In 1992, when the first exoplanet discovery was announced by astronomers Aleksander Wolszczan and Dale Frail, no one could have imagined that only 30 years later thousands of exoplanets would be known to humanity.
Yet, on Monday, NASA announced that, to date, 5,000 exoplanets have been detected.
"This is definitely a milestone to celebrate," said Sara Seager, an astrophysicist and planetary scientist at MIT. " When exoplanets first started [30] years back, people laughed and thought the field would go nowhere. Today we not only have 5,000 planets, but the discovery potential is unprecedented."
Exoplanets, worlds orbiting distant stars, were once only theorized. And even then, astronomers thought only some stars contained planets. Today, astronomers believe that, on average, a star is host to at least one planet.
And there is no shortage of different types of these far-off worlds. There are "super-Earths" — rocky planets larger than our own planet — "gas giants" larger than Jupiter, "hot-Jupiters" that are massive worlds in close orbits to their host stars, and even mini-Neptunes.
There's also a weird and wonderful assortment of these different planets. There's been a planet with the density of cotton candy; a planet with iron rain; and a planet with "sunscreen snow."
One of the most intriguing planetary systems is TRAPPIST 1.
This system of seven planets — with three that could be potentially habitable — was discovered in 2017. They lie relatively close to earth at just 40 light-years away.
Searching for signs of life
The first exoplanet was discovered orbiting a pulsar, a dense star called a neutron star, that rotates rapidly and produces millisecond-bursts of radiation. It was unlikely that any life would be able to survive these intense bursts, but it was a promising discovery.
"If you can find planets around a neutron star, planets have to be basically everywhere," Wolszczan said in a NASA statement. "The planet production process has to be very robust."
In 1995, the first exoplanet orbiting a sun-like star was discovered, though it ended up being a gas giant in close orbit to its host star.
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But the desire to search for more Earth-like planets eventually led to the 2009 launch of the Kepler space telescope. The workhorse would go on to discover the bulk of the 5,000 planets we know today, even after suffering a breakdown in 2013. It was declared dead in 2018, though analysis of its data continues.
Since then, the Transiting Exoplanet Survey Satellite (TESS), has been launched and is searching the stars for new worlds. And the James Webb Space Telescope, once it comes online, will seek to pull back the veil of the atmospheres of these planets, searching for signs of potential habitability.
"To my thinking, it is inevitable that we'll find some kind of life somewhere — most likely of some primitive kind," Wolszczan said.
And there are even more of these planet-hunting telescopes to come.
The Nancy Grace Roman Space Telescope is scheduled to launch in 2027, and the European Space Agency's Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) is expected to launch in 2029.
"It's not just a number," Jessie Christiansen, science lead for the archive and a research scientist with the NASA Exoplanet Science Institute at Caltech, said of the 5,000 exoplanets in a NASA statement.
"Each one of them is a new world, a brand-new planet. I get excited about every one because we don't know anything about them."
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