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It is generally believed that stars form millions of years before the planets that orbit them, with the leftover material from star formation accumulating to form planets. But new research on “polluted white dwarfs” adds to evidence which suggests that the building blocks of planets like Jupiter and Saturn start forming when a young star is growing.
According to the University of Cambridge, the study published in Nature Astronomy could potentially help solve a major puzzle in astronomy by changing scientific understanding of how planetary systems are formed.
Polluted white dwarf stars
To learn more about the timeline of planet formation, the researchers turned their attention to the atmospheres of white dwarfs, which are the ancient remnants of stars like our Sun. According to Amy Bonsor, the first author of the study, some white dwarfs are “amazing laboratories” because they have thin atmospheres that Bonsor compares to “celestial graveyards,” in a press statement. Bonsor is a research fellow at the University of Cambridge’s Institute of Astronomy.
Usually, telescopes cannot learn much about the interior of planets but “polluted” white dwarf systems are an exception. Polluted white dwarfs are white dwarf stars that have recently consumed a planet or asteroid that was orbiting around them. Spectroscopic observations of such polluted stars can reveal the composition of the asteroids and planets that burned up in their atmosphere.
Planetary formation
According to the current leading theory on planet formations, it is believed that planets begin forming in “protoplanetary discs,” which are mainly made of hydrogen, helium, and tiny ice and dust particles. Dust particles orbiting a young star begin sticking to each other until eventually former larger and larger bodies. Some of these will keep growing until they become planets and others will remain as asteroids.
For the study, the research team analysed spectroscopic data of the atmospheres of 200 polluted white dwarf stars from nearby galaxies. They observed that the mixture of elements seen in the atmospheres of these stars can only be explained if many asteroids had melted, causing heavy iron particles to sink to the core while lighter elements float on the surface. This process is called differentiation and it is why Earth has an iron-rich core.
The cause of the melting can only be attributed to very short-lived radioactive elements, which existed in the earliest stages of the planetary system but decay away in just a million years. In other words, if these asteroids were melted by something which only exists for a very brief time at the dawn of the planetary system, then the process of planet formation must kick off very quickly,” explained Bonsor.
According to Bonsor, this study supports a “growing consensus in the field” that planets begin forming early, at the same times that stars are formed.
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harleen 
