Scientists spy two - or maybe three - planets in the process of being born


For the first time, scientists have witnessed planets being born.

The two — or possibly three — protoplanets are in orbit around a young sun-like star that is about 430 light-years from Earth. Their discovery could give scientists new insight on how planets come to be and what they are like in their formative years.

“We are unambiguously witnessing planet formation,” a team of astronomers and astrophysicists wrote in a study published Wednesday in the journal Nature.

Nearly 1,900 planets have been found in orbit around distant stars, but all of them are grown up, like the eight planets in our own solar system. Mature planets can be detected when they pass in front of the stars they orbit, causing their light to dim.


But infant planets are too small — and too volatile — to make themselves known this way. So astronomers have tried to observe them directly, pointing their powerful telescopes toward young stars and the relatively empty space around them, a region called the disk cavity.

In late 2011, two of them finally found a possible protoplanet in the area around LkCa 15, a star estimated to be a mere 2 million years old. From what they could tell, this potential planet was about 1,000 times less bright than LkCa 15.

To learn more about this mysterious object, a team led by Stephanie Sallum, a graduate student at the University of Arizona, pointed the Large Binocular Telescope in southeastern Arizona toward LkCa 15. They used a technique called non-redundant masking, or NRM, to get high-resolution images of the region surrounding the young star. (Astronomers have such high hopes for this technique that they added NRM capability to the James Webb Space Telescope, now set to launch in 2018.)

Using NRM, the team identified two bodies — dubbed LkCa 15b and LkCa 15c — that moved in stable orbits around the star.

LkCa 15b is about 15 times farther from its star than Earth is from the sun, and LkCa 15c’s orbit is about two Earth-sun distances beyond that. Since LkCa 15b is closer, it moves faster along its orbital path.

Based on their motion around their star, the scientists calculated that the two objects are no more than 5 to 10 times bigger than Jupiter.


The team also picked up a faint signal for a possible third object, which they called LkCa 15d, but that signal wasn’t strong enough for them to be sure it was real.

To learn more about these possible protoplanets, the scientists used a different observing mode to look for the signature of H-alpha light. This is emitted by stars or even planets when the electron in a hydrogen atom downgrades its energy level in a specific way, something that can happen when dust and gas coalesce into a new planet.

In these observations, LkCa 15b was apparent, but LkCa 15c was undetectable. That didn’t mean the researchers were ready to give up on it though. Perhaps LkCa 15c happened to be in a relatively quiet period when the telescope was looking at it, and thus wasn’t producing H-alpha light, they wrote. Another possible scenario is that the telltale light was absorbed by gas and dust, so the telescope wasn’t able to pick it up.

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Either way, the researchers were convinced that the two objects they saw were planets in the making. By matching up their observations with the laws of physics, they determined that the most likely way these planets are being made is through an “accretion disk model,” in which an object accumulates the gas and dust around it. It’s like a junior-varsity version of the way stars form. The H-alpha emissions from LkCa 15b make the story very clear for this protoplanet.

As they keep watching, the scientists hope to learn more about whether planets form in a steady process or experience growth spurts.


In an essay that accompanies the study, Princeton astrophysicist Zhauhuan Zhu wrote that the observing techniques Sallum’s team used should make it possible for scientists to find nascent planets that were previously invisible because they were too far away or masked by too much dust (or both).

“This discovery has far-reaching implications for our understanding of the planet-formation process and of the properties of young planets,” he wrote.

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