Exoplanets are worlds that orbit other stars.

Exoplanets are worlds that orbit distant stars. Astronomer Nestor Espinoza of the Space Telescope Science Institute spoke to EarthSky Deborah Byrd On Monday, May 20, 2024, he talked about this diversity of worlds beyond our Sun and planets. He talked about the nearby and fascinating TRAPPIST-1 system, located about 40 light-years away. He talked about Proxima Centauri b, the closest exoplanet at 4 light-years. And he mentioned some current and future missions to exoplanets.

Types of exoplanets

The prefix exo comes from Greek and means outsideExoplanets are very, very far from 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 It was assumed for centuriesBut it wasn't until 1992 that astronomers discovered the first two exoplanets. orbiting a pulsarThen came the confirmation of the first exoplanet. orbiting a sun-like star in 1995.

Since then, astronomers have identified many types of planets in the exoplanet “zoo.” Some of them are listed below. See the Long list here for a complete classification.

• Hot Jupiters: Among the first exoplanets that astronomers discovered due to their size are: gas giant Planets. They contain the mass of Jupiter or more, are very close to their star and, in some cases, orbit around it in just a few Earth days. Assuming that such planets could not have formed in their current location, astronomers believe that they were born much farther away and emigrated 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 smaller gas giants (Neptune and Uranus) in our solar system. Astronomers believe that the composition of these planets is largely rock rather than gas. Therefore, they are 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 so-called “Earth-like planets.”Goldilocks Zone.” This zone is where water can exist in liquid form. “Earth-like” does not literally mean that a planet is a twin of Earth, possessing an Earth-like atmosphere and possibly life.
• Mini-Neptunes: An exoplanet with a mass of up to ten times that of Earth, but smaller than Neptune or Uranus. They are likely to be predominantly gaseous worlds.
• Ocean worlds: These are exoplanets that contain a substantial amount of water, either in the form of oceans on the surface or underground.
• Ice Giants: These exoplanets are made of volatile compounds such as water, methane and ammonia, instead of the hydrogen and helium found on Jupiter and Saturn, for example.

Why haven't we seen exoplanets before?

Why didn't we see them before? It's because exoplanets are so far away, several Light years At their closest point, exoplanets get farther and farther away, and this is because, unlike stars, exoplanets do not shine with their own light. Like Earth, they shine only with the light reflected from their home stars. Unlike their home stars, exoplanets are extremely dim; even the largest ones are drowned in the light of their much brighter stars.

Before the first discovery of exoplanets, most astronomers assumed that, if found, exoplanets would resemble planets in our solar system. The big surprise has been that many exoplanets are very different, and their positions and orbits are difficult to explain. If astronomers thought the solar system was in any way representative of other planetary systems in the galaxy, they have been disappointed. Of course, we don't know if our solar system is The exeption Instead of the rule.

When we interviewed him on video, the astronomer Nestor Espinoza He told EarthSky's Deborah Byrd that the Nancy Grace Roman Space Telescope, scheduled to launch in 2027, will increase the number of known exoplanets from the current 5,000 to 100,000. When that happens, we'll know a lot more.

The radial velocity method

Finding an Earth-like planet, especially one where life resides, has been and continues to be the driving force behind our searches and explorations of these distant worlds.

The detection of the first planet orbiting a main sequence star as if the sun rose in 1995. That was when Didier Queloz discovered a planet at least as massive as Jupiter orbiting F-type star 51 Pegasi, about 50 light-years from Earth. He detected it by the “wobble” of the star as an invisible planet tugged on it. For this discovery, he and his colleagues Michel Mayor and James Peebles received the Nobel Prize in Physics in 2019.

In the 1990s, available technology only allowed the detection of the largest exoplanets: those with enough gravity to induce a “wobble” in the rotation of their parent stars. This method of exoplanet detection is known as radial velocity methodand remains a very successful method for detecting exoplanets from the Earth's surface. You can read more about the radial velocity method, sometimes called Doppler spectroscopy – in this Planetary Society Page.

