Hunting for Exoplanets: Direct Imaging

Posted By on Jul 20, 2016 |


A strange and new extrasolar system was discovered by graduate research fellow and Project EOS collaborator Kevin Wagner, principal investigator for Project EOS Daniel Apai, and assistant professor of astronomy at the University of Arizona Kaitlin Kratter, announced July 7, 2016 in a paper published in the journal Science.

The system contains a total of three stars. Two stars, one sun-like in character and the other less massive, orbit around one another while also circling a much larger star many times more massive than our sun. Tucked in between this chaotic dance of stars is a giant gas planet. The planet, HD 131399Ab, was discovered using direct imaging techniques by the Spectro-Polarimetric High-Contrast Exoplanet Research (SPHERE) instrument on the Very Large Telescope in Chile.

Hunting for exoplanets is no easy task. Quite a bit of detective work goes into teasing layers of evidence out of data collected on an extrasolar system. Different hunting techniques work better for some systems than others. When possible, a combination of techniques is used to collect as much information as possible. HD 131399Ab was discovered using direct imaging techniques.

What is direct imaging and what are the many ways it can be used?

Star detectives

Astronomers begin the search by calling in the star detectives. “We need a variety of expertise in this game, people who know how to find young stars, they are crucial,” said Dr. Travis Barman, Project EOS co-investigator and associate professor at the University of Arizona.

Astronomers are most interested in young stars because they host young planets, and young planets are easier for astronomers to detect, according to Dr. Daniel Apai, principal investigator for Project EOS and co-discoverer of HD 131399Ab. This is because young planets recently formed from accretions of gas and dust from the protoplanetary disk, making them hotter than older planets.

Hotter objects glow brightly in infrared light, light just below the visible range, making these objects easier to detect than cooler objects like billion-year-old-Jupiter. Exoplanets are too hard to see using visible light because stars greatly outshine them.

Young stars are much rarer in the galaxy than old stars, so looking around these stars helps to narrow down the sample size.

Direct Imaging

Direct imaging is exactly what it sounds like: Pictures of planets and their stars are taken using extremely sensitive cameras on telescopes. The best candidates for direct imaging are stars that are near to Earth, planets that are planets, that are big and bright, and that have large orbits around stars that are not too far from the Sun. That’s why HD 131399Ab was so surprising. It’s one of the coolest and one of the least massive planets to be discovered using direct imaging.

Fine-tuning the light

Adaptive optics (AO) systems are a powerful tool used for directly imaging planets. The twinkling of stars is caused by rapidly changing currents in the atmosphere. AO systems remove the twinkling halo of starlight and focusing it back into the star. Computers manipulate small mirror actuators that act like pistons which mimic and cancel out movement in the atmosphere that cause the twinkling. What’s produced is a clearer image.

To hear Dr. Travis Barman explain adaptive optics listen here:

Watch this YouTube video for an example of adaptive optics in action!

Making the stars do the work

Proper motion is one important part of the direct imaging technique. Imagine  “driving down the road, the fence posts are going like mad but the mountains out in the distance are barely moving” Barman said in an exaggerated example. The fence has a high relative proper motion and the mountains—low. Stars that are closer to Earth, like the fence, will have a high proper motion, which is useful for finding planets.

First, astronomers take a picture of a star and a companion faint dot they suspect to be a planet. Then years, sometimes months, later they take second picture. If the faint dot has stayed still, and the star moved, then the dot was probably a background star, not a planet. If the dot moved along with the star then they’ve found a planet!

Sifting through the noise

Images taken using telescopes with adaptive optics can be improved even further when hundreds, and even thousands, of images are combined and cleaned up using statistical tricks, according to Apai.

Raw images taken with long exposures are stacked one on top of each other. It’s extremely difficult to distinguish an extrasolar planet from the glaring starlight, called speckles.

To tease out the information that’s important, astronomers average out the glare from the stars, called speckles, and then subtract it from the images. This creates a clearer picture allowing dimmer planets to be seen. This technique is called angular differentiation imaging (ADI).

There’s more to learn

Direct imaging is ideal because it can help uncover information about the planet (something that will be explored more in the next article), but the candidates are few, so to date, HD 131399Ab is only one of a few planets have been directly imaged. “One very special aspect of direct imaging is that it is currently the best, most direct, way to learn about giant planet formation, because we are studying planets at their very youngest,” Barman said.

Follow up studies of HD 131399Ab can tell us about its atmospheric properties. The discovery of this planet has pushed the boundaries of direct imaging. So far, it’s the coldest, and least massive planet found using this technique.

Listen here to find out more on why giant telescopes are needed to investigate extrasolar planets from Dr. Travis Barman.