Webb was able to reveal how small, icy objects at the edges of the solar system form planets

Webb was able to reveal how small, icy objects at the edges of the solar system form planets

The biggest blockbuster results from the first weeks of James Webb Space Telescope research has involved seeing massive galaxies over the most extreme distances and times imaginable. But some of its most revealing observations will be looking for the smallest objects yet seen on the outskirts of our solar system.

Using JSWT’s near-infrared NIRCam in a team-up with near-simultaneous optical observations from Hubble, a team of astronomers plans to look at the smallest objects ever seen in Kuiper belt — three to four times smaller than anything humanity has been able to find before at that distance. And by observing them, they hope to resolve long-standing questions about the first stages of planet formation in the early solar system before dwarf planets like Pluto and Eris were interrupted mid-formation.

What is new – JWST is extremely sensitive compared to instruments that astronomers had access to in the past, underlines Wes Fraser, an astronomer at the Dominion Astrophysical Observatory in Canada. In 2003, some members of the same team used what was then the most sensitive instrument available – the Hubble Space Telescope Advanced Camera for Surveys, installed in 2002 – to determine the size distribution of objects in the Kuiper belt.

That survey produced one of the largest, deepest images of the sky ever taken. But it could only turn up a handful of objects less than 100 kilometers wide. Even in a fairly short initial observation like this of about 40 hours, the Webb telescope can see objects almost two magnitudes (about 6x) fainter.

This means that the size of the smallest objects that can be observed in the Kuiper Belt is about to plummet from 20 kilometers to about five kilometers. And while it might not seem like a huge difference, Fraser notes that this distinction is crucial to understanding the formation of planets. “It actually seems to be crossing a relatively critical threshold,” Fraser says Reversed.

The New Horizons probe is the only probe to date to explore the Kuiper Belt. MARK GARLIC/SCIENCE PHOTO LIBRARY/Science Photo Library/Getty Images

Why it matters – It will help settle a long-standing debate. Astronomers have two different models for the formation of the first planets. The older one is a model of hierarchical growth in which pebbles, one by one, collide and form into boulders, which collide and eventually form into planetesimals, and so on. A more recent model is defined by gravitational collapse, where a cloud of gas and dust gathers and rapidly forms larger objects.

Both of these models produce similar results in the size of objects that were previously easily observed by astronomers. But the small Kuiper Belt objects that Webb will be able to see will be able to put the two models to the test.

Because of its relative inefficiency, the hierarchical growth model will produce “a huge amount of very small objects—less than one percent of the dust actually enters Pluto,” Fraser says. The gravitational collapse model is much more efficient at turning small objects into planetesimals, and he notes, “it should be very easy to tell the difference, because in one you see 30 or 50 objects and in the other you see 5 to 10.” By seeing how many small objects there are in the Kuiper Belt, astronomers will get a much better idea of ​​the processes that first formed planets.

Because they’re using JWST in conjunction with Hubble, they’ll also be able to get visual data on the surface of these objects—meaning the astronomers will be able to see whether Kuiper Belt objects look like they’ve undergone powerful collisions or whether they have relatively unscathed surfaces. They will also be able to see if they are likely to occur in binary pairs as larger objects do – one of the key pieces of evidence for gravitational collapse.

1994 JR1 as seen by New Horizons. The object, also called 15810 Arawn, was one of the first Kuiper Belt objects other than Pluto or Charon to be discovered. NASA/JHUAPL/SwRI

What comes next – Dr. Fraser notes that this is just the first set of sightings, and that more will almost certainly happen in the future. Longer observations could see even smaller objects.

But more important is sharing data between different astronomers. Other astronomers, even those who train Webb on the most distant objects rather than the solar system’s backyard, are making similar observations of the same region of space. And Fraser says this sparked a realization: “something dawned on me about a month ago that I haven’t even been able to talk to my colleagues about yet,” that “it’s like, oh my gosh, we can use this data in exactly the same way as the planned observation and get science for free! Which is great.”

After speaking with colleagues in various subfields from around the world, the team now plans to start using other observations in addition to their own. Many different astronomers will use Webb’s NIRCam to take the same types of images, with many short exposures in a row, along the solar system’s ecliptic plane. The team will be able to reuse images, Fraser indicates, just as other astronomers will be able to find distant galaxies in their images that “will just naturally fall out of our data.”

And in one case, this has meant reconnecting with old friends who have gone in different directions in the field. “He works on galaxies and cosmology, I work on the solar system. They couldn’t be more different. It just so happens that his program runs the cosmology survey. I’d never have a reason to talk to him otherwise in an actual professional context. And this is really, really cool to me.”

These other observations also come from around the solar system, meaning “now we get the dynamical distribution of objects of this size,” an unexpected result. In retrospect, it has not surprised Fraser that observations could be reused. He points to the fact that astronomers have already started reusing the original release images: “I think that’s actually one of the really neat things about a new capability like what JWST provides, that it’s going to capture all kinds of data that are useful on completely unforeseen ways that the original designers simply never thought of.”

For him, this represents a once-in-a-generation change. “I think that’s one of the really beautiful things that hasn’t been talked about yet, it’s going to bring together groups of astronomers who would never even consider talking to each other in a way that you don’t get from other facilities… It here is really, really wild for me.”

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