Q&A: a stardust sifter
A new analysis of stardust left over from the birth of the Solar System shines a high-precision light on Carl Sagan’s aphorism ‘We are made of star stuff’
What do you think happened at the birth of our Solar System? Our proposal is that dust from dying stars travels in the interstellar medium, gets incorporated in a so-called molecular cloud which collapses and forms new stars like our Sun. Around the new star is a dust disc out of which terrestrial planets like Earth will form.
This means that there is stardust from red giant stars produced before our Solar System was created, that is still around today. We see that this dust was not completely mixed in our Solar System – there is still a gradient. Earth in fact has more of this red giant dust than Mars or the asteroids further from the Sun.
It’s amazing that we can resolve this nowadays
– for a long time we couldn’t and assumed everything in the pre-Solar System was well mixed together.
How did you and your team examine the stardust composition of our Solar System?
We put our sample in a mass spectrometer and for each element in them we measured how many atoms of a specific weight (isotopes) there were. Doing this we were able to reveal the fingerprints
of stardust in normal bulk material such as a rock, like a signature.
We looked at palladium isotopes in iron meteorites that come from asteroid samples. We had already done a similar study with zirconium isotopes, but to look at palladium we needed high precision measurements, which only became available recently.
What can looking at the stardust origins of Earth tell us?
What we and other researchers have discovered is that every planet or asteroid from which we have samples has its own isotopic signature. We also see that stardust has its own signature depending on if it
is from specific stars, like red giants or supernovae.
In our study we found that Earth’s palladium
fingerprint – a specific enrichment of a certain
palladium isotope – suggests there is more red giant material.
How do you know the chemical signatures of stardust from red giants?
You can predict with nuclear synthetic models which atoms will be produced in a red giant star.
You can also test actual stardust – some of it has survived in meteorites. It’s very small – smaller than the diameter of a hair – really fine dust that you can
extract and measure.
On Earth any original stardust has been destroyed because our whole planet was once molten, but there are a few very primitive asteroids where the stardust from before the planets formed has survived.
We know the dust must have been generated before our Sun formed. Our Solar System is 4.5 billion years old, and the stardust is ancient, from before that time.
What are the wider implications of your research? When the planets formed the Sun was still quite hot and destroyed a lot of the grains that were not from red giants – those are very hard to destroy. Even though Earth is quite close to the Sun, the red giant stardust has survived better than other stardust as it is more robust at high temperatures.
In a previous study we analysed zirconium in meteorites from Mars in a previous study and saw that the red giant signature was not as strong. For our Solar System we can use this red giant signature as a tracer, as we know now that there was more of this stuff close to the Sun and less further out.
In the future we will be able to make models of how Earth and the planets formed based on asteroid material, as the asteroids are leftover planet building materials. It’s a key part of the puzzle.
How does Carl Sagan’s quote that “we are made of star stuff” stand up, what does your research add? Well, it’s correct! We are made of stardust and Earth has more red giant stardust than Mars. But our study also shows that much of this stardust was recycled many times before it was incorporated into Earth and eventually our bodies. What’s more, not a great amount of stardust survived the Solar System’s formation.