OUR KNOWLEDGE OF WORLDS BEYOND THE SOLAR SYSTEM HAS EXPLODED IN THE LAST THREE DECADES

Reported by Robert Lea

© Tobias Roetsch

Ever since humans first discovered that the stars in the night sky were bodies similar to our Sun, we’ve dreamed and speculated about the worlds that could orbit these stars. Would they be rocky terrestrial planets like Earth? Could they possess liquid water? Could the presence of this vital life-sustaining element on other worlds mean that we are not alone in the universe?

“For millennia, humans have been asking the question of whether we are alone. And tied to that question, are other planets anywhere else?” Nikku Madhusudhan, a professor of astrophysics and exoplanetary science at the Institute of Astronomy of the University of Cambridge, tells All About Space. “It’s very fundamental to being human to ask the question if there are planets elsewhere.”

With this considered, it’s almost shocking to think that before the 1990s astronomers weren’t even certain that stars outside the Solar System possessed their own planets. There was no evidence to suggest that extrasolar planets, or exoplanets for short, didn’t exist, nor were there hints that the Solar System was in any way unique in the Milky Way. But until the very end of the 20th century, astronomers had been frustrated by the lack of direct evidence of worlds beyond the influence of our star.

This is because exoplanets are notoriously difficult to detect. Historically, the most successful exoplanet detection methods have worked by inferring the tiny effects that planets have on their parent stars, like tiny dips in light or the near-imperceptible ‘wobble’ they cause in their star’s motion. “Until 30 years ago, we didn’t know of any planets outside the Solar System; all we knew of were the planets in the Solar System,” Madhusudhan continues. “But as soon as exoplanets were discovered, that opened an entirely new window into the universe and its other planetary systems.”

TYPES OF EXOPLANET

HOT JUPITERS

Mass: Up to 12 times Jupiter’s mass

Size: 0.3 to 10.0 times Jupiter’s radius

Number discovered: 1,458

Gas giants like Jupiter. The difference is these worlds orbit closer to their stars, with short orbits and blistering surface temperatures. WASP-76b is a planet so close to its host that it completes an orbit in under two days.

SUB-NEPTUNES

Mass: Up to 17 times Earth’s mass

Size: Over 2.0 times Earth’s Radius

Number discovered: 1,719

Planets similar in size to the Solar System’s ice giant Neptune, they’re believed to be the most common type of planet in the Milky Way. Discovered in 2018, Kepler-1655 b has a radius around 2.3 times that of Earth, with five times our planet’s mass.

SUPER-EARTHS

Mass: Up to ten times Earth’s mass

Size: Between 0.8 and 4.0 times Earth’s radius

Number discovered: 604

Rocky terrestrial worlds or gas planets more massive than Earth but smaller than Neptune. One example of a super-Earth is Gliese 15 A b, a rocky world 11 light years away which is over three times the size of Earth.