Protoplanetary disks made of gas and dust form around young stars, and this is where planets from.
These disks don’t last forever. Eventually, the star’s energetic output dissipates the disk through photoevaporation, the material gets taken up in planets, and the planet-forming process ceases.
All young stars are expected to have protoplanetary disks, and these dusty environments make it difficult to see young planets forming.
Astronomers recently observed a binary star with separate disks. The primary star has cleared out its dusty protoplanetary disk, while its companion hasn’t. Now that the primary star has cleared away the obscuring dust, it’s an excellent target for direct imaging of planets.
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The research is titled “Direct imaging discovery of a young giant planet orbiting on Solar System scales,” and it’s published in Astronomy and Astrophysics. The lead author is Tomas Stolker. He’s an assistant professor of astronomer at the Leiden Observatory at Leiden University in the Netherlands.
The double star system is called HD 135344 AB and it’s about 440 light-years away from Earth. A and B are both young stars, and they orbit each other widely, indicating that their protoplanetary disks evolved separately. The primary star is an A-type main-sequence star, and the secondary star is an F-type main-sequence star.
The critical aspect of this binary system is that the primary star has cleared away its protoplanetary disk, while the secondary star hasn’t.
The secondary star has been studied for decades, largely because it’s still forming planets. Observations revealed a central cavity in the disk, spiral arms, and variable shadowing, all features that suggest planet-disk interactions, even though any actual planets are shielded from observations by thick dust.
The primary star, on the other hand, appears to have no disk, and hasn’t attracted much attention. However, that lack of dust makes it a prominent location to search for exoplanets.
In the new research, the team used the Very Large Telescope (VLT) and its SPHERE exoplanet instrument to directly image a planet orbiting the primary star, HD 135344 A. It took four years of dedicated observations with powerful instruments to detect it.
“Star A had never been investigated because it does not contain a disk. My colleagues and I were curious about whether it had already formed a planet,” said Stolker in a press release.
“And so, after four years of careful measurements and some luck, the answer is yes.”
HD 135344 Ab is a young planet with about 10 Jupiter masses. It orbits at 15-20 astronomical units from its star, and its spectral type is mid-L, meaning it bridges the gap between a brown dwarf or a gas giant. It’s no more than 12 million years old, making it one of the youngest directly-imaged planets.
The fact the primary star has ceased forming planets while the secondary star is still forming planets shows that binary stars can have different planet-formation and protoplanetary disk lifetimes.
When they first detected the planet, it was unclear if it was a planet or a star. But the VLT is a powerful and flexible telescope. It’s made of four separate yet identical scopes that can be used as an interferometer, and four smaller auxiliary scopes that can be positioned independently.
This allowed the VLT and SPHERE to map the planet’s location with extreme accuracy. Over time, they saw the star and the suspected planet move together, confirming that it’s a planet.
“We’ve been lucky, though,” says Stolker. “The angle between the planet and the star is now so small that SPHERE can barely detect the planet.”
Observing and imaging exoplanets is an extremely difficult tasks. Most exoplanet discoveries are inferred from observational data and presented with artist’s illustrations which are interpretations of the data. Though the images of HD 135344 Ab don’t show any planetary detail, they are direct images rather than representations.
The researchers say that the planet likely formed near its solar system’s snow line. Scientists think that this is a key region for giant planet formation.
Different materials are available there because volatiles like water, ammonia, and methane are solids there rather than gases. The collective boost to available solid surfaces means it’s easier for dust grains to stick together and eventually grow into planets.
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It was challenging to determine that the planet was not a background star, something that hinders the direct imaging of exoplanets. Gaia astrometric data plays a big role in this.
“This study also highlights the importance of high-precision astrometric measurements to fully disentangle orbital from background motion in a region of non-stationary background stars,” the authors explain.
But it also took some lucky timing. “A good portion of luck was involved with the discovery of HD 135344 Ab, however, because we caught the planet at a favorable separation along its inclined orbit,” the authors write in their conclusion.
“In the next 10 to 20 years, the angular separation with its star will decrease to ≈10–35 mas, which means that the planet would not have been discovered with SPHERE for a large fraction of its orbit.”
Direct imaging surveys show that giant planets like this one are rare at wider separations of 20 au or greater. The detection of these planets at shorter separations is expected to increase when the ESA’s Gaia astrometric mission releases its fourth dataset in 2026. That data will guide the quest to directly image more exoplanets.
“Gaia DR4 may reveal hints of similar close-in giant planets in star-forming regions, which will guide direct imaging searches and post-processing algorithms,” the researchers explain.
“HD 135344 Ab might be part of a population of giant planets that could have formed in the vicinity of the snowline,” the authors write.
“These objects have remained challenging to detect since most surveys and observing strategies have not been optimized for such small separations.”
If there is a population of young giant planets like this one, exoplanet scientists would love to find them. They could learn a great deal about giant planet formation from them. When they do detect them, the next step is to study them in greater detail.
The upcoming Extremely Large Telescope, set to see first light in 2029, will have the power to do this. This will reveal more about these planets, their compositions, and how they form.
This article was originally published by Universe Today. Read the original article.
