Above: On October 12, 2019, the Hubble Space Telescope observed Comet 2I/Borisov at a distance of approximately 420 million kilometres from Earth. Credit: NASA, ESA, D. Jewitt (UCLA)

The Hubble Space Telescope has given astronomers their best view yet of an interstellar visitor — Comet 2I/Borisov — which is believed to have come from a planetary system outside our own.

Hubble’s observation, obtained on October 12, 2019, is the sharpest view ever of the comet, revealing a central concentration of dust around the solid icy nucleus.

Comet 2I/Borisov is only the second such interstellar object known to have passed through our Solar System. In 2017, the first identified interstellar visitor, an object dubbed ‘Oumuamua, came within 38 million kilometres of the Sun before racing out of the Solar System.

“Whereas ‘Oumuamua looked like a bare rock, Borisov is really active, more like a normal comet. It’s a puzzle why these two are so different,” explained David Jewitt of UCLA, leader of the Hubble team who observed the comet.

Alien ‘building block’

As the second interstellar object known to have entered our Solar System, the comet offers various invaluable insights.

For example, it offers clues to the chemical composition, structure, and dust characteristics of a ‘planetary building block’ presumably forged in an alien star system a long time ago and far away.

“Because another star system could be quite different from our own, the comet could have experienced significant changes during its long interstellar journey,” said Amaya Moro-Martin of the Space Telescope Science Institute in Baltimore, Maryland.

“Yet its properties are very similar to those of the Solar System’s building blocks, and this is very remarkable.”

The path comet 21/Borisov will take through our Solar System (left). The panel on the right shows the comet’s position on October 12, 2019. Credit: NASA, ESA, J. Olmsted, F. Summers (STScI).

High-speed visitor

Hubble photographed the comet at a distance of approximately 420 million kilometres from Earth.

The comet is travelling sunwards and will make its closest approach to the Sun on 7 December, when it will be twice as far from the Sun as Earth is.

It is following a hyperbolic path around the Sun, and travelling at the extraordinary velocity of over 150,000 kilometres per hour.

By the middle of 2020, the comet will be on its way back into interstellar space where it will drift for millions of years before maybe one day approaching another star system.

Chance discovery

The comet was discovered by Crimean amateur astronomer Gennady Borisov on August 30, 2019.

Following a week of observations by amateur and professional astronomers around the world, the International Astronomical Union’s Minor Planet Center computed an orbit for the comet which showed that it came from interstellar space.

Until now, all known comets have come either from a ring of icy debris at the periphery of our Solar System, called the Kuiper belt, or from the Oort cloud, a shell of icy objects that is thought to be in the outermost regions of our Solar System, with its innermost edge at about 2,000 times the distance between the Earth and the Sun.

It is likely that 2I/Borisov and ‘Oumuamua are just the beginning of the discoveries of interstellar objects paying a brief visit to our Solar System.

There may be thousands of such interstellar objects here at any given time; most, however, are too faint to be detected with present-day telescopes.