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    IntroductionBillions of miles away, you can find some of the best-preserved fossils of the beginnings of our solar system. 2014 MU69, also known as Ultima Thule, is one of those relics that has been in a deep freeze all this time, with its orbit virtually unchanged over 4.5 billion years. Circling the Sun in a distant region of icy, rocky bodies called the Kuiper Belt, MU69 may represent the building blocks of Pluto and other planet-like worlds beyond Neptune.

    Size and Distance

    Size and Distance

    One of the biggest mysteries about MU69 is whether it is really just one object, or actually two that orbit in a close binary system.

    If MU69 is one object, it is probably no more than 20 miles (30 kilometers) long. If it is two, each one may be about 9 to 12 miles (15 to 20 kilometers) in diameter.

    MU69 seems to be more than 10 times larger and 1,000 times more massive than typical comets, but less than 1 percent the size of Pluto.

    MU69 orbits the Sun 4.1 billion miles from Earth, which is also 1 billion miles beyond Pluto.

    Orbit and Rotation

    Orbit and Rotation

    MU69 orbits the Sun once every 296 years. It has a low eccentricity and low inclination in its orbit, which suggests that the gravity of giant planets has not significantly affected it since it formed.

    Formation

    Formation

    MU69 is known as a “Cold Classical” object, a group that makes up about one-third of the Kuiper Belt. Cold Classical objects have extremely circular orbits with low eccentricity and low inclination. This indicates they have not been pushed around by the gravity of giant planets, and have probably stayed at the same distance from the Sun since the formation of the solar system.

    Unlike the planets, which started as small grains of dust and got bigger as they acquired material over millions of years, MU69 may have formed directly out of the disk that surrounded the Sun in its infancy, with rock and ice collapsing together into this object relatively quickly. Gravitational instabilities in the disk of material could have caused objects like MU69 to be born in just a few years, rather than millions. A competing theory is that MU69 did amass its material over a very long time from small grains, like the planets. Further exploration may help solve this puzzle.

    Regardless, we know that while a lot of material fell into the young Sun, our star was so far away from this area of the disk that its gravity could not rip an object like MU69 apart, and its radiation could barely disturb it. MU69 probably took its current form long before there were any planets in the solar system, making it one of the most pristine objects on the planetary frontier.

    Structure

    Structure

    If there are two parts to MU69, they could be orbiting each other, or even touching. Or, if it is one object, it could be an elongated oval shape, like a football.

    MU69 may have a lot of ice, like other Kuiper Belt objects, but its exact composition is unknown. Further study may determine its ice and rock content. Additional observation is also needed to determine if it is one or two objects.

    Surface

    Surface

    Scientists do not know much about variations in the surface yet, as of summer 2018. Observations from Hubble suggest MU69 is more red than Pluto, though not as red as Mars.

    Atmosphere

    Atmosphere

    MU69 doesn’t have enough gravity to have an atmosphere.

    Potential for Life

    Potential for Life

    MU69 is much too cold to support life.

    Moons

    Moons and Rings

    It is not yet known whether MU69 has rings or moons.

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