Claire Davies from October’s Sky at Night Q&A special answers five questions she’s frequently asked about how stars and planets form
The Sky at Night TV show, past, present and future
I use some of the highest-resolution observing facilities on Earth to study how stars and their planets form. Five of the questions I’m frequently asked about my research are:
How long does it take to form a star?
The process starts with an initial collapse of material in a molecular cloud, trigged by gravity, and ends when hydrogen fusion ignites in the star’s core. How long this process lasts depends on the mass of the star. For stars like the Sun, it takes a few tens of millions of years, while higher-mass stars are formed much more quickly and lower-mass stars take much longer. Regardless of the mass, the process is rapid in proportion to a star’s lifetime: if you condensed the lifetime of a star down to the lifetime of a typical human, the formation process would only take around three months!
Do planets form at the same time as stars? Our Solar System’s planets, and most extra-solar planets that have so far been discovered, are ‘first generation’ planets: they formed from the same collapsing material as the star. During collapse, there is a battle between gravity and rotation, which results in the formation of a circumstellar disc. The material in these discs is what collapses or coalesces further to form planets.
Do all stars form planets?
It ultimately depends on the lifetime of its disc. We see some protostars in very young star-forming regions which lack discs. However, we aren’t yet able to investigate whether planets have already formed around these stars or whether the disc dispersed before they had time to.
Just because planets form in orbit around a star doesn’t always mean they’ll stay there either. In long-lived discs, planets may interact with neighbouring disc material, resulting in an orbital tug-of-war. Planets may drift inwards onto increasingly shorter orbits and some of these may end up being engulfed by their star. Others may encounter more massive protoplanets during their migration through the disc and be thrown out of their system altogether.
