Far from the bright nurseries where new stars are born, the cooling embers of dead stars are quietly keeping records of how worlds begin. Astronomers at Cambridge, studying 200 polluted white dwarfs, have read in the scorched remains of ancient asteroids a suggestion that planets start forming while their parent star is still growing, rather than waiting until it has settled into its final form. It is a shift in timing that reaches back to the first days of a solar system, and it asks us to imagine creation as something that begins in the same breath as the star itself.
Polluted White Dwarfs Suggest Planets and Their Parent Stars Grow Up Together
Stars and planets may simply grow up together
Mark
So what's the headline here? Planets start forming sooner than we thought?
Mimi
Yes. The raw material for planets like Jupiter and Saturn seems to start building while the star is still growing, not after it's finished.
Mark
And the evidence comes from dead stars?
Mimi
White dwarfs whose atmospheres are contaminated with magnesium, iron and calcium. Those came from asteroids that fell in and burned up, so the starlight carries a readout of what the asteroids were made of.
Mark
What in that readout points to early formation?
Mimi
The mix of elements only makes sense if many asteroids had melted, letting iron sink to the core. The melting is blamed on short-lived radioactive elements that decay in about a million years.
Luke
That's the step I'd slow down on. The melting is an inference from the chemistry, and the attribution to those radioactive elements is the team's conclusion, not something observed directly.
Mimi
Fair. Bonsor says it can only be attributed to them, but the paper's wording is that the early picture is likely correct, not proven.
Luke
And the claim that this applies to all extrasolar planets is described as potentially ubiquitous. That's a possibility, not a finding.
Mark
Is 200 white dwarfs a lot?
Luke
The reporting doesn't say how representative it is, so I can't tell you. It's a solid sample, but whether it covers the range of planetary systems is unstated.
Mimi
The researchers do see it as a start. More white dwarfs mean more tests, and elements like nickel and chromium could show how big an asteroid was when its core formed.
Il Polso
- Two hundred dying stars are being read like archives, and the pollution in their atmospheres is revealing a timeline researchers had not expected.
- Scientists had long assumed planets only began forming once their star was fully grown, and this evidence challenges that comfortable certainty.
- Traces of melted asteroid material point to short-lived radioactive elements, which means planet-building must have started within the first million years.
- Researchers are now tracing nickel and chromium in more white dwarfs to measure asteroid cores, and if the early-formation model holds, it could apply to every known extrasolar planet.
Far from the bright nurseries where new stars are born, the cooling embers of dead stars are quietly keeping records of how worlds begin. Astronomers at Cambridge, studying 200 polluted white dwarfs, have read in the scorched remains of ancient asteroids a suggestion that planets start forming while their parent star is still growing, rather than waiting until it has settled into its final form. It is a shift in timing that reaches back to the first days of a solar system, and it asks us to imagine creation as something that begins in the same breath as the star itself.
At the University of Cambridge, astronomers have turned to the faint, ancient remains of dead stars to argue that planets begin forming far earlier than many researchers believed. Their study, published in Nature Astronomy, examined spectra from 200 so-called polluted white dwarfs, the dim remnants of stars like our Sun, and points toward a picture in which a star and the building blocks of its planets grow together.
The central question is one of timing. Dr Amy Bonsor, the study's first author, explained that the process of planet formation is reasonably well understood, but its starting point has been uncertain. The older view held that planets emerged only after a star reached its final size. The new evidence suggests the process starts much sooner.
Polluted white dwarfs make this possible. Their atmospheres should be clean, yet they carry magnesium, iron and calcium, left behind when asteroids fell into the star and burned. Analysing that material reveals what happened inside those asteroids. Many had melted, with iron sinking to the core, much as it did in the early Earth. Such melting, Bonsor argues, can only be explained by short-lived radioactive elements present at the very beginning of a system, which decay within about a million years. If that is right, planet formation must have started quickly, leaving Jupiter and Saturn ample time to grow.
Bonsor described the work as a beginning rather than an end. Each new white dwarf adds evidence, and further measurements of nickel and chromium may reveal how large those ancient asteroids once were. She suggested that this radioactive melting could be a widespread force shaping all extrasolar planets, a possibility that future observations will be needed to confirm.
Astronomers at the University of Cambridge have used the faint, ancient remains of dead stars to argue that planets begin to form far earlier than many researchers assumed. Their study, published in the journal Nature Astronomy, rests on spectroscopic observations of 200 so-called polluted white dwarfs, and it points to a picture in which a star and the building blocks of its planets grow up at the same time.
The question at the center of the work is one of timing. Astronomers have a fairly good grasp of how planets form, said Dr Amy Bonsor of Cambridge's Institute of Astronomy, the study's first author. What has been unclear is when: does the process start early, while the parent star is still growing, or millions of years later? The older assumption was that planets only form once a star has reached its final size. The new results suggest otherwise.
To test it, Bonsor and her colleagues turned to white dwarfs, the dim remnants of stars like our Sun. The team also included researchers from the University of Oxford, Ludwig-Maximilians-Universität in Munich, the University of Groningen and the Max Planck Institute for Solar System Research in Göttingen. Bonsor described the thin atmospheres of some white dwarfs as almost like celestial graveyards, and she called them amazing laboratories.
The reason is that the interiors of planets are normally beyond the reach of telescopes. Polluted white dwarfs offer a workaround. Their atmospheres would ordinarily be clean, yet these show heavy elements such as magnesium, iron and calcium. Those elements must have come from small bodies, such as asteroids left over from planet formation, that crashed into the star and burned up in its atmosphere. Spectroscopy of that pollution lets astronomers probe the interiors of the torn-apart asteroids, and so gives direct insight into the conditions where they formed.
The leading theory holds that planet formation begins in a protoplanetary disc orbiting a young star, made mainly of hydrogen, helium, and tiny particles of ice and dust. The dust grains stick together and build ever larger solid bodies. Some keep accreting and become planets; others remain asteroids, like those that fell into the white dwarfs in this study.
When the researchers analysed the observations, they found that the mix of elements in the white dwarf atmospheres could only be explained if many of the original asteroids had once melted. In that melting, heavy iron sank to the core while lighter elements rose toward the surface. The process is called differentiation, and it is what gave Earth its iron-rich core.
The likely cause of the melting is the key to the timing. Bonsor said it can only be attributed to very short-lived radioactive elements present in the earliest stages of a planetary system, which decay away in about a million years. If asteroids were melted by something that exists only briefly at the dawn of a system, she reasoned, planet formation must have kicked off very quickly. The study concludes that the early-formation picture is likely correct, which would mean Jupiter and Saturn had ample time to grow to their present sizes.
Bonsor said the findings complement a growing consensus in the field that planet formation got going early, with the first bodies forming alongside the star. She added that polluted white dwarfs suggest this radioactive melting may be a potentially ubiquitous mechanism affecting the formation of all extrasolar planets.
She also framed the work as a beginning. Each newly found white dwarf adds evidence, and elements such as nickel and chromium can be traced to estimate how large an asteroid was when it formed its iron core. The research was supported in part by the Royal Society, the Simons Foundation and the European Research Council. Bonsor is a Royal Society University Research Fellow at Cambridge.
Citazioni salienti
If these asteroids were melted by something which only exists for a very brief time at the dawn of the planetary system, then planet formation must kick off very quickly.— Dr Amy Bonsor, Cambridge Institute of Astronomy (paraphrased)
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