What are exoplanets?
Exoplanets are planets that orbit a star other than our sun. The prefix exo comes from the Greek and means outside; these worlds are far, far outside our own solar system. Astronomers have confirmed more than 5,000 exoplanets orbiting distant stars. The existence of planetary systems other than our own had been surmised for centuries. But it wasn’t until 1992 that astronomers found the first two exoplanets orbiting a pulsar. Then came the confirmation of the first exoplanet orbiting a sunlike star in 1995.
Why didn’t we see them before? It’s because exoplanets are so far away, several light-years away at their closest. And it’s because – unlike stars – exoplanets don’t shine with their own light. Like Earth, they shine only with light reflected from their local stars. In contrast to their stars, exoplanets are exceedingly dim; even the largest drown in the light of their vastly brighter stars.
Before the first exoplanet discovery, most astronomers assumed exoplanets, if found, would resemble the planets in our solar system. The great shock has been that many exoplanets are far different, with their positions and orbits difficult to explain. If astronomers thought the solar system was in any way representative of other planetary systems out there in the galaxy, they’ve been disappointed. Our solar system may be the exception rather than the rule.
Unusual exoplanet families
As a harbinger of this realization, the very first exoplanets discovered in 1992 orbit a neutron star. In this case, it was a pulsar (a neutron star that emits beams of radio waves like a lighthouse, which may be detected from the Earth if the beams point in the right direction). Generally speaking, a neutron star is the superdense remains of the core of a massive star after it has ended its life in a supernova explosion.
It wasn’t thought possible, and it’s still not fully explained, that planets could survive such a cataclysm. Normally, the neutron stars we see as pulsars rotate with an invariance rivaling that of atomic clocks. Thus, neutron stars are some of the most accurate timekeepers in the cosmos.
Astronomers Aleksander Wolszczan and Dale Frail were trying to explain irregularities in the rotation of a particular pulsar, known as PSR B1257+12. They realized they could explain the slight variations in the star’s rotation if the gravity of two planets were pulling on it. Those planets would need to be three and four times the mass of Earth.
Historically significant as this discovery was, astronomers’ main quest in hunting exoplanets was to find one orbiting a sunlike star, not orbiting the remains of a huge star after a supernova. After all, ultimately, the quest is to find a planet like Earth, and then to find life there. Humans have always asked the question: “Are we alone in the universe?”
The radial velocity method
Finding an Earth-like planet, especially one where life resides, has been and remains the impetus for our searches and explorations of these distant worlds.
The detection of the first planet orbiting a main-sequence star like the sun came in 1995. That’s when Didier Queloz discovered a planet at least as massive as Jupiter orbiting the F-Type star 51 Pegasi, some 50 light-years from Earth. He detected it by the star’s “wobble” as an unseen planet pulled on it. For this discovery, he and colleagues Michel Mayor and James Peebles received the Nobel Prize in Physics in 2019.
In the 1990s, the available technology turned up only the largest exoplanets: those with enough gravity to induce a “wobble” in the spin of their parent stars. This method of detecting exoplanets is known as the radial velocity method, and it’s still a highly successful method for detecting exoplanets from Earth’s surface. You can read more about the radial velocity method – sometimes called Doppler spectroscopy – at this link.
The transit method
Nowadays, astronomers use another method – called the transit method, or transit photometry – with even greater success to find exoplanets. NASA’s planet-hunter spacecraft, Kepler, has discovered the most exoplanets so far, and it employs the transit method. This technique can detect smaller exoplanets. The transit method relies on the fact that, when an exoplanet crosses the face of its star as seen from Earth, it blocks the star’s light ever so slightly, dimming it. This change in brightness may only be 1%, but is nevertheless detectable with modern instruments such as those on Kepler. Read more about the transit method here.
Direct image of exoplanets
With the launch of the James Webb Space Telescope in 2022, astronomers have a new tool to help them track down exoplanets. On September 1, 2022, NASA announced that the Webb telescope had directly imaged an exoplanet for the first time. This exoplanet, HIP 65426 b, was not a new discovery. It was first found through direct imaging by the Spectro-Polarimetric High-Contrast Exoplanet Research (SPHERE) instrument in 2017. But Webb was able to look for the exoplanet and pick it up in four different filters. Read more about the direct imaging of exoplanets.
