On New Year’s Day, NASA’s New Horizons probe streaked by a tiny world dubbed MU69, or Ultima Thule, the farthest object humankind has studied up close. With most of the data still on the spacecraft waiting to be transmitted, scientists are still getting to know this distant body. We know that it’s composed of two chunks of rock loosely stuck together. We know that it doesn’t have moons or rings that New Horizons might have careened into on its close pass. And we know Ultima Thule is red.

Carly Howett, a member of the New Horizons team, said that if you were standing on New Horizons as it sped past, Ultima Thule would appear red to the human eye and very dark. But with the aid of enhanced imagery, it’s also clear that some patches are redder than others, like the rim of the large crater known as Maryland.

That redness is likely caused by a mysterious class of compounds called tholins, the New Horizons team said Monday during a mission update at the 50th Lunar and Planetary Science Conference in Houston.

So what are tholins?

Broadly speaking, tholins are complex carbon chains made when ultraviolet light strikes carbon-rich molecules like methane or ethane. The result is a reddish, tarry substance. That may not sound exciting, but it was astronomer and science communication all-star Carl Sagan who named the material after creating it in his lab (along with fellow researcher Bishun Khare). They were performing variations on the famous Miller-Urey experiment, trying to recreate the chemical conditions on early Earth to see how life might have started.

The idea is that nature can, in the total absence of biology, produce more and more complicated carbon chains, until the leap to a biological protein, and presumably, life, is in fact just a single step. Tholins are complex, organic (meaning it contains carbon) molecules that could be a key step in this process. So scientists are very interested in where it’s occurring in the universe.

In the Lab and in the Wild

Early Earth was likely a tholins-rich place. But in our current, oxygen-rich world, it exists only in labs (oxygen destroys tholins). Farther afield, researchers have found similar materials on Saturn’s moon Titan and Neptune’s moon Triton, as well as on lots of smaller icy bodies. But here is where the disagreement starts — it’s difficult to be sure how close the substances scientists create in labs are to the ones observed in space. They appear to be similar, but since we haven’t yet been able to bring back a sample from the outer solar system, appearances are all we have at the moment.

A reddish tint in certain environments is a pretty good hint that scientists might be looking at compounds that are at least similar to tholins, but there is far more than just one kind of potential tholin out there. Many different compounds can create various types of tholins, and picking out which might be present based just on their spectra, or the type of light they reflect, is nigh impossible. Mixed together and seen from afar, it’s not so easy to figure out one complex carbon chain from another.

Some researchers, such as planetary scientist Sarah Hörst of Johns Hopkins University, don’t like calling the outer-space versions tholins at all, since it’s hard to be sure what we’re looking at from so far away truly matches the substances created here on Earth. They are analogs, to be sure — but perhaps not the exact same thing.

Space Missions on the Tholin Hunt

But that’s all the more reason to study them as up-close as we can. The last decade has been filled with missions getting better looks at these far-off maybe-tholins. When the Cassini mission dropped the Huygens probe onto Titan, it revealed an entire world full of carbon-rich materials: on the ground, in the moon’s seas and lakes, and scattered throughout the atmosphere.

In 2015, the New Horizons probe flew by Pluto, painting a breathtakingly detailed view of the former planet — which was distinctly red in places, as is its sidekick moon Charon. Tholins (or something tholins-like) are the most likely culprit.

And now, as the new year dawned, New Horizons got an in-person glimpse at out an even more distant object, Ultima Thule, which is “very red,” according to New Horizons scientist Carly Howett. This puts it on par with other objects near it in the Kuiper Belt. “It’s a lot more red than things like comets,” Howett added.

Comets, in contrast to other Kuiper Belt objects, occasionally plunge into the inner solar system where they heat up and undergo chemical changes that never occur in the cold Kuiper Belt. This means that looking at Ultima Thule is a clean picture of primordial material from early in the solar system’s history.

The beautiful thing about tholins is that while they’re precious as a clue to our history as living organisms, they’re also pretty common. Carbon compounds like methane exist all over in the universe. Stars emit ultraviolet radiation every second, bathing the cosmos in it. The ingredients are easy to find. So if tholins are key to sparking life, we can look around and feel reassured that the makings for them are everywhere we look.

No matter what planet you’re on, physics remains the same. For clouds, that means they follow a peculiar law – they form only around a seed of some sort, usually a fleck of dust or salt. On Earth, with its thick atmosphere and strong air currents, it’s possible to find these lightweight particles throughout the atmosphere, forming clouds at all different altitudes.

On Mars, this process is more difficult. Scientists have struggled to understand the high-altitude clouds they routinely see in Mars’ skies, since models predict that it’s difficult to lift even Mars’ prolific dust high enough into the atmosphere to form the clouds.

But recent observations from NASA’s MAVEN spacecraft, combined with modeling from a group of researchers led by Victoria Hartwick, a graduate student at the University of Colorado, Boulder, may have cracked the case. They say that meteors crashing into Mars’ atmosphere and shredding into dust, which they call “meteoric smoke” can provide the required seeds to form high-altitude clouds. Hartwick’s team published their results Monday in Nature Geoscience.

Cloud Seeding

“We’re used to thinking about these planets as self-contained bodies,” Hartwick told Astronomy, where each world’s own climate and weather come from within. “But this is an example of the climate being impacted by its solar system environment.”

Making clouds from meteor dust isn’t unheard of. Meteors are thought to be the seeds for some of Earth’s wispy noctilucent clouds, but even this is a relatively recent discovery.

