What is the oldest thing in a meteorite?
The oldest things are tiny grains of stardust that formed around other stars before the Sun existed. The oldest material that formed within our own Solar System is calcium-aluminium-rich inclusions, dated to about 4.567 billion years.
This question has two answers, depending on whether you mean the oldest material made in our Solar System or the oldest material of any kind.
Within the Solar System, the record holders are calcium-aluminium-rich inclusions, usually shortened to CAIs. These pale, irregular white or pinkish patches, from less than a millimetre to over a centimetre across, are easy to spot on a cut slice of Allende, the carbonaceous chondrite that fell in Mexico in 1969. They are made of minerals rich in calcium, aluminium and titanium, which are among the first to condense from a hot gas. Uranium-lead dating gives them an age of about 4.567 billion years, so the moment they formed is used as time zero for the Solar System. Chondrules, the round grains in chondrites, mostly formed within the next few million years.
But some meteorites hold something older still: presolar grains. These microscopic specks of silicon carbide, graphite, diamond, oxides and silicates formed in the outflows of ageing stars and in exploding stars, then drifted through interstellar space before being swept into the cloud that formed the Sun. Scientists recognise them by isotope ratios that are wildly different from anything made in the Solar System.
How old are they? In 2020, a team studying silicon carbide grains from the Murchison meteorite measured how long they had been exposed to cosmic rays in interstellar space. Most turned out to be up to a few hundred million years older than the Solar System, and a few appear to be roughly 7 billion years old. Those grains are among the oldest solid materials ever studied, older than the Sun, Earth and everything else in our planetary system.