Our understanding of planet formation is often shrouded in theoretical models. But now, scientists have directly observed a new planet in the gaseous disk around a young star about 370 light-years from Earth.
The SPHERE instrument on the Very Large Telescope in Chile imaged the newborn planet PDS 70b.
ESO / A. Müller et al.
The search for new planets often starts with a survey of the heavenly regions where stars are born. Scientists at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany were doing just that when they focused their attention on the young star PDS 70. At 5 million to 6 million years old it is still accreting mass from its surrounding disk of gas and dust. The star —which is about the size of our Sun — was first discovered in 1992, but scientists using the Very Large Telescope’s SPHERE instrument hoped to see new details in its disk. To their surprise, they found a gap — a dark path — in the middle of the glowing gaseous swirl.
While disk gaps are often taken to be the signature of a newborn planet, the discovery isn’t guaranteed — other processes can create disk gaps, too.
To confirm the planet’s presence, scientists used SPHERE to block out the light from the star, whose glare hides surrounding features. When researchers could see the dark path more clearly, they found a point source of light exactly in the location where a planet would be expected to form, the group reports in Astronomy and Astrophysics.
In a follow-up study, scientists went a step further, using the brightness and age of the planet — dubbed PDS 70b — to derive its mass. “Mass is the main indicator that it’s a planet,” says the study’s co-author Sascha Quanz (ETH Zürich, Switzerland); indeed, PDS 70b turned out to be between 2 to 17 times Jupiter's mass, which qualifies it as a planet rather than a brown dwarf. It’s made mostly of gas and has a temperature of about 1000°C.
By analyzing the planet’s spectrum, scientists discovered that the planet’s atmosphere even contains clouds. “Clouds have a different meaning than on Earth,” warns Müller; this planet’s extreme temperatures don’t allow for water condensation. Instead, its clouds contain ingredients such as iron, sodium sulfite, potassium chloride, and even a magnesium compound called fosterite.
In addition to using new observations, scientists also revisited observations dating back to 2012 in order to track the planet’s location. While doing this, they saw an arc in the protoplanetary disk that was indicative of orbital motion. The team concluded that it takes PDS 70b about 120 years to orbit its star.
The planet is still forming by accumulating material, adds André Müller (MPIA), the second study’s leader author. The present findings “confirm theoretical models, but we need more of these systems to understand the process,” he says.
Trent Dupuy (Gemini Observatory), who was not involved in the study, warns of the “technological challenges of confirming very faint objects next to very bright stars,” which make it difficult to confirm that any point source is a new planet. But he adds: “Overall, I think this extremely exciting.”