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An exoplanet is a planet from a different star system. Most known exoplanets orbit stars within the Milky Way, but with the help of powerful space telescopes, scientists have discovered exoplanets far beyond the rim of our own galaxy.
Many types of exoplanets exist, but we often shorthand with descriptions like “super-Earth,” hot Jupiter, and mini-Neptune. But some exoplanets are so unique that they almost defy shorthand. There’s an exoplanet where molten diamond raindrops are flung from the sky by a howling maelstrom whose winds move at thousands of miles an hour.
Astronomers have discovered planets covered in ice from pole to pole and planets so close to their star that they’re baked to a crisp, with no atmosphere to speak of. Proxima Centauri, a star just over four light-years away, has at least two exoplanets confirmed by multiple instruments on Earth and in space. There’s a nearby planet with twin suns like Tatooine, a water world with a hydrogen atmosphere, and a “rogue” exoplanet with no sun at all. Scientists have found gas giants so improbably huge they should have ignited as stars, but they didn’t, and we don’t know why.
Hycean planets, like K2-18 b, are water worlds with atmospheres rich in hydrogen.
Credit: NASA, CSA, ESA, J. Olmsted (STScI)
When studying planets in our own Solar System, we can collect physical samples. To study exoplanets, scientists take readings using instruments like telescopes, spectroscopes, and interferometers—the more readings, the better our data and hypotheses. Then, they reason their way to conclusions based on the rules of physics and chemistry.
Exoplanet Characteristics
Telescopes use optical lenses to capture light from the targets they observe. Some modern telescopes, like the JWST and the upcoming Nancy Grace Roman telescope, record their sky views with a grid of sensors called a charge-coupled device (CCD). The same sensor technology powers the digital camera on NASA’s Ingenuity Mars helicopter and the one in your phone.
Spectroscopes, like the NIRSpec on Webb, tell us about what’s in the atmosphere based on the characteristics of the light reflected from a planet. Different types of molecules have different shapes, so they reflect light differently. The special prismatic lens of a spectroscope splits up that light into a spectral “fingerprint” that scientists can use to identify molecules at great distances.
Spectroscopes can also help to confirm or clarify our readings of a planet’s surface temperature. For example, many molecules behave differently at different pressures and temperatures. Nitrogen gas reflects light in a way that’s subtly different than nitrogen ice because of the way the molecules are arranged. Nitrogen ice, in turn, differs from carbon dioxide ice (“dry ice”). This is how we can tell what comets are made of while they’re still much too far from Earth to be seen by anything but a high-powered space telescope. It’s also how we know an exoplanet has nitrogen ice on its surface versus nitrogen gas.
The way a planet makes its parent star wobble also says something about the planet’s mass. Giant, high-mass planets exert more influence on a given star. The effect is even more pronounced when a large planet orbits very close to its star. These planets are the ones we call “hot Jupiters.”
A planet’s mass, taken together with its physical size, tells us the planet’s density. Density, in turn, tells us a lot about what a planet must be made of. How do we distinguish between a cold super-Earth and a warm mini-Neptune? Neptune, an ice giant (a cold gas giant with a rocky, icy core), has a density less than water’s. In contrast, as a rocky planet, the Earth’s density is more than five times the density of water. This helps scientists make sense of the readings we get from spectroscopes and the light that hits a telescope’s CCD.
How Are Exoplanets Found?
There are many methods of finding exoplanets, but a few approaches dominate the field. The “transit” method (short for transit photometry) is useful for finding star systems with planets that orbit “edge-on” with respect to Earth because, from our vantage point, the planets pass directly in front of their star. When they do, they dim their star’s light, and we can pick it up with telescopes.
In 2001, astronomers with the Hubble telescope announced the discovery of the first exoplanet ever confirmed to have an atmosphere. About 150 light-years away, a yellow, Sun-like star is visible through amateur telescopes, dubbed HD 209458. That star has an exoplanet, a gas giant, and here’s how we know. As the exoplanet passed in front of its star, the starlight changed. Why? It had to filter through the planet’s atmosphere to reach our telescopes. Traces of sodium in its nitrogen atmosphere left a telltale mark on the spectral characteristics of the yellow star’s light.
