A Breakthrough Way to See Distant Planets

Astronomers have created a one-of-a-kind image of a distant exoplanet by zeroing in on the molecules in its atmosphere.

An artist's impression of an exoplanet
An artist's impression of an exoplanet (ESO / L. Benassi)

On a cloudless night, stars provide a breathtaking view: hundreds, thousands, of milky-white specks, draped over the Earth like a sparkling quilt. But stars can be annoying for astronomers trying to observe distant planets. Because stars are so luminous, it’s extremely difficult for even the most powerful telescopes to make out the tiny planets around them, says Henriette Schwarz, an astronomer at the University of California, Santa Cruz who studies planets outside of our solar system.

“Stars are millions to billions of times brighter than the planets that orbit them, and their bright glare completely outshines the faint light of these planets,” Schwarz says. Even the youngest planets, still glowing brightly from the heat of their formations, are millions of times fainter than their stars.

Since the discovery of the first planet outside of our solar system in the 1990s, astronomers have found thousands more “exoplanets” in the Milky Way, from gas giants the size of Jupiter to rocky planets 10 times the mass of Earth. They have been found through indirect means: telescopes detect an exoplanet by the effects the planet has on its parent star, like a faint dimming as it passes in front of the star, or the star’s slight wobble as the planet tugs on it. Only a handful of exoplanets have ever been observed directly. Stars are just too bright.

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.

Hoeijmakers, et al.

“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.

This method can only be used if a planet and its star differ in their chemical compositions. Some stars can be dim enough to support the existence of some of the same molecules that can be found in planetary atmospheres. And the technique only works for characterizing exoplanets, not detecting new ones. Imagine scouring telescope images of random star systems, pixel by pixel, looking for hints of molecules that may or may not indicate the presence of a planet.

That’s fine: Astronomers don’t need more techniques for detecting exoplanets. They’ve already found more than 5,000 potential worlds and verified about half of them, including 30 Earth-sized planets orbiting inside their star’s habitable zone.

What astronomers need now are powerful telescopes and crafty analyses that can dim the starlight and reveal the characteristics of some of these worlds. Perhaps someday, with the right technology, we’ll be able to see what’s floating in their atmospheres, or swimming in their seas, or scuttling across their surfaces.