How are planets born, and how do they grow? This is an essential question in astronomy, one that researchers try to answer by observing “young planets.” That’s easier said than done. All the planets in our solar system are the same age, 4.6 billion years old. Astronomers therefore need to look elsewhere to find planets that have formed more recently—making exoplanets enticing research grounds.
Still, the vast majority of the approximately 6,000 exoplanets confirmed to date are also billions of years old. That’s not because there aren’t youthful planets out there, but old planets are easier to discover using common techniques such as the transit method, in which astronomers detect the slight dimming of a star’s light as a planet passes in front of it.
Newly formed exoplanets are often buried within disks of gas and dust, the leftover building blocks from a planet's formation, also known as a protoplanetary disk. The difficulty of just locating such a new celestial body is part of what makes the discovery of the exoplanet Elias 2-24b—now the youngest ever confirmed—so exciting.
The research team, led by Andrea Bernardi at Diego Portales University in Chile, identified the exoplanet by focusing on seven protoplanetary disks where there were notable gaps in the dense matter, since it is believed that growing planets carve out such voids. They reanalyzed “coronagraph” data, in which the central star is blocked out, making the dim reflected light from surrounding celestial bodies stand out.
Scientists had known about a gap in the disk surrounding the star Elias 2-24 for nearly a decade. The European Southern Observatory’s Very Large Telescope (yes, that’s its official name) had detected a point of light within this gap, sparking an ongoing debate over whether it was truly a planet. The researchers used data collected in 2018 and 2020 at the W.M. Keck Observatory in Hawaii to compare the dot’s positions over the years. They concluded that this bright spot was not an unrelated background star or noise from the equipment, but rather a celestial object gravitationally bound to Elias 2-24—in other words, a planet.
Elias 2-24b is located approximately 450 light-years from Earth. It orbits its star at a distance of approximately 55 astronomical units (one astronomical unit being the average distance between Earth and the sun). Its mass is estimated to be at most 1.9 to 4.0 times that of Jupiter, though researchers believe its volume is likely to be much smaller than our local gas giant. Since its host star is less than 1 million years old, Elias 2-24b is estimated to be similarly young.
Until now, the youngest exoplanets confirmed through direct observation were those orbiting the stars PDS 70 and WISPIT 2, both of which are over 5 million years old. This new discovery breaks those records by a significant margin.
And Elias 2-24b has a few attributes that require reevaluating some planetary formation models. One of the standard theories is known as the “core accretion model,” which posits that giant planets are formed when a solid core grows more massive and attracts surrounding gas. The time required for formation varies depending on the conditions of the calculation, but according to NASA, it should take about 5 million years for a Jupiter-sized planet to form. That’s also given Jupiter’s distance from the sun, approximately 5 astronomical units. For a planet with a much farther removed orbit like Elias 2-24b, accruing this mass should take even longer.
However, Elias 2-24b doesn’t seem to be following these rules. Despite being 55 astronomical units from its host star, it’s already a giant and behaving like a planet after a remarkably short 1 million years. The research team believes this planet was formed through core accretion, which would mean that this model can produce giant planets much faster than previously assumed.
Lucas Siesa, a professor at Diego Portales University and coauthor of the paper, pointed out in a press release, “Elias 2-24 b shows us that even our best planet-formation models are still missing some important processes.”
What can improve these models is finding more young planets, which may soon be more possible than ever. The Nancy Grace Roman Space Telescope, launched by NASA this past August, is equipped with an even more powerful coronagraph than the one the researchers used to locate Elias 2-24b. Astronomers therefore hope to soon detect more young planets as well as other planets obscured by their bright stars.
“This is just the beginning of a new era of discovery,” Siesa said. “It’s incredible that with modern technology, we are actually able to see planet formation in action, and Roman will take planet hunting to the next level.”
This story originally appeared on WIRED Japan and has been translated from Japanese.
