Scientists shed new light on fate of giant planets

As gas giants get closer to their sun, they undergo a dramatic transformation: their atmospheres expand rapidly, become unstable and start to burn off into space, now a team of scientists from
University College London (UCL) has worked out just how close a gas giant can get to its star before suffering this fate.

The work, which is published in the latest edition of the journal Nature, was presented at the European Forum on Science Journalism in Barcelona, Spain, on 4 December.

‘We know that Jupiter has a thin, stable atmosphere and orbits the Sun at five Astronomical Units (AU) – or five times the distance between the Sun and the Earth. In contrast, we also know that
closely orbiting exoplanets like HD209458b – which orbits about 100 times closer to its sun that Jupiter does – has a very expanded atmosphere that is boiling off into space. Our team wanted to
find out at what point this change takes place, and how it happens,’ said Tommi Koskinen, one of the authors of the paper.

Many gas giants migrate towards their sun over millions of years. As a planet approaches its star, the atmosphere warms up, and as things warm up, they expand. However, the atmospheres of these
giant planets also have a built-in thermostat in the form of a molecule called H3 . This electrically charged form of hydrogen is created by sunlight, so the closer a planet is to the Sun, the
more H3 is produced. H3 strongly radiates sunlight back into space, effectively lowering the temperature of the planet’s atmosphere.

The researchers developed a complex, three-dimensional model which took into account the heating effect of the Sun and the cooling effect of H3 . The model revealed that even at 0.16AU from the
sun, the planet’s atmosphere remains stable and Jupiter-like. However, moving the planet just a little bit closer to the sun triggers a breakdown of the H3 cooling mechanism and the atmosphere
expands.

‘We found that 0.15AU is the significant point of no return,’ said Professor Alan Aylward. ‘If you take a planet even slightly beyond this, molecular hydrogen becomes unstable and no more H3
is produced. The self-regulating, ‘thermostatic’ effect then disintegrates and the atmosphere begins to heat up uncontrollably.’

Presenting his team’s results in Barcelona, Professor Steve Miller explained that the phenomenon of large planets migrating inwards to the centre of their solar systems appears to be relatively
common. ‘Of the 260 plus extra-solar planets we have found, around 100 to 120 are orbiting within one astronomical unit, so they may have gone through or be going through this process,’ he
explained.

However, Professor Miller was quick to reassure the conference participants that Jupiter was not going to do this.