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Exoplanets are planets beyond our own solar system. Thousands have been discovered in the past two decades, mostly with NASA's Kepler Space Telescope.

These worlds come in a huge variety of sizes and orbits. Some are gigantic planets hugging close to their parent stars; others are icy, some rocky. NASA and other agencies are looking for a special kind of planet: one that's the same size as Earth, orbiting a sun-like star in the habitable zone.

The habitable zone is the range of distances from a star where a planet's temperature allows liquid water oceans, critical for life on Earth. The earliest definition of the zone was based on simple thermal equilibrium, but current calculations of the habitable zone include many other factors, including the greenhouse effect of a planet's atmosphere. This makes the boundaries of a habitable zone "fuzzy."

Related: 15 places on Earth that look exoplanetary

Astronomers announced in August 2016 that they might have found such a planet orbiting Proxima Centauri. The newfound world, known as Proxima b, is about 1.3 times more massive than Earth, which suggests that the exoplanet is a rocky world, researchers said. The planet is also in the star's habitable zone, just 4.7 million miles (7.5 million kilometers) from its host star. It completes one orbit every 11.2 Earth-days. As a result, it's likely that the exoplanet is tidally locked, meaning it always shows the same face to its host star, just as the moon shows only one face (the near side) to Earth.

Most exoplanets have been discovered by the Kepler Space Telescope, an observatory that began work in 2009 and is expected to finish its mission in 2018, once it runs out of fuel. As of mid-March 2018, Kepler has discovered 2,342 confirmed exoplanets and revealed the existence of perhaps 2,245 others. The total number of planets discovered by all observatories is 3,706.

Early discoveries

While exoplanets were not confirmed until the 1990s, for years beforehand astronomers were convinced they were out there. That wasn't just wishful thinking, but because of how slowly our own sun and other stars like it spin, University of British Columbia astrophysicist Jaymie Matthews told Space.com. Matthews, the mission scientist of occasional exoplanet telescope observer MOST (Microvariability and Oscillations of STars), was involved in some of the early exoplanet discoveries.

Astronomers had an origin story for our solar system. Simply put, a spinning cloud of gas and dust (called the protosolar nebula) collapsed under its own gravity and formed the sun and planets. As the cloud collapsed, conservation of angular momentum meant the soon-to-be-sun should have spun faster and faster. But, while the sun contains 99.8 percent of the solar system's mass, the planets have 96 percent of the angular momentum. Astronomers asked themselves why the sun rotates so slowly.

The young sun would have had a very strong magnetic field, whose lines of force reached out into the disk of swirling gas from which the planets would form. These field lines connected with the charged particles in the gas, and acted like anchors, slowing down the spin of the forming sun and spinning up the gas that would eventually turn into the planets. Most stars like the sun rotate slowly, so astronomers inferred that the same "magnetic braking" occurred for them, meaning that planet formation must have occurred for them. The implication: Planets must be common around sun-like stars.

For this reason and others, astronomers at first restricted their search for exoplanets to stars similar to the sun, but the first two discoveries were around a pulsar (rapidly spinning corpse of a star that died as a supernova) called PSR 1257+12, in 1992. The first confirmed discovery of a world orbiting a sun-like star, in 1995, was 51 Pegasi b - a Jupiter-mass planet 20 times closer to its sun than we are to ours. That was a surprise. But another oddity popped up seven years earlier that hinted at the wealth of exoplanets to come.

A Canadian team discovered a Jupiter-size planet around Gamma Cephei in 1988, but because its orbit was much smaller than Jupiter's, the scientists did not claim a definitive planet detection. "We weren't expecting planets like that. It was different enough from a planet in our own solar system that they were cautious," Matthews said.

Most of the first exoplanet discoveries were huge Jupiter-size (or larger) gas giants orbiting close to their parent stars. That's because astronomers were relying on the radial velocity technique, which measures how much a star "wobbles" when a planet or planets orbit it. These large planets close in produce a correspondingly big effect on their parent star, causing an easier-to-detect wobble.

Before the era of exoplanet discoveries, instruments could only measure stellar motions down to a kilometer per second, too imprecise to detect a wobble due to a planet. Now, some instruments can measure velocities as low as a centimeter per second, according to Matthews. "Partly due to better instrumentation, but also because astronomers are now more experienced in teasing subtle signals out of the data."

