Science —

First images of a planet in the act of forming

Hydrogen-alpha light is a dead giveaway that the planet is still growing.

An artist's conception of the still-growing exosolar system.

A study released today describes a new technique that has allowed scientists to observe a planet forming for the first time. Using adaptive optics, the researchers observed light coming from the planet that gave a clear indicator that the body is still growing and forming.

Planets in general are pretty hard to observe, and forming planets are even harder. Normally, dips in the light of a star indicate to researchers that there’s a planet orbiting it. But that technique doesn’t work in young exosolar systems where the planets are still forming. Young stars, having recently finished forming themselves, are typically pretty chaotic, with their light output varying. And it’s nigh-impossible to tell the difference between these natural variations in a star’s light and a dip caused by a transiting planet.

Fortunately, a forming planetary system has its own observational advantages: the planets are putting out a lot of energy in the form of light. They're especially bright in hydrogen-alpha emissions, a particular shade of red that is very often useful in astronomy because it’s strongly emitted where hydrogen gas is ionized. It’s thought that the region around a still-forming planet can become hot enough to cause the hydrogen atoms to emit hydrogen-alpha light.

The planet’s magnetic field is thought to play a strong role in this process. It can draw in material from the disk of material around the star, which heats up as it falls into the planet, emitting hydrogen-alpha light.

Using this light, researchers have created an actual photograph (a composite of hydrogen-alpha light and a few other wavelengths) of the star system in question, LkCa 15. The image appears to show two planets, with a possible third as well. The first planet, LkCa 15b, is a strong hydrogen-alpha emitter, confirming it as a forming planet. The other two (LkCa 15c and d) are not as strong, and they are visible mostly in different wavelengths.

Researchers have been observing these planets for the past few years, watching them move through their orbits over that time. Their motion can be seen in this video as an artist’s rendition.

The images provide important data for models of planet formation. One point of interest is that LkCa 15b is about 15 to 16 astronomical units from its star and has been forming for about two million years. It wasn’t expected that a body would still be growing after all that time, given the amount of material present at this distance.

Also, the amount by which the planet’s magnetic field has eaten into the circumstellar disk gives an estimate on the strength of the magnetic field. It must be at least 20 times the present-day magnetic field of Jupiter. In gas giants like Jupiter and LkCa 15b, the magnetic field is controlled by the churning inner structure of the planet. So measuring the magnetic field provides an indirect glimpse at what’s going on inside the forming planet.

Future work is needed to determine the nature of the other two planets and to learn more about the system in general. Its discovery is a great opportunity for researchers, as it provides a perfect laboratory for the study of planet formation.

The researchers’ technique—looking for the hydrogen-alpha signature of a forming planet—is not exactly new. The same basic idea has been used to observe hydrogen-alpha light coming from a forming star. But applying it to a smaller, planet-sized scale and actually finding planets with it is a first. It’s an important technique that can be used to discover other baby planets, just as the technique of looking for dips has been doing with mature ones.

That, in turn, will provide more detailed understandings of how exosolar systems grow and become mature like our own Solar System.

Nature, 2015. DOI: doi: 10.1038/nature15761 (About DOIs)

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