In just thirty years, a wide variety of exoplanets have been found, including rocky ones the size of Earth.
The first two exoplanets, as the worlds outside our solar system are called, were discovered in 1992 around a pulsar, a strange neutron star. Since then, the list has not stopped growing thanks to new missions, instruments, and detection techniques. From time to time, a team of researchers announces the discovery of one or several new planets, some of them forming part of the same system. Today, thirty years later, more than 5,000 extrasolar worlds have already been confirmed, a scientific milestone that NASA has just confirmed. And, surely, there are many more waiting.
The mark was broken on March 21, when a set of 65 exoplanets was added to the Exoplanet Archive.
of the US space agency. The archive records discoveries of extrasolar worlds that have been confirmed using multiple detection methods or analytical techniques.
Among the 5,005 planets found so far, there are small, rocky ones like Earth, gas giants many times larger than Jupiter, and ‘hot Jupiters’ in scorchingly close orbits around their stars. There are also super-Earths, which are possible rocky planets larger than our own, and mini-Neptunes, smaller versions of our system’s Neptune. Some planets orbit two stars at once and others stubbornly orbit the remains of collapsed stars.
“It’s not just a number,” says Jessie Christiansen, scientific director of the archive and a research scientist at NASA’s Exoplanet Science Institute at the California Institute of Technology (Caltech) in Pasadena. “Each of them is a new world, a new planet. I get excited about each one because we don’t know anything about them », she says.
Astronomers believe that our galaxy probably contains hundreds of billions of such planets. The steady pace of discovery began in 1992 with strange new planets orbiting an even stranger star. It was a type of neutron star known as a pulsar, a rapidly spinning stellar corpse that pulsates with millisecond bursts of searing radiation. Measuring slight changes in the timing of the pulses allowed scientists to reveal planets in orbit around the pulsar.
“Finding just three planets around this spinning star opened the floodgates,” says Alexander Wolszczan, lead author of the paper that, 30 years ago, revealed the first confirmed planets outside our solar system. “If you can find planets around a neutron star, the planets have to be basically everywhere,” says Wolszczan. “The process of creating planets has to be very robust.”
Primitive life
Wolszczan, who continues to search for exoplanets as a professor at Penn State, argues that we are opening an era of discovery that will go beyond simply adding new planets to the list. The Transiting Exoplanet Survey Satellite (TESS), launched in 2018, continues to discover new exoplanets. But soon, innovative telescopes with highly sensitive instruments, like the recently launched James Webb Space Telescope, will capture light from exoplanets’ atmospheres, deciphering what gases are present in them to identify telltale signs of habitable conditions.
The Nancy Grace Roman Space Telescope, due to launch in 2027, will make new exoplanet discoveries using a variety of methods. The European Space Agency’s (ESA) ARIEL mission, due to launch in 2029, will observe exoplanet atmospheres; a piece of NASA technology onboard, called CASE, will help study the clouds and haze of exoplanets.
“In my opinion, it is inevitable that we will find some kind of life somewhere, most likely of some primitive kind,” says Wolszczan. “The close connection between the chemistry of life on Earth and the chemistry found throughout the universe, as well as the detection of organic molecules, suggests that the detection of life itself is only a matter of time.”
How to ‘hunt’ exoplanets
The first exoplanet detected around a Sun-like star, in 1995, turned out to be a hot Jupiter: a gas giant about half the mass of Jupiter, in an extremely close (four-day) orbit around its star. A year on this planet, in other words, lasts only four days.
Over time, more such planets appeared in ground-based telescope data once astronomers learned to recognize them: first dozens, then hundreds. They were found using the ‘wobble’ method: tracking slight back-and-forth motions of a star, caused by gravitational tugs from orbiting planets. But even so, nothing seemed potentially habitable.
Finding small, rocky planets more like our own required the next big leap in exoplanet search technology: the ‘transit’ method. Astronomer William Borucki came up with the idea of attaching extremely sensitive light detectors to a telescope and then launching it into space. The telescope would watch a field of more than 170,000 stars for years, looking for small dips in starlight caused when a planet crosses in front (from our perspective) of a star. That idea materialized in the Kepler Space Telescope.
Borucki, the principal investigator of the Kepler mission, now retired, says that its launch in 2009 opened a new window on the universe. “I have a real sense of satisfaction and really awe of what is there. None of us expected this enormous variety of planetary systems and stars. It’s just amazing », he admits.
