According to a widely-held theory on how giant gas planets can form — known as core accretion theory — pebbles, rocks, and gases clump together in a rotating disc of dense gas and dust, forming planet-size cores. Depending on how much material a core can accumulate, it could become a small rocky planet like Mercury or a massive gas giant like Jupiter. While scientists believe this broad picture is generally true, many of the details of the planet formation process remain uncertain.
PDS 70 is the only star that harbors two confirmed protoplanets (PDS 70 b and PDS 70 c), newborn planets still in the process of forming. These planets have cleared a large gap in the protoplanetary disk by gobbling up nearly all materials around them. By imaging these planets, scientists reveal that their formation process is still very active as there are telltale signs that the planets are still accreting new materials. A collaboration between three 51 Pegasi b Fellows – Jason Wang, Sivan Ginzburg, and Peter Gao – is beginning to determine the basic properties of these protoplanets: their mass, their rate of accretion, and their composition.
In a paper with Wang as lead author, a team of researchers combined innovative technological techniques and theoretical models to yield new insights on these protoplanets. One of the major challenges in imaging planets is the process of disentangling the faint signal of the planet from the bright glare of its host star. However, the research team used the Keck Observatory’s unparalleled technology (enhanced near-infrared camera combined with an upgraded adaptive optics system) to snap a stable image of the dusty red star system.
Wang applied this new hardware and developed advanced data processing algorithms to obtain unrivaled images of the two protoplanets in the thermal infrared. He then used these images to measure the planets’ thermal emission and trace out possible orbits for the planets.
Simultaneously, Ginzburg worked on new planet evolution models specifically designed for young protoplanets like PDS 70 b and c. Using Wang’s measurements, Ginzburg calculated that the two protoplanets have masses between one and four times that of Jupiter, and that they formed by accreting between 1 and 8 x10-7 Jupiter masses per year. These remarkable results provide insight into how Jupiter may have formed in our own Solar System and make PDS 70 b and c some of the lowest mass planets that researchers have imaged so far.
Gao then used the accretion results from Ginzburg to understand how fast these protoplanets are gathering materials that might linger as dust in their atmospheres. Gao calculated that newly acquired material arrives rapidly enough that dust clouds should completely shroud the planets. Although this adds an extra level of complexity, there may still be windows to see through this shroud of dust to unveil the protoplanets’ composition. One possibility is to use new instrumentation like the Keck Planet Imager and Characterizer (KPIC) to search for the spectral lines of key molecules in the atmosphere. Led by Professor Dimitri Mawet at Caltech, KPIC is anticipated to be fully operational in early 2021 to help unravel this mystery.
“Now that we have established some of the basic properties of these protoplanets, we are even more excited to study them in detail with state-of-the-art instrumentation like KPIC to understand the planet formation processes at work,” said Wang.
The team’s results are published in today’s issue of The Astronomical Journal. The Foundation is humbled to support such talented and creative 51 Pegasi b fellows and is eager to see how their insights will transform the way we think about planet formation.
51 Pegasi b Fellows left to right: Jason Wang, Caltech; Sivan Ginzburg, UC Berkeley; Peter Gao, UC Berkeley