How do astronomers find exoplanets?

Artist’s concept of a distant planet transiting in front of its star. Many exoplanets are found via the tiny dip in the star’s light that happens during planet transits. Image via SciTechDaily.

Since the TRAPPIST-1 news hit the media on February 22, 2017, exoplanets have become an even hotter topic than they already were. The 7 known planets in the TRAPPIST-1 system are only 40 light-years away, and they are ripe for exploration via Earth- and space-based telescopes. But several thousand other exoplanets – planets orbiting distant suns – are known to astronomers. The artist’s concept above is a bit misleading because it doesn’t show how very, very bright stars are in contrast to their planets. It’s this brightness of stars that makes exoplanets so hard to find. Follow the links below to learn more about how astronomers find exoplanets.

Most exoplanets are found via the transit method

Some exoplanets are found via the wobble method

A few exoplanets are found via direct imaging

A few exoplanets are found via microlensing

Artist’s concept of the TRAPPIST-1 system as viewed from Earth. Image credit to NASA/JPL-Caltech.

Most planets are found via the transit method. That was the case for the TRAPPIST-1 planets. In fact, the word TRAPPIST stands for the ground-based TRAnsiting Planets and PlanetesImals Small Telescope, which – along with the NASA’s Spitzer Space Telescope and other telescopes – helped reveal the planets in this system.

We know most exoplanets via the transit method in part because our world’s chief planet-hunter telescope – the space-based Kepler mission – uses this method. The original mission, launched in 2009, found 4,696 exoplanet candidates, of which 2,331 are confirmed exoplanets, according to NASA. Since then the extended Kepler mission (K2) has discovered more.

Transit via NASA.
Light curve of Kepler-6b. The dip represents the transit of the planet. Image via Wikimedia Commons.

How does the transit method work? A solar eclipse, for example, is a transit, occuring as the moon passes between the sun and Earth. Exoplanet transits occur when a distant exoplanet passes between its star and Earth. When a total solar eclipse takes place, our sun’s light goes from 100% to almost 0% as seen from Earth, then back to 100% as the eclipse ends. But when scientists observe distant stars in search of transiting exoplanets, a star’s light might, at most, dim by only a few percent, or fractions of a percent. Still, assuming it happens regularly as the planet orbits its star, that minute dip in a star’s light can reveal an otherwise hidden planet.

So the dip in a star’s light is handy tool for revealing exoplanets. To use it, though, astronomers have had to develop very sensitive instruments that can quantify the light emitted by a star. That’s why, although astronomers looked for exoplanets for many years, they didn’t begin to find them until the 1990s.

The light curve obtained by graphing the light of a star over time also allows scientists to deduce the tilt of an exoplanet’s orbit and its size.

Click on the name of an exoplanet to see an animated light curve here.

And note that we don’t actually see the exoplanets discovered with the transit method. Instead, their presence is inferred.

The wobble method. The blue waves have a higher frequency than the red light waves. Image via NASA.

Some planets are found via the wobble method. The second-most-used path to discovering exoplanets is via Doppler spectroscopy, sometimes called the radial velocity method, and commonly known as the wobble method. As of April 2016, 582 exoplanets (about 29.6% of the total known at the time) were discovered using this method.

In all gravitationally bound systems involving stars, the objects in orbit – in this case, a star and its exoplanet – move around a common center of mass. When an exoplanet’s mass is significant in comparison to its star’s mass, there’s the potential for us to notice a wobble in this center of mass, detectable via a shift in the star’s light frequencies. This shift is essentially a Doppler shift. It’s the same sort of effect that makes the vroom of a race car’s engine sound high-pitched as the car zooms toward you and low-pitched as the car races away.

The wobble of a star being orbited by a very large body. Image via Wikimedia Commons.

Likewise, when viewed from Earth, the slight movements of a star and its planet (or planets) around a common center of gravity affects the star’s normal light spectrum. If the star is moving towards the observer, then its spectrum would appear slightly shifted towards the blue; if it is moving away, it will be shifted towards the red.

The difference isn’t very big, but modern instruments are sensitive enough to measure it.

So when astronomers measure cyclic changes in the light spectrum of a star, they may suspect a significant body – a large exoplanet – is orbiting it. Other astronomers may then confirm its presence. The wobble method is useful only for finding very large exoplanets. Earth-like planets couldn’t be detected in this manner because the wobble caused by Earth-like objects is too small to be measured by current instruments.

Also note that, again, using this method, we don’t actually see the exoplanet. Its presence is inferred.

The star HR 87799 and its planets. Read more about this system via Wikiwand.

A few planets are found via direct imaging. Direct imaging is fancy terminology for taking a picture of the exoplanet. It’s the third-most-popular method of discovering exoplanets.

Direct imaging is a very difficult and limiting method for discovering exoplanets. First of all, the star system has to be relatively close to Earth. Next, the exoplanets in that system must be far enough from the star so that astronomers can distinguish them from the star’s glare. Also, scientists must use a special instrument called a coronagraph to block the light from the star, revealing the dimmer light of any planet or planets that may be orbiting it.

Astronomer Kate Follette, who works with this method, told EarthSky that the number of exoplanets found via direct imaging varies, depending on one’s definition of a planet. But, she said, anywhere from 10 to 30 have been discovered in this way.

Wikipedia has a list of 22 directly photographed exoplanets, but some weren’t discovered via direct imaging. They were discovered in some other way and later – via excruciatingly hard work and painstaking cleverness, plus advances in instrumentation – astronomers have been able able to obtain an image.

The microlensing process in stages, from right to left. The lensing star (white) moves in front of the source star (yellow) magnifying its image and creating a microlensing event. In the fourth image from the right the planet adds its own microlensing effect, creating the two characteristic spikes in the light curve. Image and caption via The Planetary Society.

A few exoplanets are found via microlensing. What if an exoplanet isn’t very large and absorbs most of the light received by its host star? Does that mean we’re just not able to see those?

For smaller dark objects, scientists use a technique based on an awesome consequence of Einstein’s General Relativity. That is, objects in space curve spacetime; light traveling near them bends as a result. This is analogous to optical refraction in some ways. If you put a pencil in a cup of water, the pencil appears broken because the light is refracted by the water.

Although it wasn’t proven until decades later, the famous astronomer Fritz Zwicky said as early as 1937 that the gravity of galaxy clusters should enable them to act as gravitational lenses. In contrast to galaxy clusters, or even single galaxies, though, stars and their planets aren’t very massive. They don’t bend light very much.

That’s why this method is called microlensing.

To use microlensing for exoplanet discovery, one star must pass in front of another more distant star as seen from Earth. Scientists may then be able to measure the light from the distant source being bent by the passing system. They may be able to differentiate between the intervening star and its exoplanet. This method works even if the exoplanet is very far away from its star, an advantage over the transit and wobble methods.

But, as you can imagine, it’s a difficult method to use. Wikipedia has a list of 19 planets discovered by microlensing.

Exoplanets discovered per year. Note that the two predominant discovery methods are transit and radial velocity (wobble method). Image via NASA’s Exoplanet Archive.

Bottom line: The most popular methods of discovering exoplanets are the transit method and the wobble method, also know as radial velocity. A few exoplanets have been discovered by direct imaging and microlensing. By the way, most of the information in this article comes from an online course I’m taking called Super-Earths and Life, given by Harvard. Interesting course!

March 6, 2017

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