Infant planets coming together in a system 370 light-years away are shedding light on the early lives of Saturn and Jupiter in our solar system.
At only 5 million years old, PDS 70 is an infant among stars. Its nascent planets are still in the process of forming — and now astronomers have caught that process in action. Sebastiaan Haffert (Leiden University, The Netherlands) and colleagues report the new observations June 3rd in Nature Astronomy.
PDS 70 is so young that it hasn’t yet ignited fusion in its core; nor has its radiation chased away the dust and gas around it. Its protoplanetary disk is 140 astronomical units (a.u.) wide, but with a large, 20–40 a.u. dust-free gap. While not all disk gaps come from planets, in this case it’s pretty clear that planets have swept clear a large region of the disk. The massive planet PDS 70b was directly imaged last year orbiting inside this gap.
Now, Haffert’s team used the same Very Large Telescope in Chile to catch the signature of hydrogen gas flowing onto the planet. What’s more, they’ve discovered a similar gas flow onto what they’re calling another planet: PDS 70c.
This team is using a different instrument than the one used to discover PDS 70b: the Multi Unit Spectroscopic Explorer (MUSE). The Very Large Telescope’s adaptive optics system helped MUSE obtain crystal-clear images of this far-away system. Each of MUSE’s image pixels does double-duty, noting not only brightness in a given waveband, but also recording the spectrum of that light. (These hard-working “spatial pixels” are known as spaxels, which is just fun to say.)
Haffert and colleagues focused on an individual spectral line, the one emitted by hydrogen-alpha, and created an image showing only whatever was emitting that line. After removing the star from the image, two dots remain. One is exactly where the previously discovered PDS 70b should be. The other is on the other side of the star, at the outer edge of the disk gap.
“PDS 70 b is certainly the most definitive case for a protoplanet,” says Thayne Currie (Subaru Telescope / National Astronomical Observatory of Japan), who was not involved in the study. “The PDS 70 c object looks very encouraging: hopefully, future studies will clearly show evidence for orbital motion.”
Both planets have masses many times Jupiter’s, but from these observations, it seems that PDS 70c is less massive than PDS 70b. Both planets are still growing, but at current rates, it would take them 50 million to 100 million years to grow a Jupiter. Given that the star is only 5 million years old, and protoplanetary disks usually only stick around for 10 million years total, it’s likely that the planets have experienced growth spurts in the past.
Based on the planets’ positions, they seem to be in or near a 2:1 resonance, where PDS 70c completes a single orbit for every two orbits of PDS 70b. That means that in these two planets, we may be seeing a scenario play out that affected the evolution of the early solar system.
According to the Grand Tack hypothesis, Jupiter formed earlier than Saturn and was in the process of migrating inward, almost reaching Earth’s orbit. However, when Saturn came together, it too migrated inward, locking the two gas giants into a 3:2 resonance. The resonance reversed their migration, sending them out to their current orbits in the outer solar system.
The scenario sounds chaotic compared to the staid planetary order we learn in elementary school, but this migratory switcheroo can explain several solar system features, such as the existence of an asteroid belt and the absence of super-Earths. Future observations should be able to confirm whether PDS 70b and c are orbiting in resonance. If so, we’ll get a glimpse of how a Grand Track scenario can play out.