Richard A Lovett
October is planetary science month. That’s when the American Astronomical Society’s Division for Planetary Sciences holds its annual meeting, with hundreds of researchers discussing planetary bodies large and small. Want the latest on the search for Planet 9? Here’s the place to find out. (Hint: nobody’s found it.) Wondering what counts as a planet? Forget the controversy over Pluto. Planetary science welcomes objects that would fit tidily within a football stadium.
So what did we learn in this year’s meeting?
First and foremost, the proper name is Division for Planetary Sciences. Not “science” but “sciences”, and not “of” but “for”, as DPS chair Amy Mainzer, of the University of Arizona, said in a humorous welcoming address.
Her comedic riff underscored another thing: planets are fun. Here are a half-dozen of the latest nifty findings.
1. Planets can be hot enough to vaporise lava
We’ve all heard of rocky planets like Earth and Mars. But exoplanet WASP 76b has a rocky atmosphere.
More interestingly, it’s hot enough to vaporise rock.
Scientists already knew it was hot. It orbits so close to its star that its “year” is 1.8 Earth days, and its sun is hotter than ours. They’d also discovered it’s so hot it can vaporise iron and (probably) produce iron rain.
But vaporised rock? That’s hotter yet. Nevertheless, there appears to be a lot of ionised calcium (a common constituent of rocks) in its atmosphere, says Emily Deibert, a graduate student at the University of Toronto.
“That tells us the atmosphere is even more extreme than anticipated,” she says – possibly substantially hotter than 2200°K (~1900°C). To put that in context, that’s hot enough for its air to glow red-hot.
2. Missing Plutos
Our Solar System is believed to have been born with a great many Pluto-like worlds, orbiting in the cold, dark reaches beyond Neptune, says Nathan Kaib of the University of Oklahoma.
But early in the Solar System’s evolution, the orbits of the giant planets (particularly Neptune) shifted, throwing the vast majority of these Plutos off into interstellar space. Today, only two remain: Pluto, and its near-twin Eris, which is nearly the same size but two-thirds farther out.
To figure out how many Pluto-like bodies might once have been there, Kaib’s team modeled the Solar System’s evolution, looking to see how many primordial Plutos were needed for two (give or take a bit) to be retained. “We ran 10 simulations, varying the number from 2,500 to 200,” he says.
Based on that, he says, it appears that the primordial number was somewhere between 1,000 and 200. If he’s right, Pluto and Eris are the only survivors of what was once a huge cohort of dwarf planets. The rest are now drifting in interstellar space, along with who knows how many others thrown away by other solar systems. Who ever said that interstellar space was a void?
3. NASA has cool new missions up its sleeve
One is DART (Double Asteroid Redirection Test), which launches in late November, with the intent to smack a rocket into an asteroid and see what it takes to change its course.
Its target is a 160-metre-wide moonlet of a 780m-wide asteroid called Didymos, chosen because it is typical of the type of asteroid that might hit Earth with enough punch to cause widespread damage.
Also, says team member Cristina Thomas of Northern Arizona University, targeting a moonlet of a larger asteroid makes it easier to measure the effect of the impact by measuring the change in its orbital period, something that can be done much more quickly “than if we were to impact something orbiting the Sun”. (Not to mention that there’s no chance of knocking the moonlet out of orbit and inadvertently diverting it toward Earth.)
Also up for launch, possibly as soon as 16 October, is Lucy, which will visit Jupiter’s Trojan asteroids.
These are asteroids that Jupiter has captured in a gravitational resonance with the Sun, trapping them in two clusters, one about 120 degrees ahead of it in its orbit, and the other 120 degrees behind.
Lucy, says Simone Marchi, the mission’s deputy project scientist, from the Southwest Research Institute in Colorado, will visit six of these asteroids, using an “amazing trajectory” that allows it to loop outward from Earth, swing through the leading swarm in 2027–28, then fall back toward the Sun and loop back outward just in time to encounter the trailing swarm in 2033.
On the way out, he adds, it will even be able to visit a seventh asteroid in the main asteroid belt, along with its recently discovered moon, meaning that Lucy will get eight asteroid flybys for the price of one.
