The planet that took us beyond the solar system

Astronomers are always brainstorming new ways to observe exoplanets, and Schwarz and her fellow researchers recently did it in a very clever way: by making stars disappear.

The team, led by Jens Hoeijmakers, an astronomer at the University of Bern in Switzerland, collected archival images, taken by the Very Large Telescope in Chile, of a star called Beta Pictoris, located about 63 light-years from Earth. Beta Pictoris is orbited by a planet several times the mass of Jupiter, named Beta Pictoris b.

The telescope observations had captured the light coming from the Beta Pictoris system. Through a method known as spectroscopy, the astronomers split this light into different wavelengths, known as a spectrum, in the same way a prism splays light into a rainbow of colors. This process can reveal all sorts of properties about a source, including its chemical composition.

The team compared the archival images, pixel by pixel, to the known signals of four kinds of molecules: carbon monoxide, water, methane, and ammonia. A match indicated the presence of a given molecule in the star system. When the astronomers searched for methane and ammonia, Beta Pictoris b remained invisible, suggesting these molecules aren’t present in its atmosphere. When they looked for water or carbon monoxide, the planet bloomed into view.

The astronomers had teased out a direct image of an exoplanet, molded not out of light, but of the molecules drifting in its atmosphere.

“I was scrolling [through the images] and the planet just popped up,” Hoeijmakers says. Which is rarely the case with exoplanet data. “All your signals usually are very, very small, and you have to make a lot of effort to tease them out of the data. You’re lucky if you see something. But in this case, it was completely clear. It was crystal clear.”

In all four scenarios, the star showed no evidence of the four molecules, which meant it remained invisible. Here, the researchers have added a star-shaped marker to indicate its location. The nearby orb, seen as red through the lens of carbon monoxide and as blue through water, is the planet.

“The star is totally gone,” says Matthew Kenworthy, an astronomer at the Leiden Observatory who was not involved in the study. “It’s pretty spectacular.”

The “molecule maps” were published in Astronomy & Astrophysics this summer. In addition to the pretty pictures, the technique reveals some information about the conditions of the planet. The absence of all four molecules in Beta Pictoris means that the star’s temperatures are too hot to support these molecules. By the same measure, Beta Pictoris b is too hot to maintain methane and ammonia, but cool enough to support carbon monoxide and water. Cool is relative, of course. Based on this information, the astronomers estimate the planet’s temperature to be 1,700 degrees Celsius, or more than 3,000 degrees Fahrenheit.