Hartwick’s work is mostly based on modeling. Previously, it was difficult to recreate with computers the clouds that spacecraft have observed in the Red Planet’s middle and upper atmosphere. But with the addition of the meteor particles to the models, the matches were much closer.

The theory is strengthened by results from the MAVEN mission. In 2017, the spacecraft observed ionized metal particles in Mars’ upper atmosphere. Researchers see this as proof that meteors entering Martian skies are being heated and evaporating into tiny particles.

This is a common process on Earth, where as much as 100 tons of meteoric material rains down every day. Mars gets a relative trickle, no more than 3 tons, and less than half a ton is converted into the fine particles that might serve as cloud seeds. “That’s about the same amount as a lion or a harbor seal being distributed across the planet per day,” says Hartwick. But even that smaller amount still comprises billions of individual particles, which have a noticeable cumulative effect.

While the evidence is somewhat indirect, the modeling combined with MAVEN’s findings is as much proof as researchers are likely to get. “On Earth we were able to discover [cloud seeds] by flying through the material and capturing meteoric smoke. It’s very difficult to do that on Mars.”

As far as the clouds’ impact, they’re thin and often barely visible. But Hartwick says they can still affect the middle atmosphere, where the temperature can fluctuate by 10 degrees a few times over the course of the day, with the clouds’ formation and dissipation being part of the temperature cycle.

But Hartwick points out that ancient Mars would have had much heavier rains of meteors. “When you rewind the clock, this could have a bigger impact on the formation of clouds on the early climate.”

If there’s one thing we known from the slew of exoplanets detected over the past few decades, it’s that giant planets are not afraid to cozy up to their stars. However, because the region near an active young star is not the ideal place to build large planets, astronomers tend to think oversized exoplanets first form far from their host stars before migrating inward as they age.

Now, new research suggests the biggest planet in our solar system, Jupiter, likely underwent its own great migration early in its life. And it turns out it was quite the trip.

According to the study, which has been accepted for publication in the journal Astronomy & Astrophysics, although Jupiter now sits an average distance of 5.2 astronomical units from the Sun (1 AU is the average Earth-Sun distance), the core of the gas giant likely formed some 18 AU away. That’s about twice as far as present-day Saturn is from the Sun. Furthermore, Jupiter apparently made the entire journey in less than about a million years, which is just a blink of the eye in astronomical terms.

Although the idea of a wandering Jupiter is not new, “This is the first time we have proof that Jupiter was formed a long way from the Sun and then migrated to its current orbit,” said lead author Simona Pirani, a doctoral student at Lund University, in a press release. “We found evidence of the migration in the Trojan asteroids orbiting close to Jupiter.”

Meet the Trojans

Jupiter’s Trojan asteroids are a mysterious bunch. Sharing an orbit with the giant planet, these dark and reddish bodies are divided into two main groups: the “Greek Camp,” which leads Jupiter, and the “Trojan Camp,” which trails behind.

Though the Minor Planet Center database currently lists 7,190 known Jupiter Trojans, they are not divided equally between the two camps. Instead, according to a preprint of the study available on arXiv.org, the leading Greek Camp has anywhere between 40 and 100 percent more asteroids than the trailing Trojan Camp (the imbalance is more pronounced for smaller asteroids).

“The asymmetry has always been a mystery in the solar system,” said co-author Anders Johansen, a professor of astronomy at Lund University.

In order to investigate why Jupiter has more Trojans in its vanguard than at its flank, the researchers ran advanced computer simulations that marched the early solar system through millions of years of evolution by 50-day increments.

Based on numerous simulations, the researchers say the inward migration of the giant planets always resulted in a larger swarm of Trojans in front of Jupiter instead of behind it. This is due to the fact that as Jupiter travels inward, it creates a wider zone of gravitational stability ahead of the planet, leading to a surplus of Greek Camp asteroids. Other evolutionary models that rely on Jupiter forming in its current position, however, result in equal numbers of Trojans in both camps.

Additionally, the study shows that Jupiter’s great migration occurred pretty darn quick in cosmic terms. Within just a few million years of Jupiter beginning as a small, icy asteroid, the planet grew large enough to capture the majority of its Trojans. Then, over the next roughly 700,000 years, Jupiter and its tag-along Trojans made their push closer to the Sun, propelled by gravitational interactions between the fledgling gas giant and the young Sun’s protoplanetary disk.

But the Trojans aren’t just interesting because they drifted inward with Jupiter. According to the paper, „In our scenario explored here, the Jupiter Trojans are a primordial population in which Jupiter’s core formed. Therefore, they hold precious information about the building blocks of our giant planets’ cores.”

This means, Johansen said, “We can learn a lot about Jupiter’s core and formation from studying the Trojans.” And that’s exactly what an upcoming NASA mission plans to do.

Lucy in the sky

To help researchers better understand Jupiter’s Trojan asteroids — and by extension Jupiter’s mysterious core and the early solar system — NASA plans to launch the Lucy mission in October 2021. Named after a famed 3.2-million-year-old hominin fossil that helped us explore human evolution, Lucy is a Discovery-class robotic spacecraft that aims to shed light on how our solar system came to be.

Over the course of more than a decade, Lucy will venture out to the orbit of Jupiter to explore six different Trojans — targeting asteroids in both the Greek and Trojan camps — along with a main-belt asteroid for good measure.

With the help of Lucy, astronomers will soon explore the dim diamonds in the sky known as the Trojans asteroids. And because these ancient relics likely hold valuable information concerning Jupiter’s core and the infant solar system, the first results of the mission cannot come soon enough.