New Tool Allows Astronomers to Directly Observe Exoplanets

In the two decades since the discovery of the first exoplanet in 1995, another 3500 exoplanets have been found ranging in size from double Earth's moon to almost twice the size of Jupiter. While this "exoplanet bonanza" is undoubtedly exciting, astronomers know remarkably little about the exoplanets being discovered.

However our exoplanet ignorance may soon be over thanks to CHARIS, a new instrument for Hawaii's Subaru telescope which recently allowed astronomers to directly observe large exoplanets 50-light years away. By isolating light reflecting off of these exoplanets, CHARIS will provide insight into the planets' size, age, and atmosphere—details that were difficult if not impossible to obtain with previous methods of exoplanet observation.

Prior to the early 2000s, astronomers seeking exoplanets almost exclusively relied on Doppler spectroscopy for their discoveries. This method indirectly detects exoplanets by measuring shifts in a star's light spectrum, which slightly changes as the star responds to the gravitational pull of an orbiting planet. To date, this method accounts for roughly 30 percent of all exoplanet discoveries, but gives astronomers almost no information about the planet itself beyond its minimum mass.

Over the last decade, the exoplanet detection method à la mode has become transit photometry. This method involves measuring changes in a star's brightness—when an exoplanet passes in front of its host star, the brightness of that star drops for the observer on Earth. This method reveals the size of the planet, which can be combined with measurements of the planet's mass via Doppler spectroscopy to determine the density of the planet and thus some aspects of its physical structure.

Of course the crown jewel of exoplanet detection is direct observation, which is what makes the CHARIS announcement so exciting. The first direct detection of an exoplanet happened in 2015, when astronomers at Chile's La Silla Observatory directly observed Dimidium, the first exoplanet ever discovered. This direct observation was surprising, given that the telescope at La Silla was optimized for Doppler spectroscopy.

CHARIS, on the other hand, was built for direct observation. The instrument sits inside a large 500 pound case kept at -370 F and is comprised of nine mirrors, five filters, two prism assemblies and a microlens array. CHARIS sits behind another device called a coronagraph, which the astronomers use parse the light that is coming directly from the star versus the light that is being reflected off the planet. This is a remarkably subtle difference, given how dim a planet's reflection is compared to its host star, and the researchers liken it to measuring "the light reflecting from a speck of tinsel floating in front of a spotlight hundreds of miles away."

The first direct exoplanet observations using CHARIS was of the star system HR 8799, which is located about 130 light years from earth and host to four planets that are all larger than Jupiter. These planets were discovered back in 2008 and were the first whose orbital motion was confirmed through direct imaging.

Following its successful test run, CHARIS will become available to astronomers at the Subaru telescope starting in February, who will use it to directly observe previously discovered exoplanets and potentially discover new ones.