Kuiper Belts around other Stars

The green light signal arrived from New Horizons earlier today as it traverses the Kuiper Belt. It tells the NH team that the spacecraft (while mostly asleep) is still safely on its way to the next object on its itinerary. That’s another tiny step in the exploration of the outer solar system.

While this study of our own trans-Neptunian region is underway, keep in mind that it’s probably not the only such region in the galaxy. Other stars that have dusty disks around them are pretty likely to have their own versions of this collection of objects. These “extrasolar” disks and belts could be populated with many of the same types of object that we see in ours.

Why Study these Regions?

A number of stars have dusty planet-forming disks around them. Studying those distant Kuiper Belts helps us understand the region where Pluto, Quaoar, Makemake, and Haumea now orbit. Most are pretty young. These “baby” disks are likely a bit narrower and flatter than the older ones, and their materials are still sorting themselves out. Planets form in the disk, some closer to the star than others. The icy “leftovers” eventually migrate to the outer regions where they won’t be destroyed by the heat of the star.

Without going into gory details about disk dynamics, that’s more or less how our planets got sorted out in the early solar system. The rocky bodies formed close to the Sun. The gas giants may well have formed closer to the Sun than where they are now. However, through the disk’s evolution, they made their way out to cooler regions where their gases and ices could exist. The cometary bodies and dwarf planets (icy or ice-covered bodies for the most part) also settled out in the deep freeze.

Plumbing the Depths of our Own Kuiper Belt

Astronomers use many tools to study the regions beyond Neptune. Joining Hubble Space Telescope in the hunt for worlds “out there” is a myriad of observers using ground-based telescopes such as the Palomar Observatory in California, the European Southern Observatory in Chile, the Keck telescopes in Hawai’i, and orbiting observatories such as the Spitzer Space Telescope (which is sensitive to infrared light). Using infrared telescopes helps astronomers detect the “warmth” of these objects as they orbit in the much-colder outer solar system. Even cold bodies such as Pluto can glow in the infrared, giving off clues about them. Granted, they’re very distant and small. So, even with well-equipped facilities, they can and will continue to be difficult to find.

What tales will they tell us about themselves and maybe our own Kuiper Belt? For one thing, studying them long-term will allow astronomers to watch them evolve (slowly but surely). Look at their structures can reveal gaps where giant planets form. As those worlds coalesce, they can nudge smaller objects out to alien Kuiper Belts. That helps astronomers understand how own Belt and Oort Cloud formed billions of years ago. Finally, studies of the distant regions of those disks may reveal water ice and other frozen constituents of outer-system worlds. They could be very similar to what we see at Pluto and beyond. On the flip side, our own Kuiper Belt can tell astronomers what to look for at other circumstellar disks. It’s win-win all the way around.

This week’s green light signal from New Horizons tells us that it’s continuing its mission through the Kuiper Belt. At the present time, the spacecraft is 40.55 astronomical units from the Sun and just under 3 AU from its next target, 2014 MU69. It’s moving at 14.19 kilometers per second and at that rate will get to MU69 on December 31, 2018. To give you some perspective on where it is in the Kuiper Belt, the inner edge of this distant region is about 30 AU and the outer edge is around 50 AU. So, the spacecraft is well within the third “regime” of the solar system. (Note: an astronomical unit is the distance between Earth and the Sun, 150 million kilometers.)

Deducing the Kuiper Belt

Planetary scientists haven’t always known the Kuiper Belt existed. However, the discovery of Pluto suggested that there might be more bodies beyond Neptune, populating a region between that planet and the Oort Cloud (the most distant reaches of the solar system). That got people to thinking about a theoretical population of objects dubbed “Trans-Neptunian Objects”, or TNOs. Today, some are also referred to as Kuiper Belt Objects (KBOs). Several astronomers suggested that such objects were out there, but of course, their distance, size, and likely dim surfaces would make them hard to find from Earth.

Eventually, astronomer Kenneth Edgeworth suggested that materials left over from the original proto-solar system nebula of gas, dust, and ice existed “out there”. He theorized they’d be too thinly spread out over the region to form large planets. Instead, this area could be populated by smaller worlds, like Pluto. In addition, some of its materials would be cometary nuclei. They occasionally get knocked out of their orbits and sent sunward. Another astronomer, Gerard Kuiper, also suspected a disk of material once orbited “out there”, but questioned whether it still existed. He thought that gravitational influences from a once Earth-sized Pluto would have knocked all the region’s bodies out of orbit.

Both men were partially right. There is a huge torus-shaped belt of material “out there”. It turns out there are also a number of fairly large worlds (although none so far are Earth-size). However, the idea that Pluto was once an Earth-sized world is wrong. Further discoveries, particularly in studies of short-period comets, confirmed the existence of bodies in this region. Eventually, it was named the Kuiper-Edgeworth Belt for the two men. Another astronomer, Julio Fernández, suggested a “comet belt” between 30 and 50 AU could account for many of the comets observed from Earth. That region coincides with the Kuiper Belt, and some astronomers suggest that the region really should include his name, as well.

The Kuiper Belt Today

Today, the region of space beyond Neptune is called the Kuiper Belt. It is now known to be home to a handful of dwarf planets. Some astronomers suspect there is at least one larger, massive world out there, but so far nothing has been directly seen. They have only gravitational influences on other objects to go by to suggest it even exists. Continuing observations will undoubtedly uncover it, not only from the ground, but by space-based observatories.

Exploring the Kuiper Belt In Situ

Due to its distance, the Kuiper Belt is not well-studied by spacecraft. Much of what we know about it comes from ground-based observations. Astronomers David Jewitt, Jane Luu, and more recently Michael Brown and others have made many observations. Their efforts paid off in the discovery of such worlds as Quaoar, Eris, Haumea, and others in the trans-Neptunian region. Further studies of cometary bodies now suggest that the Kuiper Belt is not exactly the home to all short-period comets. Instead, they come from a region called the “scattered disk” that lies outside of the orbit of Neptune as well. Some of its comets have approached the Sun, others remain out in this region of interplanetary space.

While some spacecraft (notably the Voyagers and Pioneers) are beyond Neptune, no missions explored the Kuiper Belt until New Horizons got there. As we all know, it imaged and studied Pluto and its moons in 2015 in a spectacularly successful flyby. That was one of its main goals. The others focus it on the conditions in the Kuiper Belt as it flies along to its next target. This is the kind of “scouting” mission needed to understand what the Belt is really like. The data it sends back will, one day, inform a mission that will circle Pluto and study that world extensively. And, who knows, maybe other missions will traverse the Kuiper Belt at different points, charting this otherwise unknown territory of the solar system.