Telescope, reveals the orbital motion of the planet Fomalhaut b. Based on these observations, astronomers calculated that the planet is in a 2,000-year-long, highly elliptical orbit. The planet will appear to cross a vast belt of debris around the star roughly 20 years from now. If the planet's orbit lies in the same plane with the belt, icy and rocky debris in the belt could crash into the planet's atmosphere and produce various phenomena. The black circle at the center of the image blocks out the light from the bright star, allowing reflected light from the belt and planet to be photographed. The Hubble images were taken with the Space Telescope Imaging Spectrograph in 2010 and 2012.
On the same day that the imaging of the planet around Fomalhout was made public, another group of astronomers announced the imaging of 3 planets orbiting the star HR 8799. Unlike Fomalhaut b, which was imaged in visible light, these planets were detected in the infrared range of the spectrum. This is because HR 8799 is a young star and the planets around it still retain some of the heat of their formation, which registers in the infrared range. Furthermore, whereas in the visible range the reflected light from the planets would be swallowed up by the brilliance of the star, the independent heat of the planets stands out far more clearly in the infrared range.
With repeat observations, astronomers can see planets move in their orbits.
HR 8799 time-lapse animation
The HR 8799 system hosts 4 super-Jupiter planets with orbital periods ranging from 40 to more than 400 years. This animation includes 7 images taken with the Keck telescope over a period of 7 years. Right-click on the video and select "loop" from the menu to make the video repeat.
There are plans for future missions and projects that would make direct imaging easier. Ground-based telescopes with adaptive optics systems obtain sharper images, helping astronomers separate planet and star light. Ground-based or spaceborne telescopes equipped with coronagraphs can block the light from the star just like you might use your hand to shade your eyes from strong sunlight, making it easier to spot planets. And missions have been proposed to fly a starshade in formation with the telescope, blocking starlight before it ever gets to the imaging instrument.
NASA / JPL-Caltech
Artist's concept of a Starshade
An artist’s depiction of the fully-deployed Starshade spacecraft (left) next to the space telescope it supports. The two spacecraft must fly in almost perfect alignment to allow the telescope to stay in the shadow created by the Starshade.
For humans there is something magical about the ability to actually see an object. "Seeing is believing," as they say, and that applies to extrasolar planets as much as for anything else.
Direct imaging can provide scientists with valuable information about the planet. In the case of Fomalhaut b, for example, the planet's interaction with the protoplanetary disk and the fact that it is invisible in the infrared provided strong limits to its mass, and its exceptional brilliance led scietists to theorize that it is surrounded by a massive ring system. In the case of HR 8799, the infrared radiation from the objects combined with models of planetary formation provides a rough estimate of the planets' mass.
Direct imaging works best for planets that orbit at a great distance from their stars so that they are not lost in the star's glare. It also works best for planetary systems that are positioned face-on when observed from Earth. This makes it complimentary to the radial velocity method, which is most effective for planetary systems positioned edge-on to Earth and planets orbiting close to their parent star.
With current observation technology, direct imaging is possible on very rare occasions. It is most likely to succeed when conditions are just right, namely when a bright planet orbits at a great distance from a nearby star. Because of these strict limitations, direct imaging is not a good candidate for large-scale surveys searching for new exoplanets.
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