Cause of Spiral Arms Uncovered in New Exoplanet Discovery

Depictions of the Milky Way show a pattern of spiral arms which extend outward from the centre. This coiling pattern is filled with stars, the journal Nature Astronomy reported.

Similar patterns have been observed in the swirling clouds of gas and dust surrounding some young stars. These protoplanetary disks – the birthplaces of young planets – offer scientists a glimpse into what the solar system may have looked like in its infancy.

They also have the potential to shed light on how a planet may form in general.

Scientists have long thought that spiral arms in these disks could be caused by nascent planets. However, none had been detected until now.

In the paper, ‘Direct images and spectroscopy of a giant protoplanet driving spiral arms in MWC 758,’ the researchers made a giant exoplanet discovery dubbed MWC 758c. This exoplanet may be the cause of spiral arms in its infant planetary system.

The astronomers from the University of Arizona propose possibilities as to why scientists have previously struggled to make this exoplanet discovery.

They also show how their methods may apply to detecting other concealed planets in similar circumstances.

“Our study puts forward a solid piece of evidence that these spiral arms are caused by giant planets,” said Kevin Wagner, lead author of the paper and a postdoctoral researcher at the University Arizona Steward Observatory.

“And with the new James Webb Space Telescope, we will be able to further test and support this idea by searching for more planets like MWC 758c.”

The planet’s star is located around 500 light-years away from Earth and is only a few million years old. Therefore, the system still has a protoplanetary disk.

This is because it takes about 10 million years for the circling debris to either be ejected out of the system, ingested by the star, or formed into planets, moons, asteroids, and comets.

The spiral pattern in this system’s debris was first discovered in 2013. Astronomers were quick to point out the connection to theoretical simulations of forming giant planets.

“I think of this system as an analogy for how our own solar system would have appeared less than 1% into its lifetime,” Wagner said.

“Jupiter, being a giant planet, also likely interacted with and gravitationally sculpted our own disk billions of years ago, which eventually led to the formation of Earth.”

Most of the visible protoplanetary disks in stellar systems have been imaged by astronomers. Out of around 30 identified disks, about one-third feature spiral arms.

Spiral arms are prominent swirls within the gas and dust particles of the disk.

They “can provide feedback on the planet formation process itself,” Wagner said.

“Our observation of this new planet further supports the idea that giant planets form early on, accreting mass from their birth environment, and then gravitationally alter the subsequent environment for other, smaller planets to form.”

Spiral arms are generated due to the orbiting companion’s gravitational pull on the material orbiting the star. This means that the presence of a massive companion, like a giant planet, was expected to trigger the spiral pattern in the disk.

However, previous attempts to detect the responsible planet have not worked.

“It was an open question as to why we hadn’t seen any of these planets yet,” Wagner said.

“Most models of planet formation suggest that giant planets should be very bright shortly after their formation, and such planets should have already been detected.”

The researchers made the exoplanet discovery by using the Large Binocular Telescope Interferometer – a University of Arizona instrument.

The LBTI is a very sensitive infrared telescope, and can even outperform Webb when detecting planets very close to stars due to its large size.

“We propose two different models for why this planet is brighter at longer wavelengths,” said Steve Ertel, co-author of the paper and LBTI lead instrument scientist.

“Either this is a planet with a colder temperature than expected, or it is a planet that’s still hot from its formation, and it happens to be enshrouded by dust.”

“If there is a lot of dust surrounding this planet, the dust will absorb shorter wavelengths, or bluer light, making the planet appear bright only at longer, redder wavelengths,” said co-author Kaitlin Kratter, a UArizona theoretical astrophysicist.

“In the other scenario of a colder planet surrounded by less dust, the planet is fainter and emits more of its light at longer wavelengths.”

Large amounts of dust in the planet’s vicinity may tip off that the planet is still forming.

If the planet follows the colder model, there might be something going on in these early stellar systems that causes planets to form colder than expected. This might cause scientists to revise their planet formation models and exoplanet discovery strategies.

“In either case, we now know that we need to start looking for redder protoplanets in these systems that have spiral arms,” Wagner said.

The team believe that once they observe the discovered exoplanet with the James Webb Space Telescope, they will be able to identify which of the two scenarios is occurring in the system.

They have been granted time to use Webb in early 2024 to complete these observations.

“Depending on the results that come from the JWST observations, we can begin to apply this newfound knowledge to other stellar systems,” Wagner said, “and that will allow us to make predictions about where other hidden planets might be lurking and will give us an idea as to what properties we should be looking for in order to detect them.”

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