Of the thousands of new exoplanetsidentified in the past few years, none has been quite like this one—because it's still forming. Astronomers led by Stephanie Sallum at the University of Arizona and Katherine Follette at Stanford University have discovered the first planet still in the process of formation.

The hatching planet is nestled between a massive ring of dust and gas and a young 2-million-year-old star named LkCa 15. That star is roughly 450 light years away from Earth and is similar in size to our Sun. While scientists previously had discovered other immature star systems like LkCa 15, where infantile planets are hypothesized to be growing, Sallum and Follette are the first to directly observe such a planet. Their discovery is published today in the journal Nature.

"It's an exciting observation," Sallum says, "now we can finally watch planetary formation as it's happening," fueling new theories and even refining our understanding of how our own Earth was born.

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We've been finding planets beyond our solar system for two decades now, but there are good reasons why it's taken so long to find the first forming world. For one thing, Sallum says, planets spend only a brief period of their long lives in formation. Simply looking at the the odds, "it's unlikely that you'll come across a planet when it's still forming," she says.

Secondly, young star systems like LkCa 15 are inherently gassy and dusty, with stars that shine relatively chaotically. That messiness rules out the popular observational method that's detected most recently discovered planets: waiting for stars to dim as planets pass in front of them.

Instead, Sallum and Follette found the forming planet by looking at the star system from "above," focusing on a relatively empty gap between the star and and a swirling outer ring of dust and gas. (That gap is caused by the accretion of planets.) But even knowing where to look, "it's just a really difficult observation to make—450 light years is far enough away that the separation between the planet and the star we're seeing is really, really small. And keep in mind, the planet is much fainter than the star," Sallum says.

The scientists overcame this challenge by cleverly combining two types of astronomical observation for a new insight. The researchers overlaid infrared images of the LkCa-15 star system taken by the Large Binocular Telescope in Arizona with a highly specific measurement of a certain wavelength of red light—called hydrogen-alpha emissions—from the Magellan Telescopes in Chile. That light is a strong sign of a forming world; it's given off when hydrogen atoms fall through the magnetic field of a blazing new planet.

Together, the two observations allowed the scientists to "unambiguously detect one planet," Sallum says, which is currently named LkCa-15b. Oddly, the data suggest that there's actually two otherforming planets in addition to the one they found. But for unknown reasons, the other two are not giving off hydrogen-alpha emissions, making their existence hard to confirm conclusively.

Adaptive optics observations from the Large Binocular Telescope and the Magellan Adaptive Optics System (color scale) show multiple sources in the cleared region of the LkCa 15 transition disk (greyscale). Steph Sallum

As you might imagine, the new discovery is a boon to researchers studying how tiny particles of dust accumulate into massive worlds. Despite the deluge of new worlds, the mechanics behind how exactly these planets are forged from dust and gas is still guesswork. It's clear that after a star is born, the surrounding matter can eventually accumulate into planets, but that process had never been observed before.

Sallum and Follette's discovery "provides important clues about when, where, and how young planets are born," writes Zhaohuan Zhu—an astrophysicist at Princeton University who was not involved in the discovery—in an essay accompanying the research in the journal Nature.

That's not just because planetary formation theories will have to match what was found in the real cosmos. Zhu says the clever detection method of combining infrared and red hydrogen-alpha emissions will quickly lead the way to astronomers conforming the discovery of many other proposed, still-forming planets. "[Sallum and her colleagues] have demonstrated a powerful technique to find young planets. . . one that will discover many such planets in the future," Zhu says.

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