Astronomers have discovered more than 4,000 exoplanets using instruments like Kepler and the aging Hubble Space Telescope, but only a tiny handful have been imaged directly. One of those planets is PDS 70b, which is a very young gas giant planet orbiting a star some 370 light-years away. Hubble recently took a peek at this world using a new observational technique to reveal previously unseen details. This marks the first time scientists have been able to directly observe a still-forming exoplanet.
The star PDS 70 (also known as V1032 Centauri) is an orange dwarf star with about 82 percent the mass of our Sun. Because it only formed about 10 million years ago, it’s dimmer than most stars of its type, and that allowed the European Southern Observatory to directly image two exoplanets forming around it. Usually, the only way we can detect exoplanets is to infer their presence from gravitational effects or the way they block starlight. Even young stars are thousands of times brighter than exoplanets, so getting a glimpse of the planets themselves is a big deal, even if we can’t resolve much detail.
PDS 70b was first discovered in 2018, but researchers from the University of Texas recently had the opportunity to scan it again using the Hubble Space Telescope. At a mere 5 million years old, PDS 70b is the youngest exoplanet we’ve been able to image directly. That means it could teach us a great deal about planetary formation, if only we could make out some more details. That’s where astronomer Yifan Zhou comes in. Zhou devised a new processing technique that filters out more starlight in the ultraviolet spectrum, leaving just the light reflected from the exoplanet. You can only do this with space-based telescopes because very little UV light makes it through the atmosphere.
This young solar system is still dominated by a disk of dust and gas, which feeds the formation of its planetary system. We now know that PDS 70b is five times more massive than Jupiter, and Hubble has managed to measure its growth rate for the first time. The material spiraling into the enormous planet from the protoplanetary disk appears to follow magnetic field lines, creating “hotspots” that glow brightly in UV. The team speculates these areas could be ten times hotter than the rest of the exoplanet. However, the rate at which material is falling into the planet would only add another one percent of Jupiter’s mass if it continued for another million years. That suggests PDS 70b is at the tail end of its formation process.
The team hopes that this same technique will work in similar solar systems, which could vastly improve our understanding of planetary formation. After more than three decades, Hubble is still surprising us. Just imagine what the James Webb Space Telescope will do if it ever gets off the ground.
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