Not so long ago, we lived in a universe with only a few known planets, all orbiting our sun. But a new string of scientific discoveries has been a high point: More than 5,000 other planets have now been confirmed to exist outside our solar system.
The planetary odometer changed on March 21, with the latest batch of 65 exoplanets – planets outside our solar system’s immediate family – added to NASA’s Exoplanet Archive. This archive records discoveries of exoplanets that appear in peer-reviewed scientific articles and that have been confirmed using various detection methods or using analytical techniques.
More than 5,000 planets have been found to date that include small, rocky worlds like Earth, gas giants several times larger than Jupiter, and “hot Jupiters” in very close orbits around their stars. There are “terrestrial giants,” which can be rocky worlds larger than our own, and “minor Neptunes,” which are smaller versions of Neptune in our system. Also in this mix are planets orbiting two stars simultaneously and planets stubbornly orbiting the remnants of dead, collapsing stars.
“It’s not just a number,” said Jesse Christiansen, senior archival scientist and research scientist at the NASA Exoplanet Science Institute at California Tech (Caltech) in Pasadena. “Every one of them is a new world, a whole new planet. I’m excited for each one because we don’t know anything about them.”
We know this: our galaxy probably contains hundreds of billions of these planets. The constant announcement of these discoveries began in 1992 with the emergence of strange new worlds orbiting an even more exotic star. This was a type of neutron star known as a pulsar, a rapidly spinning stellar body pulsing with millisecond-filled bursts of scorching radiation. Measuring subtle changes in the timing of the pulsations allowed the scientists to detect planets in orbit around the pulsar.
Finding only three planets around this rotating star has essentially led to a flurry of discoveries, said Alexander Walchzan, lead author of the research paper that revealed the first confirmed planets outside our solar system 30 years ago.
“If we can find planets around a neutron star, the planets have to be basically everywhere,” Walchzan said. “The planet’s production process must be very robust.”
We’re opening an era of discovery that will go beyond simply adding new planets to the list, says Wolszczan, who continues to research exoplanets as a professor at Penn State University. The Transiting Exoplanet Survey Satellite (TESS), launched in 2018, continues new exoplanet discoveries. But soon the powerful next-generation telescopes and their highly sensitive instruments, starting with the recently launched James Webb Space Telescope, will be picking up light from the atmospheres of exoplanets, reading the gases present to determine the clear signs of habitable conditions.
The Nancy Grace Space Telescope, scheduled to launch in 2027, will make discoveries of new exoplanets using a variety of methods. The European Space Agency’s (ESA) ARIEL mission, scheduled for 2029, will monitor the atmospheres of exoplanets; A piece of technology from NASA, called CASE, will go aboard that mission and help focus on clouds and haze from exoplanets.
“In my opinion, it is inevitable that somewhere we will find some form of life, perhaps of a primitive kind,” said Walchzan. He added that the close relationship between the chemistry of life on Earth and the chemistry found throughout the universe, as well as the detection of organic molecules in general, indicates that the discovery of life itself is only a matter of time.
How do you find other worlds
The picture was not always this clear. The first planet discovered around a Sun-like star, in 1995, turned out to be hot Jupiter: a gas giant about half the mass of our own Jupiter in a very close orbit of its star for four days. In other words, a year on this planet lasts only four days.
More of these planets appeared in ground-based telescope data as soon as astronomers learned to identify them, first in the tens and then in the hundreds. It was found using the “oscillation” method: tracking the slight back-and-forth motions of a star, caused by the gravitational pull of orbiting planets. But nonetheless, nothing appears habitable.
Finding rocky little worlds like ours required the next big leap in exoplanet search technology: the “crossover” method. Astronomer William Borucki came up with the idea of attaching highly sensitive light detectors to a telescope, and then launching it into space. The telescope will observe a field of more than 170,000 stars for years, looking for slight dips in starlight as a planet passes in front of them.
This idea was revived on the Kepler space telescope.
Borucki, the principal investigator for the Kepler mission, which is now retired, says its 2009 launch opened a new window into the universe.
“I have a real sense of satisfaction and a real sense of awe for what’s out there,” he said. None of us expected this huge diversity of planetary and star systems. It is absolutely amazing.”
Written by Pat Brennan
Jet Propulsion Laboratory, Pasadena, California
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