Kepler, TESS and other observatories

Kepler launched in 2009 on a prime mission to observe a region in the Cygnus constellation. Kepler performed that mission for four years - double its initial mission lifetime - until most of its reaction wheels (pointing devices) failed. NASA then put Kepler on a new mission called K2, in which Kepler uses the pressure of the solar wind to maintain position in space. The observatory periodically switches its field of view to avoid the sun's glare. Kepler's pace of planetary discovery slowed after switching to K2, but it is still found hundreds of exoplanets using the new method. Its latest data release, in February 2018, contained 95 new planets.

(Image credit: Space.com Store)

Kepler has revealed a cornucopia of different types of planets. Besides gas giants and terrestrial planets, it has helped define a whole new class known as " super-Earths": planets that are between the size of Earth and Neptune. Some of these are in the habitable zones of their stars, but astrobiologists are going back to the drawing board to consider how life might develop on such worlds. Kepler's observations showed that super-Earths are abundant in our universe. (Oddly, our solar system doesn't appear to contain a planet of that size, although some believe a large planet nicknamed " Planet Nine" may be lurking in the outer reaches of the solar system.)

Kepler's primary method of searching for planets is the "transit" method. Kepler monitors a star's light. If the light dims at regular and predictable intervals, that suggests a planet is passing across the face of the star. In 2014, Kepler astronomers (including Matthews' former student Jason Rowe) unveiled a new "verification by multiplicity" method that increased the rate at which astronomers promote candidate planets to confirmed planets. The technique is based on orbital stability - many transits of a star occurring with short periods can only be due to planets in small orbits, since multiply eclipsing stars that might mimic would gravitationally eject each other from the system in just a few million years.

As Kepler wraps up its mission, a new observatory called the Transiting Exoplanet Survey Satellite (TESS) is expected to launch in spring 2018. TESS will orbit the Earth every 13.7 days and will perform an all-sky survey over two years. It will survey the Southern Hemisphere in its first year, and the Northern Hemisphere (which includes the original Kepler field) in its second. The observatory is expected to reveal many more exoplanets, including at least 50 that are around the size of Earth.

Other prominent planet-hunting observatories (past and present) include:

The HARPS spectograph on the European Southern Observatory's La Silla 3.6-meter telescope in Chile, whose first light was in 2003. The instrument is designed to look at the wobbles that a planet induces in a star's rotation. HARPS has found well over 100 exoplanets itself, and is regularly used to confirm observations from Kepler and other observatories.

The Canadian Microvariability and Oscillations of STars (MOST) telescope, which started observations in 2003. MOST is designed to observe a star's astroseismology, or starquakes. But it also has participated in exoplanet discoveries, such as finding the exoplanet 55 Cancri e.

The French Space Agency's CoRoT (COnvection ROtation and planetary Transits), which operated between 2006 and 2012. It found a few dozen confirmed planets, including COROT-7b - the first exoplanet that had a predominantly rock or metal composition.

The NASA/European Space Agency Hubble and NASA Spitzer space telescopes, which periodically observe planets in visible or infrared wavelengths, respectively. (More information about a planet's atmosphere is available in infrared.)

The European CHaracterising ExOPlanets Satellite (CHEOPS), which is expected to be ready for launch in 2018. The mission is designed to calculate the diameters of planets accurately, particularly those planets that fall between super-Earth and Neptune masses.

The NASA James Webb Space Telescope, which is expected to launch in 2020. It is specialized to observe in infrared wavelengths. The powerful observatory is expected to reveal more about the habitability of certain exoplanets' atmospheres.

The European Space Agency's PLAnetary Transits and Oscillations of stars (PLATO) telescope, which is expected to launch in 2024. It is designed to learn how planets form and which conditions, if any, could be favorable for life.

The ESA ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) mission, which will launch in mid-2028. It is expected to observe 1,000 exoplanets and also do a survey of the chemical compositions of their atmospheres.

(Image credit: NASA/Tim Pyle)

Notable exoplanets

With thousands to choose from, it's hard to narrow down a few. Small solid planets in the habitable zone are automatically standouts, but Matthews singled out five other exoplanets that have expanded our perspective on how planets form and evolve:

51 Pegasi b: As mentioned earlier, this was the first planet to be confirmed around a sun-like star. Half the mass of Jupiter, it orbits around its sun at roughly the distance of Mercury from our Sun. 51 Pegasi b is so close to its parent star that it is likely tidally locked, meaning one side always faces the star.