Star systems that don’t have an orbital plane oriented edge-on toward Earth lend themselves to a tactic called the “wobble” method, also known as the radial velocity method. The mass of an orbiting planet tugs on its star, making the star appear to wobble slightly in the sky. (This happens in our star system. Jupiter is so massive that it drags the Sun around, detectably, in a little circle whose diameter is just larger than the Sun’s actual diameter.)
An illustration of newly discovered exoplanet LP 791-18 d
Credit: NASA’s Goddard Space Flight Center/Chris Smith
The radial velocity method is best for finding exoplanets that are relatively nearby because it requires us to be able to resolve relatively fine features on the star. For this method, we need to see more detail than just a single pixel of light. The way planets tug on their stars doesn’t just move the star physically about a common center; it also subtly affects the speed at which the star spins. One side of a star will be approaching Earth, while the other side will be rotating away from us. Consequently, light from different regions of a star will be Doppler shifted toward the blue or the red, depending on whether the star is coming toward us or moving away. Gravitational pull from an orbiting planet changes the radial velocity of the star’s rotation, which telescopes like Webb can detect.
Exoplanets also reflect light that’s Doppler shifted according to how the planet rotates. This is how we can tell if an exoplanet is tidally locked, with one side facing toward its star and one side facing away. This is how we learned about the newly discovered exoplanet LP 791-18 d, pictured above. LP 791-18 d (the fourth planet in its system) is a rocky exoplanet whose mass is just a little less than Earth’s. It’s tidally locked to its star, which means that while one side is always a frozen wasteland, the other is always a searing hellscape.
Credit: NASA
Direct imaging is another important method of finding exoplanets relatively close to Earth. For example, in October 2013, scientists using the Pan-STARRS terrestrial telescope detected PSO J318.5-22 via direct imaging. Just 80 light-years away, it’s a rogue planet that doesn’t orbit a star. (All nightlife, all the time.) And yet, this inhospitable world has clouds of molten iron droplets. Where did it come from? How does it stay so hot? We simply don’t know yet.
Yet.
Proxima b: The Closest Exoplanet
The closest exoplanet to Earth is just over four light-years away, in a star system called Proxima Centauri. Proxima Centauri has at least two confirmed exoplanets, and one of them is of great interest to researchers because it’s in the system’s so-called Goldilocks zone.
Every star has an orbital zone beyond which, even in ideal circumstances, not enough solar radiation can reach a planet’s surface for its water to stay liquid. The region of space that can sustain liquid water on a planet’s surface is sometimes referred to as the habitable zone or the Goldilocks zone—because, like Goldilocks’ porridge, it’s not too hot, not too cold. Just right. This means, in turn, that the planet could harbor alien life.
However, Proxima Centauri is a red dwarf, a type of star known for violent outbursts of X-rays and gamma rays that could easily strip away a planet’s gaseous atmosphere. Unlike the Sun, a relatively even-tempered star midway through its life, red dwarf stars may have a habitable zone that’s too close to the star to be safe from its radiation. Even relatively low-energy ultraviolet flares could mean no liquid water would persist on the surface because any atmosphere would have been driven off long ago.
Proxima b is too far away for us to visit on human timescales, at least with the sub-lightspeed technology currently available to us. (Sadly, no warp drives just yet.) For a sense of scale, it would take the Voyager probes about 75,000 years to make it to Proxima b. But that doesn’t mean we can’t point our telescopes at the closest exoplanet in the entire sky that isn’t gravitationally bound to our own star. Breakthrough Listen, and Breakthrough Starshot are research initiatives aimed at observation and, eventually, visiting Proxima b. One day, we may have the most direct evidence we can get: observations by human visitors on an exoplanet. One small step for man; one giant leap for mankind.