I love stories like this: Hubble spotted a small iceball -- only about a kilometer (0.6 miles) across -- orbiting the Sun 6.7 billion kilometers (4.2 billion miles) away. And the best part? It was an accident. But it was found on purpose.
The object wasn't seen directly; it passed in front of a star, momentarily blocking the star's light. The star is one that Hubble uses to point the telescope itself; it's one of several thousand guide stars used to keep the telescope aimed at astronomical objects, a bit like stellar benchmarks.
Hubble's Fine Guidance Sensors are telescopes used to lock onto these guide stars. They don't take pictures, but instead use a technique called interferometry to (extremely) precisely measure the starlight. If the telescope starts to drift, even a teeny amount, the FGS will sense this and the motion can be corrected.
If a small object passes in front of a guide star it will momentarily block (what we call occult) the starlight, causing a dip in brightness as well as a diffraction pattern, a fluctuation in the starlight caused as the light bends around the object. Knowing this, astronomers dug through 4.5 years of archived data from the FGSs and found the tell-tale sign of such an occultation.
Using this data, the astronomers were able to determine the distance and size of the object. The distance puts it in the Kuiper Belt, the torus of icy comet nuclei orbiting the Sun out past Neptune. The size is remarkable: the smallest known Kuiper Belt Object (or KBO to those in the know) before this was about 20 kilometers across. This is therefore the smallest KBO ever found! Assuming a typical color for the object, it would be about 35th magnitude, or one-trillionth as bright as what you can see with your unaided eye!
Yowza. That's also far dimmer than anything directly detected by Hubble, by a factor of about 50.
Again, this object was found by going through 4.5 years of archived Hubble FGS data; however, all the FGS data since launch (in 1990) is available, so it's possible more events like this are waiting to be discovered. There's real science in this, too: KBOs this small probably come from the collisions and grinding together of bigger objects; getting a handle on the members of this population will let us know more about the history of the Kuiper Belt and the solar system itself.
Also, it's good to remember that not everything Hubble sees is on purpose! The fact that all the observations are archived means that people will be able to go treasure hunting for years to come. Hubble is the gift that keeps on giving.
Artwork credit: NASA, ESA, and G. Bacon (STScI)