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Kissing Kuiper belt objects produce a planetesimal

Ultima Thule's two lobes came together in a gentle collision that allowed them to stick together without breaking up

Ultima Thule's two lobes came together in a gentle collision that allowed them to stick together

This composite image of the primordial contact binary Kuiper Belt Object 2014 MU69 (nicknamed Ultima Thule) combines enhanced colour data close to what the human eye would see. (Roman Tkachenk/NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)

The first scientific report from the flyby of the distant Kuiper belt object (KBO) Ultima Thule, shows two objects gently joined together in a cosmic kiss that is believed to be the process that formed planets like Earth.

Ultima Thule is a small, icy body resembling a red snowman flying through space more than a billion kilometres beyond Pluto. It is the most distant object ever visited by a spacecraft, as the New Horizons probe zoomed past it last January 1. The encounter was the second flyby for the spacecraft, having passed close to Pluto in 2015.

The snowman appearance is quite appropriate because scientists believe it was formed by two two giant ice balls floating through space that stuck together the way you would stack a smaller snowball on top of a larger one to make the head and body of a snowman on Earth. This process of packing small objects together to make larger ones is thought to be the process that formed all the planets in our solar system, including Earth.

More than four billion years ago, when our sun was a newborn star, it was surrounded by an enormous, flat disk of gas, dust and debris known as the solar nebula. Within the cloud, collisions brought small bits together the way you grab handfuls of snow and pack them together to make a snowball. There was so much material in the cloud that the process made some objects grow larger and when they grew to about a kilometre in size, they developed gravity, which attracted more stuff from the cloud that fell onto their surfaces, adding even more to their growth. These planetesimals were the seeds that in some cases, cleared a path through the cloud, growing ever larger until they became full-fledged planets.

Artist's conception of the dust and gas surrounding a newly formed planetary system. (NASA)

Closer in toward the sun, the ingredients of the primordial nebula were heavier materials such as metals and silicates, which gave us rocky planets like Mercury, Venus, Earth and Mars, while farther out, there was a higher proportion of gas and ices. Ultima Thule, along with Pluto, are on the outer fringes of the solar system, in a frozen region known as the Kuiper belt. Out there, the planet-building process did not proceed as far, which is one reason Pluto and its relatives are known as dwarf planets.

Usually, when objects in space collide, the speeds are so great they blow each other to bits. But occasionally, if they come together more slowly, and just kiss each other, they can stick together. Kisses have a way of doing that. The scientists are delighted to find that Ultima Thule is an object where that kiss actually happened.

Computer models reportedly show that the two lobes came together at a leisurely 8.9 km/h (5.5 mph) about half the speed of a bicycle, so they stuck without destroying each other. Fortunately for us, that's about as far as the process went, and Ultima Thule has remained in this embryonic stage ever since, giving us a glimpse into the very early process of planet formation. It's like seeing an image of yourself when you were still in the womb.

Scientists often try to look back in time to figure out how we got here, whether it be the process of planet formation or the evolution of life. Theories are developed, but until we see those processes in nature, there is a chance the theories could be wrong. That's why finding a fossil of life from the past, or in this case, a remnant of the process that formed our solar system is so important. They are windows into our deep past.

There is still more to learn from Ultima Thule as more data continues to stream down from the spacecraft. Then, the scientists hope that this remarkable New Horizons probe, no larger than a grand piano, which left Earth in 2006, and has already performed beyond expectations, will continue on to encounter another Kuiper belt object even farther out. Who knows what mysteries lie beyond?

About the Author

Bob McDonald is the host of CBC Radio's award-winning weekly science program, Quirks & Quarks. He is also a science commentator for CBC News Network and CBC-TV's The National. He has received 12 honorary degrees and is an Officer of the Order of Canada.