What is the largest comet? There have been some large ones in history, such as Comet Hale-Bopp, which was 60 miles in diameter. It was a brilliant comet, the biggest one in my lifetime that I have seen. However, it was clear across the Solar System. If it had been near us, like Comet Hyakutake in 1996 at 9 million miles, it would have been an incredible sight.
The asteroid Chiron is about the same size, and it has shown signs of developing a coma. If so, then Chiron is another comet. But it is also a straggler Kuiper Belt object (KBO), which makes one think of the other KBOs. They are icy worlds, and if one were placed in the inner solar system, the ices off the world would stream off to produce a comet. So does that mean that all KBOs are comets? Here we go with Pluto again. The International Astronomical Union has ruled that it is not a planet, but rather a KBO and a "dwarf planet". Could it really be a comet? If Pluto got into the inner Solar System, it would be one big whopper of a comet, perhaps. The same holds for Eris, too, and the other large KBOs. But this leads to the question: what is a comet anyway?
To me a comet is any body that produces a bright center and a faint to bright tail trailing it, and lasting at least an hour (to exclude meteors). The bodies that produce the traditional comets we observe are much like small asteroids or KBOs. They are solid bodies. The comets differ in that they have icy surfaces, unlike the rocky surfaces of Mars-Jupiter asteroids. So to me anything that is capable of producing such a brilliant tail in the sky is a comet. That makes Eris and Pluto comets. So is Eris the largest comet?
No. Recently Astronomy magazine reported an even larger comet. This one is larger than Jupiter. That's correct, larger than Jupiter. They reported that a large body, TrES-4, orbits the star GSC 02620-00648 in Hercules. This body is considerably bigger than Jupiter, maybe twice as big. But it is less massive. Bodies that are between 1 and 80 Jupiters in mass are all of about the same size, with the difference being their density. But this one is considerably larger. It must be a puffball of a planet, with gases on the surface that are heated up by the central star and blown away by the stellar wind, to produce an enormous tail. To me, TrES-4, although it is a gas-giant planet, is also a comet. A really huge comet! But is this the largest comet? No.
There is one much larger than that one, and this object has been known since the 1600s. It is a favorite with amateur astronomers. It is Mira, the wonderful variable star in Cetus. This star varies in magnitude from 2.5 (about the same as Phecda in the Big Dipper) to 10 (a faint dot in an 8-inch telescope), over an irregular period that averages 331 days. It has long been known to be a red giant star about 1.2 times as massive as the Sun, and as large as Mars' orbit. The maxima and minima have been studiously observed since the 1600s. Astronomers have discovered something new with Mira. It is producing a tail in ultraviolet light that makes it look like a comet. If it looks like a comet, then it is one. To me this star qualifies as a comet, as it consists of a bright object with a long tail behind it. The tail is certainly long. Instead of 100 million miles (typical of a Solar System comet), it is 13 light years long! The star is moving fast, and it is ejecting gas, which is forming this tail. So this is a comet, in a planetary system where the central star is itself a comet.
This certainly has to be the largest comet ever discovered.
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