The same ratio of two varieties of titanium has been found in a range of meteorites, hinting that the cloud of gas and dust that formed the solar system was well-mixed before the first solids formed (Illustration: NASA)

The solar system emerged from a well-blended soup of dust and gas despite being cobbled together from the remains of multiple exploded stars, new meteorite measurements suggest.

Meteorites form a fossil record of the conditions that existed when they formed. By looking at the chemical makeup of some rocks, evidence has mounted in recent years that sun and the rest of the solar system formed from a cloud of debris blasted away from a number of supernovae.

But it is still unclear what that cloud – the solar nebula – looked like or how many stars might have been involved in the Sun’s birth. Now, a team led by Martin Bizzarro of the Natural History Museum of Denmark has found one clue.

Bizzarro and colleagues measured the levels of titanium in meteorites from the moon and Mars as well as inclusions in some meteorites that are thought to be the oldest rocks in the solar system.


Stable forms

Titanium is a good probe for conditions billions of years ago because it does not evaporate easily. It also has a number of stable isotopes – forms of the element that contain different numbers of neutrons – that can be used to cross-check each other.

Although the concentration of titanium varied from rock to rock, the team found that two isotopes of titanium – titanium-50 and titanium-46 – were always found in the same ratio.

“It is quite astonishing,” since these two different isotopes probably formed in different stellar explosions, Bizzarro told New Scientist. Titanium-46, which contains 22 protons and 24 neutrons, is created inside the cores of massive collapsing stars.

Titanium-50, which contains 22 protons and 28 neutrons, is commonly created when white dwarf stars explode as type Ia supernovae after gorging on a companion star.

Well mixed

If these two types of titanium come from two sources but are always found in the same ratio, the solar nebula must have been very well mixed. The level of mixing seems to exceed what meteorite researchers have seen in the isotopes of other elements, Bizzarro says.

“People thought that the isotope anomalies typically reflected that the cloud from which the solar system formed was not very well homogenised,” says Bizzarro. He suspects the differences that are seen between the planets, asteroids, and other rocks came later, when the young sun was more active, sending out vaporising solar flares.

Stray cloud

But there may be alternative explanations for the seemingly universal ratio of titanium concentrations.

The mix could also be explained if a stray cloud of dust containing both varieties of titanium hit the early solar system, says Jeff Hester of Arizona State University in Tempe.

“Then you could have wild inhomogeneity in how the dust was distributed in the solar disc, while preserving the association between the two isotopes of titanium,” he says.

Journal reference: Science (vol 324, p 374)