PICTURED: A comparison of the orbit and size of Earth to its solar system, with that of Kepler-i86f. PC: NASA/JPL Caltech.
Story at a glance…
5,000 planets outside of our solar system have been discovered.
The science has advanced rapidly since the 1990s when the first exoplanet was discovered.
Planets thought to be ocean-covered, others that have all the structural characteristics of Earth, and other interesting gas worlds have shown the remarkable diversity of planetary formation.
From super-Earths and mini-Neptunes to planets orbiting two stars and others orbiting dead stars, the number of alien worlds known to science reached, and then exceeded, 5,000.
Standing officially at 5,005, the NASA Exoplanet Archive celebrated the 5,000th exoplanet after a recent batch of 65 confirmed worlds were added to the archive’s list, including a system of 5 Earth-sized planets orbiting around a M-dwarf star.
It was only 30 years ago that the number of known planets stood at 9. After Alexander Wolszczan announced the discovery of two rocky Earth-like planets orbiting around a pulsar, advances in the availability of telescopes to search for these worlds have seen the field of exoplanet astronomy rapidly progress.
“It was a great surprise to everybody,” Dr. Wolszczan, who still seeks exoplanets today at the age of 75, told WaL. “You need a supernova explosion to make a neutron star, so how do you make a planet survive something like this? Of course it’s almost impossible, so you have to make planets after a neutron star formation and that requires some sort of special circumstances”.
“If you can make planets around neutron stars, then they should be, sort of, everywhere. 5,000 exoplanets later, we now know that planets really are a very common byproduct of star formation”.
Future tools like the James Webb Space Telescope, which recently finished aligning its mirrors, as well the European Space Agency’s ARIEL Mission, both of which will help astronomers gather more details about exoplanets such as the chemical contents of their atmospheres.
Exoplanets are naturally more exciting objects to study since they are the places in which life will likely be found, as well as the only chance of humanity finding another home in the distant future.
“30 years ago if you said that super-Earth systems would be very, very common among exoplanets in the exoplanets world, you’d probably be called out of your mind,” Wolszczan told WaL. “It was like a preview of what would happen in terms of the kind of planetary systems we would find later on”.
Stepping into the light
Exoplanets can be found mainly by one of two ways. The first is by using a telescope like the Kepler Space Telescope, Humanity’s most successful planet catcher, to focus in on a star for a fixed period. Called the “transit method,” scientists look for small consistent dips in the observable light of a star, indicating an orbiting planet briefly eclisping it.
This method allowed Kepler to discover 2,700 exoplanets, most of them in the constellation Cygnus, where Kepler was pointed for the first 4 of its 9 years in operation. This was the method, but not the telescope, that allowed Wolszczan to confirm the first two exoplanets as he observed a pulsar, the bright, rapidly-spinning remains of a star that stream out cosmic radiation. Pulsars are excellent host stars for finding exoplanets for this reason.
“The expectation [in 1990] that we should find planets soon was strong because of the abundant evidence that came from the existence of disks around young stars,” Wolszczan told WaL. “There was a lot of evidence that places like that existed and we knew our own planetary system must have originated from a disk and there’s a lot of evidence for that too”.
Another way exoplanets can be detected is through the “wobble” method, which can be combined with the transit method to reinforce scientific findings. The wobble method states that minute movements in a star’s position can occur along a planet’s orbit with it. Owing to a slight gravitational tug from the proximity of the orbiting planet, the star appears to wobble under a telescope.
Another serious planet hunter has been TESS, the Transiting Exoplanet Survey Satellite. Confirming 203 out of 5,459 candidates, TESS has already completed its two-year mission of surveying 75% of the sky. This massive mosaic is being constantly studied for more planetary confirmations, and is currently still flying as part of its extended mission.
The Very Best of…
Not all the worlds discovered by scientists are a jumble of numbers and letters in a digital ledger somewhere. Some are extremely interesting, and offer incredible potential for discovering microbial life. One of the best examples of this is the TRAPPIST system.
At a tiddling 40 light-years from Earth in the constellation Aquarius, the 7 planets of the TRAPPIST-1 System, named after the telescope in Chile that discovered it, has several planets that could be of great interests to exoplanet researchers in the next decade.
All rocky Earth-like worlds, TRAPPIST-1 b through h together orbit closer to their dwarf star than Mercury does to ours. But being a dwarf star (about as big as Jupiter) the penalty for closer orbit is much less severe. Three of these worlds, e, f, and g, are in the star’s habitable zone, and e in particular is thought to be quite habitable in some regards.
As they are too small to see, researchers detected the existence of the planets around TRAPPIST-1 by looking at the dimming of the star as the flat-orbits of the planets pass across our view. Through information gleaned during each such transiting, scientists at the American Astronomical Society have deduced their make-up; as rocky planets containing similar mineral profiles to Earth.
Reasoning they contain about 10% less iron than Earth, TRAPPIST-1 planets are thought to potentially contain water based on calculations of their mass.
The lower densities of planets d, f, g, and h can allow for two to three times as much water than for planets b, c, and e., which also correlate to their position in the habitable zone. Beyond hypotheses, further observations by the Hubble Space Telescope has found evidence of huge plumes of atmospheric water vapor coming from evaporated water on planets a and b, which are closest to the sun.
“The inner planets could have lost more than 20 Earth-oceans-worth of water during the last eight billion years,” report the telescope’s handlers. “However, the outer planets of the system — including the planets e, f and g which are in the habitable zone — should have lost much less water, suggesting that they could have retained some on their surfaces”.
“It has galvanized the exoplanet community to study this system in detail, both observationally and theoretically, and has fueled hopes that atmospheric signatures (or even biosignatures) might be detected with the James Webb Space Telescope,” wrote the authors of the paper.