Astronomers May Have Found the Galaxy’s Youngest Planet

Over the last 30 years, astronomers have found more than 5,000 exoplanets, an eclectic menagerie of worlds far from our stellar neighborhood. The latest may be a mere infant.

In the journal The Astrophysical Journal Letters, scientists on Tuesday announced compelling evidence for a world just 1.5 million years old, making it one of the youngest planets ever found, perhaps the youngest.

This world — 395 light-years from Earth in the constellation Ophiuchus — is so young that its building blocks of gas and dust are still coming together. This planet is a newborn being cradled in the arms of its parent star.

“It is like looking at our own past,” said Myriam Benisty, an astronomer at the Institute of Planetology and Astrophysics of Grenoble in France and a co-author of the study.

As the suspected planet is shrouded by the matter that is making it, further telescopic observations will be required to confirm its existence. Presuming it isn’t rocky detritus masquerading as a planet, scientists can use it to better understand how worlds are made.

The torrent of newly discovered exoplanets has complicated or disproved longstanding theories of planet formation. But the location of this baby planet — firmly within the disk of primordial matter around its star — supports the idea that most planets spend much of their time growing up in a similar sort of nursery.

The discovery of the celestial pip suggests “all planetary systems have a common formation process,” said Anders Johansen, an astronomer at Lund University in Sweden who was not involved with the study. Despite the chaos of the cosmos, he said, “there is actually a lot of order” when it comes to crafting planets.

The team of scientists used the Atacama Large Millimeter/submillimeter Array (ALMA), a collection of 66 antennas acting in unison in Chile, to gather evidence of this exceedingly young world. Gas and dust orbits certain stars in so-called circumstellar disks. This material, which clumps together to form planets within these disks, emits radio waves that ALMA can detect.

Last year, Dr. Benisty and her colleagues used ALMA to make the first unambiguous detection of ahalo of gas and dust orbiting an exoplanet: a circumplanetary foundry still making the world it shrouded, and perhaps a few moons too.

For the latest study, they pointed ALMA at AS 209, a star just a tad heavier than the sun. Just 1.5 million years of age, it has only recently started to burn hydrogen — the stellar equivalent of a toddler uttering its first words.

AS 209’s circumstellar disk was found to have several gaps. And in one such gap, ALMA detected the radio-wave signature of a planet-making tempest, gas that was presumably enveloping a Jupiter-like world still under construction.

The planet’s precise age won’t be resolved soon, but it’s likely to be very similar to its nascent star. But its youth is not the only thing piquing astronomers’ interests. It is also bafflingly far from its star. Neptune, the outermost planet in our solar system, is roughly 2.8 billion miles from the sun. This exoplanet is almost 19 billion miles away from its own star.

That raises questions about our own neck of the woods.

The size of the debris disk that forged Earth and the other planets is uncertain. “Maybe the disc was only slightly larger than Neptune’s orbit, and that is why Neptune is the outermost planet,” Dr. Johansen said. But perhaps our hub of planet-making matter was more like AS 209’s. If so, “we also cannot rule out that our own solar system has a planet beyond Neptune,” he said — perhaps the hypothesized Planet 9 that some astronomers suspect is lingering in distant darkness.

In the coming days, the James Webb Space Telescope will determine the mass of the planetary newborn and study its atmospheric chemistry. And by painting a detailed portrait of one of the youngest worlds known to science, these observations will inch us all closer to answering the ultimate question, said Jaehan Bae, an astronomer at the University of Florida and an author of the study: “Where did we come from?”