Most exoplanet discoveries are no longer Big News, since the excellent Paris Observatory exoplanet site now lists nearly a planet for each day of the year. (Currently 360, to be exact.) But the discovery of ex-exoplanets still warrants special mention, and gets it from Sky & Telescope (plus a discussion thread at SFConsim-l, including a link to the original paper). A star with the catchy name HD 172555, a young, Vega-like Class A star 95 light years away in the constellation Pavo, is surrounded by what researchers using the Spitzer Space Telescope expected to be a protoplanetary disk.
Instead it turned out to be just the opposite – a debris ring from the collision of two protoplanetary bodies, estimated to be the size of Mercury and the Moon. The observed IR radiation comes from enough glassy silica dust to form an asteroid over 300 km across, along with about a hundred times that much SiO2 gas that hasn't condensed back into particles yet. Nothing was said about larger pieces of wreckage, which wouldn't be detectable with current instruments. The smashup was pretty recent, too, perhaps less than a thousand years ago, which is why we get to observe wreckage skidding across the freeway lanes, as it were.
A demolition derby phase is in fact an expected stage of planet formation, so all is not lost for HD 172555. Earth probably took a more glancing whack from a Mars-sized object in its early history, which wrote finis for the unlucky intruder in our orbital space, but knocked enough chrome detailing off Earth to coalesce into the Moon.
For your added entertainment value, Sky & Telescope links to this NASA video of what happens when protoplanets go bump in the night.
Related Links: In the early days of this blog I wrote twice about non-wrecked exoplanets.