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An image of the newborn planet PDS 70b (bright spot right of center) forming in a gap in the protoplanetary disk around its star, which is blacked out by the camera’s coronagraph mask.

ESO/A. Müller et al.

In a first, astronomers witness the birth of a planet from gas and dust

In the first convincing observation of its kind, astronomers have directly imaged a newborn planet still forming around its star. The planet, hotter than any in our solar system, supports what astronomers have long believed: that such bodies are born of the disks of gas and dust that coalesce around young stars.

“After decades of speculation, it’s nice to actually see one. It’s very comforting,” says astronomer Kevin Heng of the University of Bern in Switzerland, who was not involved in the work.

The initial discovery was led by astronomer Miriam Keppler of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany. Since 2015, her team has been regularly surveying hundreds of young stars for signs of exoplanets with a next-generation imaging instrument called SPHERE (for Spectro-Polarimetric High-contrast Exoplanet Research), attached to the European Southern Observatory’s Very Large Telescope (VLT) on Cerro Paranal in Chile. One of those stars was PDS 70, a 10-million-year-old dwarf star about 370 light-years from Earth.

In 2016, the team looked back at 2015 observations of PDS 70. After additional processing of data, they saw signs of a bright point of light close to the star and clearly visible in a gap in the disk. “That’s the dream of every astronomer looking for planets,” says MPIA team member André Müller. Given that such detections are often controversial, the team followed up with new observations using other VLT instruments and with archived data from instruments on the VLT and the Gemini South telescope on Cerro Pachón in Chile. As they describe in a paper published today in Astronomy & Astrophysics, the planet, dubbed PDS 70b, remained visible in such a wide range of wavelengths of light that the team is confident it's not a mirage.

“It’s a very interesting and pretty convincing result—a solid detection,” says Bruce Macintosh of Stanford University in Palo Alto, California, who leads a rival instrument called the Gemini Planet Imager. Macintosh says there have been tentative detections of very young planets before, but this one stands out. The newborn exoplanet "is very solidly bright and has very politely cleared a gap for itself,” making it even more visible. 

“It’s a very interesting and pretty convincing result—a solid detection.”

Bruce Macintosh, Stanford University

In another paper published today in Astronomy & Astrophysics, a separate group led by Müller estimated the size, mass, temperature, and orbit of the planet by plugging all the information into various orbital and atmospheric models. The group predicts that PDS 70b is several times more massive than Jupiter, with a cloudy atmosphere that hits temperatures of around 1000°C, despite orbiting its star at roughly the same distance as Uranus from the sun. Their models further suggest that PDS 70b itself has a circumplanetary disk of material that is accreting to its surface. The ability to make such predictions opens “a new chapter” in the science of planet formation, Müller says.

Other researchers say such estimates are uncertain because it requires a lot of extrapolation to apply these models to such youthful stars. “It’s extremely tricky, especially when the planet is so young,” Heng says.

But he and Macintosh agree that the detection of PDS 70b holds promise for unraveling the mysteries of how planets form and evolve. “It’s a good sign that planets at these ages are still bright [and so detectable] and can pull the disk apart to become visible,” Macintosh says.

Keppler says the team will continue to observe the planet with SPHERE and with other instruments, hoping to detect it moving around its 120-year orbit. They’ve also applied for time on the Atacama Large Millimeter/submillimeter Array, a collection of radio dishes in Chile. With that telescope’s ability to detect dust, they hope to get a glimpse of that circumplanetary disk.