It should be noted that a famous and beloved exoplanet, Fomalhaut b – the first ever to be directly imaged – turned out not to be an exoplanet, after all, but instead a cloud of dust. Read more about the sad disappearance of Fomalhaut b.
Types of exoplanets
Twenty-five years after the discovery of the first exoplanet orbiting a sunlike star, astronomers have identified many types of planets in the exoplanet “zoo.” Some of these are listed below. See the lengthy list here for a complete classification.
- Hot Jupiters: Among the first exoplanets astronomers discovered because of their size, these are gas giant planets. They contain the mass of Jupiter or more, in close proximity to their star, and in some cases, orbiting it in just a few Earth days. Assuming such planets could not have formed in their current location, astronomers think they were born much further out and migrated inward. The study of Hot Jupiters has shed much light on the formation of the solar system.
- Super-Earths: These are planets with a mass between that of Earth and the smallest gas giants – Neptune and Uranus – in our solar system. Astronomers think the composition of such planets is largely rock rather than gas. Therefore, they’re more likely to be like our terrestrial planets. Astronomers use the term “Earth-like planets” for exoplanets that are rocky rather than gaseous and orbit in the so-called “Goldilocks Zone.” This zone is where water can exist in liquid form. “Earth-like” does not literally mean a planet is a twin of Earth, possessing an Earth-like atmosphere and possibly life.
- Mini-Neptunes: An exoplanet with up to ten Earth masses, but smaller in size than Neptune or Uranus. These are likely to be predominantly gaseous worlds.
- Ocean Worlds: These are exoplanets that contain a substantial amount of water, either as oceans on the surface or underground.
- Ice Giants: These exoplanets are made up of volatile compounds such as water, methane and ammonia, rather than the hydrogen and helium of Jupiter and Saturn, for example.
Looking for Earth’s twin
The quest for a true twin of the Earth continues. In June 2019, astronomers announced the discovery of the most Earth-like planet discovered at that time, orbiting Teegarden’s Star, a red dwarf just 12.5 light-years away. The exoplanet, Teegarden b, has a rating of 95% on the Earth Similarity Index.
But new exoplanets are turning up all the time. In fact, astronomers announced in December 2022 that they’ve discovered two possibly Earth-like worlds just 16 light-years away.
Kepler, for which so many exoplanets are named, is no longer active (although its data are still being analyzed). But the planet-hunting spacecraft TESS has been discovering planets since 2018. TESS is using citizen scientists to help it find worlds beyond our own.
In December 2019, the European Space Agency (ESA) launched the spacecraft CHEOPS to better characterize already-known exoplanets. The new generation of Earth-based telescopes such as the European Extremely Large Telescope (ELT), the world’s largest telescope, currently under construction in Chile, will be able to analyze the atmospheres of exoplanets directly and identify biosignatures such as oxygen and methane.
Thus the ancient dream of finding life elsewhere in the universe may soon be a reality. Stay tuned!
By the way, the cool video below shows all of the multi-planet systems from Kepler’s original mission as of the announcement of Kepler’s end of life: October 30, 2018. Astronomer and planet-hunter Ethan Kruse, who created this visualization using data derived from Kepler, wrote:
The systems are shown together at the same scale as our own solar system (dashed lines). The size of the orbits are all to scale, but the size of the planets are not. For example, Jupiter is actually 11 times larger than Earth, but that scale makes Earth-sized planets almost invisible (or Jupiters annoyingly large). The orbits are all synchronized such that Kepler observed a planet transit every time it hits an angle of 0 degrees (the 3 o’clock position on a clock). Planet colors are based on their approximate equilibrium temperatures, as shown in the legend.
Bottom line: Exoplanets are worlds orbiting distant stars. The history of our knowledge of exoplanets, the various types of exoplanets, how astronomers find them, and more, here.