The transit method

Today, astronomers use another method, called transit methodeither transit photometry – with even greater success in finding exoplanets. NASA's planet-hunting spacecraft Kepler It has discovered the largest number of exoplanets so far and 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 very slightly, dimming it. This change in brightness may be only 1%, but it is detectable with modern instruments such as those on Kepler. Read more about the transit method here.

Direct imaging of exoplanets

With the launch of the James Webb Space Telescope In 2022, astronomers will have a new tool to help them track exoplanets. On September 1, 2022, NASA will Announced that the Webb telescope had captured a direct image of an exoplanet for the first time. This exoplanet, HIP 65426bwas not a new discovery. It was first discovered through direct imaging by the Spectro-Polarimetric High-Contrast Exoplanet Research (SPHERE) in 2017. But Webb was able to search for the exoplanet and detect it in four different filters. Read more about direct imaging of exoplanets.

It is worth noting that one famous and beloved exoplanet, Fomalhaut b (the first to be directly imaged) turned out not to be an exoplanet after all, but a cloud of dust. Read more about the sad disappearance of Fomalhaut b.

In search of Earth's twin

The search for a true twin of Earth continues. In June 2019, astronomers discovered Announced the discovery of the most Earth-like planet ever discovered, orbiting Teegarden's star, a Red dwarf Just 12.5 light-years away, the exoplanet Teegarden b has a 95% rating on the Earth Similarity Index.

But new exoplanets appear all the time. In December 2022, astronomers announced that they had discovered Two worlds possibly similar to Earth Only 16 light years away.

Kepler, which gave its name to so many exoplanets, is no longer active (although its data is still being analyzed). But the planet-hunting space probe Tess has been discovering planets since 2018. TESS is using citizen scientists to help you find worlds beyond our own.

In December 2019, the European Space Agency (THAT) thrown out Spaceship CHEOPS to better characterize already known exoplanets. And the new generation of ground-based telescopes, such as the European Extremely Large Telescope (Teaching English), the largest telescope in the world, currently under construction in Chile, will be able to analyze the atmospheres of exoplanets directly and identify biosignatures like oxygen and methane.

So the long-standing dream of finding life elsewhere in the universe could soon become a reality. Stay tuned!

Visualizing exoplanets

By the way, the interesting video below shows all the multi-planet systems from Kepler's original mission up to the time of Kepler's end-of-life announcement on October 30, 2018. Astronomer and planet hunter Ethan Krusewho 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 sizes of the orbits are to scale, but the sizes of the planets are not. For example, Jupiter is actually 11 times larger than Earth, but that scale makes Earth-sized planets nearly invisible (or Jupiter's annoyingly large). The orbits are all synchronized such that Kepler observed a planet transit every time it reached an angle of 0 degrees (the 3 o'clock position on a clock). The planets' colors are based on their approximate equilibrium temperatures, as shown in the legend.

A little history about exoplanets

The first exoplanets discovered in 1992 orbit around a neutron starIn this case, it was a pulsar (a neutron star that emits beams of radio waves like a lighthouse, which can be detected from Earth if the beams are pointed in the right direction). In general terms, a neutron star is the super-dense remains of the core of a massive star after it has ended its life in a supernova burst.

It was not thought possible, and is still not fully explained, that planets could survive such a cataclysm. Normally, the neutron stars we see as pulsars rotate with a regularity that rivals that of atomic clocksNeutron stars are therefore some of the most precise timekeepers in the cosmos.

Astronomers Alexander Wolszczan and Give it fragile They were trying to explain irregularities in the rotation of a particular pulsar, known as PSR-B1257+12They realized that they could explain the slight variations in the star's rotation if the gravity of two planets were pulling on it. Those planets would have to be three and four times the mass of the Earth.

As important as this discovery was historically, astronomers’ primary mission in the search for exoplanets was to find one orbiting a Sun-like star, not the remnants of a massive star after a supernova. After all, ultimately the mission is to find a planet like Earth, and then find life there. Humans have always asked themselves the question, “Are we alone in the universe?”

In short: Exoplanets are worlds orbiting distant stars. Watch a video with an astronomer who studies exoplanets and learn the history of our knowledge about them, among other things.

Explore: NASA's exoplanet archive

Visit: NASA's exoplanet exploration site

Have fun: NASA's Office of Exoplanet Travel