The University of Arizona’s Magellan Adaptive Optics Xtreme instrument, combined with a six-and-a-half meter ground-based telescope, has imaged protoplanets in sharp detail. By capturing images of the PDS 70 planetary system, these observations revealed characteristics of planets in their early stages of development.

For the first time, astronomers were able to observe these planets glowing H-alpha as they accrete (grow in mass) and the changing rate at which they consume hydrogen. These astronomers were also the first to observe the rings of dust around these planets.

“We’re making the highest resolution images of outer space,” Laird Close, a professor of astronomy at the UA, said. “And that includes space telescopes like the famous [James] Webb and HST [Hubble Space] telescopes.”

According to Close, their technology has deformable mirrors, allowing the team to guarantee the removal of blurring and turbulence from the cold and hot air mixing above the telescope.

“We basically sense the aberrations in the atmosphere coming from the bright star in the center, and we then put perfect glasses, if you will, in front of our camera, so that it makes perfect images, and we’re changing that power. We’re changing that optical prescription 2000 times a second,” Close said. 

Close explained that with MagAO-X, they can remove the twinkling stars in the night sky and produce optimal images.

The PDS 70 planetary system is 370 light-years from Earth. There are two confirmed protoplanets, gas giant exoplanets, inside this system: planets b and c. 

Close said the big gap in the gas and dust disk around the star in PDS 70 was the first aspect which clued astronomers in that this was an important solar system. Once they looked inside, they could see these H-alpha-emitting planets carving out this gap.

“These two big planets are orbiting around, and they’re acting like vacuum cleaners, and they’re getting rid of all the dust that’s around the star,” Close said.

According to Close, H-alpha is a special frequency and wavelength of light associated with hydrogen gas. 

Hydrogen falls onto a planet quickly, and when it hits the surface, it generates a large amount of heat. When that heat creates a plasma, the hydrogen gas in that hot temperature glows at this H-alpha wavelength as the planets grow, Close said. 

“We can see them [the young planets] eating, if you will, hydrogen gas,” Close said. 

H-alpha emission confirms that these are growing baby planets. Astronomers using MagAO-X target this wavelength with a narrow band filter.

Since the astronomers working with this technology are trying to observe the accretion signatures from the baby planets and the emission line of H-alpha light is small, the narrow band filter allows the accretion signature light to pass through while excluding other types of light, Jialin Li, a graduate student at the UA, said.

“That really gives us that precision in terms of isolating the planet’s signal compared to its surroundings,” Li said.

On the other hand, astronomers also observed considerable changes in the planets’ brightness.

According to their paper in The Astronomical Journal, “It is now well established that some gas giant protoplanets, pass through a period of high luminosity as they accrete hydrogen gas from their circumplanetary disks (CPDs) producing detectable Hα emission.”

Over the years, PDS 70 b’s light has gotten fainter, and PDS 70 c’s has gotten brighter. Close said that the difference is that planet c is consuming more hydrogen gas than planet b.

MagAO-X technology led to the first observation of this phenomenon in protoplanets.

At the same time, rings of dust and material orbit around the planets.

“People theorize that there’s this dust orbiting around the planet, but no one’s actually observed it in scattered light, and we are the first ones to do that,” Close said. “Over time, they’ll collapse down and they’ll form their own moons.”

According to Close, if astronomers could look at Jupiter when it was 5 million years old, the solar system would have looked a bit like PDS 70. Planets b and c could resemble, for example, Jupiter and Saturn, as the circumplanetary disks around those planets collapsed and formed moons. 

Because our solar system is around 5 billion years old, MagAO-X is seeing what a solar system looks like when it’s 1000 times younger.

In line with these compact rings of dust, the team is also commissioning a new observational mold with MagAO-X: a polarimeter.

A polarimeter would help the system detect polarized light better, Li said. While the planets and the star of PDS 70 emit direct, non-polarized light, the starlight hitting and reflecting off the dust particles, the scattered light, is polarized. MagAO-X is collecting all sorts of light now, but the polarimeter would be another technique to see the extended structures better.

“It should really allow us to characterize these smaller scale circumplanetary disks around these baby planets,” Li said.

With the current MagAO-X instrument, the photon-counting technologies they utilized have also made it possible to observe these baby planets.

While it can be difficult for other observatories or noisier types of cameras, the cameras this team uses are working at the theoretical limit, where every photon that makes its way to the detector can be more or less recorded and captured, Close said. 

As few photons come from these baby planets, it’s important to be able to count them.

Their published research paper reported that “the flux from these planets at Hα is very low; indeed, we only expect approximately ∼3 Hα planet photon to be detected by a given pixel every minute.”

In the future, the next big step is building a bigger telescope with better resolution. 

According to Close, the university plans to complete the Giant Magellan Telescope — 25 meters in size — and get it operational and equipped with an extreme adaptive optics system. Although the completion is far from now, the GMT instrument could give more insight into the universe.

“The next big step is to start looking for baby Earths,” Close said. “[GMT] could maybe even find life on other planets in reflected light.”

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