HD 209458 b: This was the first planet found (in 1999) to transit its star (although it was discovered by the Doppler wobble technique) and in subsequent years more discoveries piled up. It was the first planet outside the solar system for which we could determine aspects of its atmosphere, including temperature profile and the lack of clouds. (Matthews participated in some of the observations using MOST.)

55 Cancri e: This super-Earth orbits a star that is bright enough to see by eye, meaning astronomers can study the system in more detail than almost any other. Its "year" is only 17 hours and 41 minutes long (recognized when MOST gazed at the system for two weeks in 2011). Theorists speculate that the planet may be carbon-rich, with a diamond core.

HD 80606 b: At the time of its discovery in 2001, it held the record as the most eccentric exoplanet ever discovered. It is possible that its odd orbit (which is similar to Halley's Comet around the sun) may be due to the influence of another star. Its extreme orbit would make the planet's environment extremely variable.

WASP-33b: This planet was discovered in 2011 and has a sort of "sunscreen" layer - a stratosphere - that absorbs some of the visible and ultraviolet light from its parent star. Not only does this planet orbit its star "backward," but it also triggers vibrations in the star, seen by the MOST satellite.

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Exoplanets: Worlds Beyond Our Solar System

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Astronomers have discovered thousands of extrasolar planets. Researchers hope to find a habitable planet someday.

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Most of the exoplanets discovered so far are in a relatively small region of our galaxy, the Milky Way. (“Small” meaning within thousands of light-years of

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Astronomers unveiled new insights by NASA's TESS this week at the American Astronomical Society meeting in Honolulu, Hawaii.

The satellite spotted a new Earth-like exoplanet within its star's habitable zone, a new Tatooine star system, and a strange eclipse.

TESS has been working hard to uncover new corners of the universe since its launch in 2018.

Exoplanet enthusiasts, rejoice! This week, scientists revealed a series of new discoveries made by NASA's Transiting Exoplanet Survey Satellite (TESS). The telescope has spotted a number of strange new worlds circling star systems near and far, the scientists announced at the 235th meeting of the American Astronomical Society in Honolulu, Hawaii.

From Tatooine-like systems to habitable worlds to starry eclipses, TESS has been busy uncovering the universe's oddities since launching in 2018. A follow-up to the prolific Kepler mission, which discovered about 70 percent of the approximately 4,000 exoplanets known to science today, TESS is scheduled to scan 200,00o of the night's brightest stars for insight into their solar systems. It's expected to conduct this survey over the course of two years, focusing on small chunks of the night sky at a time.

The satellite, which has four cameras that snap pictures once every 30 minutes, has a leg up on Kepler, and can study stars that are 30 to 100 times brighter. Ultimately, scientists hope the project will help us understand the composition of different solar systems, uncover mysteries about potentially habitable worlds, and unlock secrets about life elsewhere in the universe.

A "Second Earth"

TESS has discovered its first Earth-sized planet in the habitable zone of a distant star. Its orbit sits in the sweet spot, where liquid water could exist on its surface. The exoplanet is called TOI 700d (TOI stands for Tess Object of Interest) and it's a short distance from Earth-101.5 light-years away, to be exact-which means it's easier for other instruments to study the planet. NASA's other exoplanet hunter, Spitzer Space Telescope, confirmed the finding.

The star, an M dwarf star, is about 40 percent of the sun's size and mass and nearly half as hot at the surface, according to a NASA press release. The planet sits just inside the habitable zone, orbits around its star every 37 days, and is about 20 percent larger than Earth. It doesn't get as much energy from its own star as we do from Earth, receiving about 86 percent of the solar energy felt here on Earth. Researchers also suspect the planet is tidally locked, meaning only one side faces its star during orbit.

NASA's Goddard Space Flight Center

It's difficult to say what conditions might be like on the exoplanet, but scientists believe it sits within the realm of habitability. Using the information currently available about the exoplanet's orbit, size and other solar system conditions, scientists are able to create models of likely environments. A tidally locked planet, for example, would have vastly different cloud systems than what you might find here on Earth.

In the 20 computer models that simulated TOI 700 d's environment, several of them were habitable. One model, according to NASA, revealed a watery world with an atmosphere chiefly composed of carbon dioxide, when another described a scenario in which the exoplanet could be rocky and have landscapes shaped by strong winds that sweep from the exoplanet's night side to its sunny side. A tidally locked planet, for example, would have vastly different cloud systems than what you might find here on Earth.

