This is the first observed object from outside our solar system.
A/2017 U1, as it is formally named, appears from its trajectory to have originated from the constellation Lyra. It is travelling so fast, 15km per second, that it will escape our solar system and continue back into interstellar space, in the direction of Pegasus.
Update 21st Nov 2017: Observers are now reporting that the ‘comet’, which has been named Oumuamua (meaning “a messenger from afar arriving first” in Hawaiian – it was first observed by an Hawaian telescope), is unusually cigar-shaped, about 400 metres long, 40 metres wide, a dark red colour, with no coma (dust or gas around it – unlike most comets as they get heated by proximity to our sun), and rocky, with a little metal mixed in.
Update 22nd Nov 2017: Further reports say that the comet rotates every 7 hours and twenty minutes – this would be enough to disperse a rubble-pile, so it is clear that it is a solid monolithic shard. From its trajectory, it may be another 10 quadrillion years before it travels close by another star. (By that time all the stars in the galaxy will have burnt out, mostly cold white dwarfs, so it may never see sunlight or feel warmth ever again!)
It seems statistically likely that the comet has been travelling for a billion or more years, and unless its arrival is an extreme fluke then there must be a vast sea of interstellar shards, for us to be able to observe one in just the few years that we have been detecting comets. However, for a shard this size to gain escape velocity to be ejected from a solar system would require a gravity-assist from a substantial planet far away from its sun, like Neptune, Uranus. But fragments in that region would be icy, and this shard has shown no coma from vaporised ice – so it is more likely to have been ejected by a very massive hot inner planet. However, such planets are rare, and unlikely to be able to create a vast sea of shards. Curiouser and….
Update 29th Nov 2017: A study released today (click for study), states that it would be feasible to send a SpaceX BFR rocket mission, using gravity-assists from Jupiter and the Sun, to catch up this interstellar comet! Apparently the rocket would achieve 70km/s, and launched in 2025 could intercept the comet at about 85 AU in 14 years.
Update 11th Dec 2017: Two more pieces of news today. Firstly the suggestion that it could be a piece of shredded inner planet, but there are still gravity-assist issues with such a theory. Secondly the unspoken possibility that it may have an artificial origin has been recognised by a 10 hour long radio-listen on a wide range of frequencies by the 100-meter Green Bank Radio Telescope to see if there are any radio transmissions associated with it, scheduled for two days time. (This telescope could detect even a mobile phone signal from its distance.)
Update 30th Dec 2017: The object as found to be emitting no radio signals. But definitely worth the listen.
Update 27th June 2018: The object was found to deviate slightly from its expected departing trajectory. All causes for this were ruled out except comet de-gassing, so the object is now officially thought to be a comet rather than asteroid. Since no cyanide gas was detected, the ratio of cyanide to water in its parent solar system must be, or have been, much lower than in our own.
Update 26th Sept 2018: Two further pieces of news today. Astronomers were somewhat puzzled that it showed no coma, as a comet normally would, but now think that the smaller grains of dust on its surface which would contribute to the coma might have eroded during its interstellar travel. Secondly, calculations of its trajectory, taking into account the small adjustments due to de-gassing, now point to its origin being one of four stars: red dwarf HIP 3757, sunlike star HD 292249 and two others. It is known that the star must have at least one large gas giant in order for the comet to have been ejected, so the candidate stars might be narrowed down in future if observations can reveal planets (none are currently identified).
Update 27th Sept 2018: There has been a comment in Scientific American today suggesting that the out-gassing theory explaining Oumuamua’s slight velocity change may be incorrect, because the velocity change should be accompanied by a change in the period of its spin, and no such change in spin was observed!
Update 6th Nov 2018: Because no change in spin was observed, a study out today makes the alternative suggestion that the observed change in velocity could be powered by sunlight if the object is rather less than a millimetre thick. Of course an object that thin but 400 metres long would not be natural – it would be fabricated. The suggestion is that it could be an alien light-sail! The argument for this interpretation is bolstered by statistics. For this to be an object ejected by an alien solar system, the chance of us detecting it suggests trillions of such objects would have to be ejected by each solar system. This is counter to expectations from studies of our own solar system. And of course the shape is extraordinary. The study is on arXiv.org.
Update 16th April 2019: A new study now suggests that Oumuamua may in fact not be the first interstellar visitor that we have observed. Inspired by the facts around Oumuamua, astronomers Loeb and Siraj made a study of recorded information on meteors, looking for objects that were travelling fast enough to suggest that they came from outside the solar system, and on trajectories which eliminate the possibility that the speed originated from a gravitational slingshot within the solar system. Indeed they found one, a one metre wide meteor detected on Jan. 8, 2014, at an altitude of 18.7 kilometers near Papua New Guinea at a speed of 216,000 km/h. The plan now is to alert spectrographic analysing telescopes to such meteors in the future so that they may be able to analyse the elemental composition of the burn-up in the atmosphere.
Update 13th Sept 2019: Another comet, C/2019 Q4 (Borisov), has been observed with high speed and trajectory strongly suggesting it is interstellar. Excitingly for astronomers it has been identified on its way in to the solar system, so plenty of scope for study.
The comet’s origin, from outside the solar system, brings to mind the Voyager 1 and 2 spacecraft that were launched in the 1970s, and are the only objects we know to have left our solar system, and intentionally, carrying a gold (for longevity) phonograph recording (as in vinyl) of information about Earth and Humans.