Has the birth of a giant planet been caught on camera?

Young planet
The opportunity to directly observe the conditions under which a planet is forming will enable us to extrapolate this process to other circumstellar discs that possess similar conditions. This object has the potential to teach us about some of the conditions that are sufficient for a new planet to form.
Dr Sascha Quanz
Astronomers using the European Southern Observatory’s (ESO) Very Large Telescope (VLT) believe that they have made the first ever direct observation of a planet in the process of being formed. If confirmed, this discovery could significantly improve our understanding of planet formation by allowing researchers to test their theories against an observable target.

The candidate planet appears to be orbiting HD 100546 – a relatively young star located 335 light years from Earth. The body in question is embedded in the disc of material that surrounds its star, and experts believe it to be a gas giant similar to Jupiter. Computer simulations suggest that the object, which was spotted in the form of a faint blob picked up by the VLT’s NaCo adaptive optics instrument, is most likely a planet in its natal stages.

Further research will be necessary in order to provide a conclusive answer as to the status of this object, but the team is cautiously optimistic that the VLT has captured the birth of a gas giant. To find out more about what this discovery could mean in terms of our understanding of planet formation, I spoke to the study’s lead astronomer Dr Sascha Quanz, Senior Research Associate at ETH Zurich’s Institute for Astronomy. I began by asking why he and his colleagues suspect that this is indeed a planet in the throes of birth.

"So far, we have only been able to measure the luminosity of the object," Dr Quanz explained. "At present, this is the only measure that we have. When we look at the luminosity of the body, we see that it is surprisingly bright. In fact, the data would suggest that its mass is 20 to 30 times greater than that of Jupiter. However, if this were true, the disc of matter around the star would be disrupted tremendously. There would be large scabs, holes, spiral waves, and all manner of weird phenomena that occur when you put a massive body into a circumstellar disc.

"Our observations show that these structures are not present," he continued. "We therefore require an alternative explanation for the combination of high luminosity and low mass. The most likely reason for this is that we are looking at a forming gas giant. During a planet’s formation, matter is transferred from the disc to the object and this boosts its luminosity. The VLT has not only measured the intrinsic luminosity of the planet. It has also picked up luminosity generated by the accretion process."

Whilst early indications suggest that the ‘new planet’ hypothesis represents the likeliest explanation, the astronomers are not yet able to confirm that this is the case. Another possible explanation is that this is a fully formed planet that has been ejected from its original orbit. Alternatively, the signal picked up by the VLT might have originated from a background source. I asked Dr Quanz how he and his colleagues intend to test their protoplanet theory.

"The answer to that question is simply that we must collect more data," he answered. "We are going back to the VLT in April to see whether the object is still in the same location. This is our big hope. Statistically, it is very, very unlikely that a background source is responsible for our finding, but this explanation cannot be completely ruled out at this stage. In light of this, the first thing that we have to do is measure its position relative to the star. If a background source is responsible, we should find that its position has changed by a measurable amount. This is because whilst the star will move, the background source will remain stable. However, if the relative positions are the same as they were when we last measured them, we can eliminate this possibility once and for all.

"After this, we must continuously monitor the object," continued Dr Quanz. "This will allow us to see whether or not it has orbital motion; a property that you’d expect a forming planet to exhibit. Our calculations suggest that this planet’s orbit is approximately 360 years. This means that we would expect to see one degree of rotation per year. In five years, therefore, the planet should have moved through five degrees of its orbit. This is something that we can accurately measure; it should allow us to prove that this object really is bound to its star."

Whilst an ejected planet would still be of interest to the ESO team, the astronomers are hoping to demonstrate that they have made the first ever direct observation of a protoplanet. Confirmation of this hypothesis would provide a launching pad for researchers to test their theories against a tangible example. My final question for Dr Quanz concerned what such an example might reveal about planet formation. Which theories is he most interested in testing?

"It would be extremely interesting to learn more about the interaction between the planet and its star’s surrounding disc," he replied. "We will not be able to witness the full formation process as it takes far too long. According to our models, the planet is probably a few hundred-thousand years old. It could therefore take up to a million years for the formation process to run its full course. Even so, we are able to constrain the parameters of the disc with currently available technology. Instruments such as the VLT, the Atacama Large Millimeter/submillimeter Array (ALMA) and the James Webb Space Telescope (JWST) will allow us to measure the gas and dust contained within the disc, its temperature, its density, etc. We will be able to precisely characterise the disc in terms of its physical and chemical properties.

"The opportunity to directly observe the conditions under which a planet is forming will enable us to extrapolate this process to other circumstellar discs that possess similar conditions," Dr Quanz concluded. "This object has the potential to teach us about some of the conditions that are sufficient for a new planet to form. For the first time, we would be able to empirically test hypotheses relating to planet formation."


Image: An artist’s impression portraying the formation of a gas giant planet in the ring of dust around the young star HD 100546.

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