TESS Spots a Tatooine System

The same sky that Luke Skywalker once saw could be what hypothetical visitors to TOI 1338b see. The star system in which TOI 1338b is found resides in the constellation Pictor and is just 1,300 light-years away from Earth.

The exoplanet, scientists announced, lives within a two-star system and is a type of exoplanet called a circumbinary planet. These types of planets are extremely difficult to spot. TOI 1338 b is almost seven times larger than earth-between the size of Neptune and Saturn, according to NASA. It also has a very strange orbit. The depth and duration of the orbit fluctuate and its transit in front of the two stars is odd, occurring every between 93 and 95 days.

The two stars, which haven't been named yet, orbit each other every 15 days, according to NASA. One of them is 10 percent more massive than the sun, and other is about one-third the mass of the the sun. No other planets have been observed in the solar system.

During its mission, Kepler discovered 12 circumbinary planets in a total of 10 systems, but most of the exoplanets found in those systems were larger; scientists believe these are easier to spot because they're more visible when they slide in front of their stars. It's likely TESS will spot a lot more of these strange systems

A bonus discovery! TESS data also revealed evidence that the well-studied star Alpha Draconis and a nearby companion star regularly eclipse each other. According to NASA, it's one of the brightest examples of what's called an eclipsing binary system, where two stars are situated far from each other, but still interact gravitationally.

The super star is located about 270 light-years away in the constellation Draco. About 5,000 years ago, the star was considered to be a guiding light in the night sky-a North Star directing the ancient Egyptians and other ancient civilizations. Thanks to Earth's precession, or its cyclical 26,000-year wobble, that role is now played by Polaris.

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Astronomers unveiled new insights by NASA's TESS this week at the American Astronomical Society meeting in Honolulu, Hawaii. The satellite spotted a new Earth-like exoplanet within its star's habitable zone, a new Tatooine star system, and a strange eclipse. TESS has been working hard to uncover new corners of the universe since its launch in 2018.

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There are even more potentially habitable planets near Earth than we ever imagined. A research team discovered two Earth-like planets in our cosmic backyard, and they're located in the perfect zone for water to form on their presumably rocky surfaces.

The planets orbit a sun known as "Teegarden's star," which is only 12.5 light-years from Earth. (A light-year is the distance that light travels in a year, or roughly 6 trillion miles or 10 trillion kilometers.) The two planets look an awful lot like Earth and our neighboring worlds, the researchers said.

"The two planets resemble the inner planets of our solar system," lead author Mathias Zechmeister, a research scientist at the Institute for Astrophysics at the University of Göttingen in Germany, said in a statement. "They are only slightly heavier than Earth and are located in the so-called habitable zone, where water can be present in liquid form."
Related: Our Interstellar Neighbors: 5 Potentially Earth-Like Planets Nearby
The results were obtained as part of the search for exoplanets; CARMENES stands for "Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs."

According to that project's observations, the newly found worlds orbit their parent star with periods of roughly five days and 11 days, respectively. That's very quick compared to planets orbiting our own sun (even Mercury takes 88 days for a single circuit), but Teegarden's star is an - a type of star that produces less light and energy than our own sun. Any potentially habitable worlds would be found huddled closer to this star than Earth is to the sun, or their water would freeze. Thus, their orbits would be quicker.

More planets could be lurking in Teegarden's star's solar system, the research team added, as many stars have more than a couple of planets orbiting them. The research team tried to find more evidence of planets using the "transit" method, which looks for subtle dips of brightness as a world passes in front of its star.

The scientists didn't detect any transits, but they did point out a coincidence of cosmic geometry: Any potential inhabitants on the newfound planets could use the transit method to see Earth. That's because from the vantage point of Teegarden's star, Earth orbits its sun at just the right angle to transit across the face of our star, allowing any astronomers "out there" to spot us as we pass by.
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A paper based on the research was published in the journal Astronomy and Astrophysics.

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Two Potentially Earth-Like Alien Planets Found Around Nearby Star

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There are even more potentially habitable planets near Earth than we ever imagined. A research team discovered two Earth-like planets in our cosmic backyard, and they're located in the perfect zone for water to form on their presumably rocky surfaces. The planets orbit a sun known as "Teegarden's star," which is only 12.5 light-years from Earth .

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Eyes On Exoplanets - Exoplanet Exploration: Planets Beyond our Solar System