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Brand new comet spotted by Hubble, shows signs of activity

Scott Sutherland
Meteorologist/Science Writer

Thursday, September 28, 2017, 5:21 PM - A brand new comet has been spotted by the Hubble Space Telescope, and remarkably, even though it is still far enough away from the Sun that its water ice would be as hard as rock, it is already showing signs of activity.

That makes this new comet, called C/2017 K2 (PANSTARRS), or just K2 for short, the farthest active comet astronomers have ever seen - at a distance of 2.4 billion kilometres from the Sun, which is beyond the orbit of Saturn.

Discovered in May in 2017, using Hawaii's Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), astronomers used the Hubble Space Telescope to get a better look at "K2".

Hubble revealed that the comet's coma - the gassy, dusty "atmosphere" that surrounds the comet's solid nucleus - was already around 115,000 km across. That's roughly the diameter of the planet Saturn!

Comet "K2" and its Saturn-sized coma. Credit: NASA, ESA, and D. Jewitt (UCLA)

In contrast, the solid nucleus of K2 is probably more like 20 kilometres wide.

"K2 is so far from the Sun and so cold, we know for sure that the activity - all the fuzzy stuff making it look like a comet - is not produced, as in other comets, by the evaporation of water ice," David Jewitt, an astronomer at UCLA. said in a Hubble news release.

"Instead, we think the activity is due to the sublimation [a solid changing directly into a gas] of super-volatiles as K2 makes its maiden entry into the solar system's planetary zone. That's why it's special. This comet is so far away and so incredibly cold that water ice there is frozen like a rock."

Related Video: Three comets swinging by Earth in 2017 and 2018.

According to Jewitt, this activity is likely the comet "shedding its outer skin" of volatile gases, such as oxygen, nitrogen, carbon dioxide and carbon monoxide, as those gases are heated by light from the distant Sun.

This schematic of the solar system shows the position and orbit of Comet C/2017 K2 (PANSTARRS). Credit: NASA, ESA, and A. Feild (STScI)

"Most comets are discovered much closer to the Sun, near Jupiter's orbit, so by the time we see them, these surface volatiles have already been baked off," he said. "That's why I think K2 is the most primitive comet we've seen."

In fact, based on the observations that Jewitt and his team gathered, they believe that K2 is a pristine remnant of the primordial solar system, which has already been travelling for several million years, and that this is its first and only trip through the solar system.

Also, after looking back in the image archives from the Canada France Hawaii Telescope, they estimated that K2 has already had its dusty coma for at least 4 years - since it was over 3 billion km away, which would put it between the orbits of Uranus and Neptune.

Comets are frozen debris, left over from the formation of our solar system. They are mainly composed of water ice, rock and dust, with a mixture of frozen gases, including (but not limited to) carbon dioxide, carbon monoxide, ammonia and methane. Some comets orbit the Sun within the confines of our planetary system. Comet 1P/Halley's orbit, for example, takes it all the way out to just beyond Neptune before it turns around and dives back in, to pass between the orbits of Mercury and Venus. It takes around 75 years for it to make the entire journey. Others, like Comet Encke, have even shorter orbits, barely making it out to Jupiter before falling back for extremely close passes around the Sun, every three years or so.

The vast majority of comets - likely billions of them - exist outside the orbits of the known planets, though, forming a large, diffuse spherical "shell" around our solar system, called the Oort Cloud. Comets periodically fall in towards the Sun from the Oort Cloud, probably due to two of these objects knocking into one another, or possibly even after receiving a gravitational "nudge" from a passing rogue planet or star. Since the Oort Cloud is so distant - roughly 100 times the distance to Saturn from the Sun - it takes these comets anywhere from hundreds to millions of years to reach the point where we can actually see them. The objects in the Oort Cloud are some of the oldest, and most primitive, in the entire solar system.

After the lengthy journey it has already made, K2 will take another 5 years to reach its closest point to the Sun, which will bring it to just beyond the orbit of Mars in the year 2022. If it behaves like other comets, we will likely see it sprout tails somewhere between the orbits of Jupiter and Mars, and it will grow progressively brighter during its approach. Comets are notorious for surprising us, though, and with this being K2's first pass through the solar system, it represents a great opportunity for astronomers to study a new comet in detail.

"We will be able to monitor for the first time the developing activity of a comet falling in from the Oort Cloud over an extraordinary range of distances," Jewitt said.

Sources: Hubble | NASA

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