Better yet, there’s no reason the spacecraft can’t loop back and forth between the leading and trailing Trojans again and again, until something breaks or it runs out of manoeuvring fuel. “We may be able to access the Trojan clouds for many years,” Marchi says.
4. Most objects in the Kuiper Belt may be binaries
When NASA’s New Horizons spacecraft flew by the object now known as Arrokoth, its team was startled to discover that it appeared to have been formed from the gentle merger of two smaller bodies.
Now, says Hal Weaver, a planetary scientist at Johns Hopkins University, New Horizons has found that Arrokoth might not be a one-off. As many as two-thirds of far outer Solar System bodies might actually be similar, he says, though not quite so tightly spaced that they have fused the way Arrokoth did.
Since its flyby of Arrokoth in 2019, Weaver says, the New Horizons spacecraft hasn’t been idle. Even though astronomers haven’t yet found another object close enough to its course to fire up its engines to steer for a close flyby, they have found four others similar to Arrokoth, close enough for its cameras to get images with 10 times better resolution than anything possible from Earth, even with the Hubble Space Telescope.
In one case, the spacecraft failed to get a good image. But in two of the three others, Weaver says, it found that they were “tight twins,” meaning that they were pairs of roughly equal-sized bodies orbiting within hundreds of kilometres of each other – far too close for them to be seen as twins from Earth.
“That may be telling us something,” he says.
Specifically, it provides support for a model in which the earliest protoplanets formed not by a cascade of massive smashups that created ever-growing survivors, but by the gentle collapse of swirling groups of pebble-sized objects.
For some yet unknown reason, he says, this process tended to form tight binaries that sometimes merged, like Arrokoth, and sometimes didn’t, with everything, at least in the outer Solar System, occurring very sedately – “on the order of how fast you can walk”.
5. Boulder-studded asteroids are sponges
One of the biggest surprises Japanese and American scientists discovered when their sample-return spacecraft, OSIRIS-REx and Hayabusa2, reached their respective asteroids, Bennu and Ryugu, was that both were covered with boulders.
“We expected a smooth surface rich in fine-grained materials,” says Saverio Cambioni, a postdoctoral researcher at Massachusetts Institute of Technology. “We found big rocks. That was a bit of a shock.”
Now, he thinks he knows why. While OSIRIS-REx was preparing to take its sample, he says, it studied changes in infrared (heat) emissions from the asteroid’s surface, watching how its boulders change temperature during the course of its day.
From the way they heat and cool, he says, it appears that they are highly porous. That means that when they are hit by micrometeorites, they don’t break into dust and sand, they simply compact. “A high-porosity rock acts like a sponge,” he says.
This, he says, is why both missions found bouldery landscapes from which it was difficult to scoop up the type of fine material they were designed to obtain.
Luckily, both found a way around the problem and obtained their precious samples. But in the process, Cambioni says, they discovered things about asteroidal rock that will be useful to future missions.
6. Juno’s propulsion failure was an unexpected gift
NASA’S Jupiter-orbiting Juno mission was planned to last less than 18 months, orbiting once every two weeks for 32 orbits before plunging into the giant planet on a final, fatal dive.
But when it braked into Jupiter orbit, an engine malfunction stranded it in what was supposed to be a temporary 53-day orbit, meaning that 32 orbits would now take nearly 4½ years. Furthermore, the damaging effect of Jupiter’s radiation belts, which it encounters briefly on each orbit, has proven less than feared, and the mission has been extended by 42 additional orbits.
That means Juno is still going strong. And over the course of the extra years, its orbit has slowly evolved to allow flybys of moons that haven’t been seen close-up for decades.
Already, it’s done two unhoped-for flybys of Jupiter’s largest moon, Ganymede. “We got within 1,000 kilometres,” says Candice Hansen, of the Planetary Science Institute.
Next up is the ice world Europa, then the hyper-volcanic moon Io. And while Juno wasn’t designed to study these moons (because nobody expected it to last long enough to come close to them) its instruments are flexible enough to be up to the task.
“It’s just great, after such a long hiatus, to be looking [again] at our favorite moons,” Hansen says.