The Sciences

For the First Time, Astronomers Witness a Planet's Birth

D-briefBy Shannon StironeNov 18, 2015 9:04 PM
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An artist's conception of planets forming in a transition disk like LkCa 15. (Credit: NASA/JPL-Caltech) A fledgling star system is giving astronomers the cosmic version of the “birds and the bees talk.” For the first time astronomers directly observed a planet growing in its very early stages of life, and that’s quite a rare find: Of the nearly 1,900 planets discovered outside of our solar system the infant planet, known as LkCa 15 b, is the only one known to be forming as you read this. It’s a first-of-its-kind opportunity to study a planet in this stage early of development.

The Birth of a Solar System

When stars are born a disk of gas and dust envelops them, forming what is known as a circumstellar disk. You can think of this disk like a doughnut. The ring, or doughy part, consists of dust and gas that’s being kicked out by the forming star in the center, or doughnut hole. Scientists believe the leftover dust and gas particles from this disk serve as the material to form planets. Dust accumulates and begins drawing in pebbles, then larger rocks, boulders, and ultimately the mash-up of debris coalesces to form a rocky core. These cores can grow so large that they have enough gravitational mass to start sucking up the hot gases surrounding the star. “New stars have so much dust and gas around them, streaming off in any direction, that some of its material is bound to run into these rocky cores,” says Kate Follette an astronomer at Stanford University a co-author of the study. Because these young star systems have so much material around them, it makes it hard to discern if any planets are forming in them. Plus, these young planets are still relatively small, adding yet another challenge to locating them. With these impediments in mind, Scientists at the University of Arizona and Stanford University devised a new method for directly observing this elusive process.

And LkCa 15 b, located 450 light years from Earth in a star system just 2 million years old, was the baby they were looking for.

A Planet in Its Infancy

This wasn’t the first time scientists had observed LkCa 15 b and its host star LkCa 15. Evidence of the protoplanet was first detected in the system in 2012. Still scientists didn’t know what was happening at its center. To cut through the noise, Stephanie Sallum, Follette and their team decided to search for optical signatures that they would expect to find in forming planets.

(Credit: by Steph Sallum. Greyscale image generated from sub-millimeter data published in Isella et al. 2014) When planets form, a disk of gas and dust builds up around it, similar to the birth of a star. As the disk of material feeds the growing planet it generates intense heat that can reach 17,000 degrees Fahrenheit releases energy and glows in a distinctive reddish hue. The red glow is caused by hydrogen-alpha photons, which only occur when a circumplanetary disk accretes onto a planet. “This is what is what happens when hydrogen gas gets shocked as it falls onto the planet, it emits H-alpha photons,” says Sallum. The team observed the forming system through the Magellan telescope in Chile, but added a special filter to allow only these telltale H-alpha photons to reach the telescope’s mirrors. What they found was LkCa 15 b growing right before their eyes. It was the first time photons from a growing planet had traveled 450 light years to scientists’ computer screens, allowing them to see a the process unfold. They published their findings Wednesday in the journal Nature. “This observation tells us the where, when and how planets form when they are young. We now know that they can form in really young star systems and clear out a large cavity in a short amount of time,” says research team member Zhaohuan Zhu, an astrophysicist at Princeton University. There are two other protoplanets forming in this system as well, but they are not emitting H-alpha photons. This could be because there is still too much material blocking the detection of that signature, or they could be in a different process of their growth. This method of detection has been used to observe the disks of forming stars before, but never for finding planets in their infancy. Now that this method has been tested and successfully produced an image of a planet early on in its formation, it is sure to be used to examine other young stars that might host growing protoplanets in their system. The LkCa 15 system is only two millions years old, and understanding what’s going on in that active system may reveal new insights into the way planetary systems mature into “adults,” like our aged, 4.6-billion-year-old solar system.

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