Scientists
are working out how cosmic dust turns
into hard, spherical pebbles
which can then
develop into planets.
Image credit - NASA/JPL-Caltech
Detailed
simulations of planetary formation are revealing how tiny grains of
dust turn into giant planets and could shed light on where to find new
Earth-like worlds.
Scientists
theorise that planets form from rotating discs of gas that surround
newly formed stars, known as proto-planetary discs. Pebble-sized objects
in these discs then clump together to form cores of would-be planets.
Professor Anders Johansen from Lund
University in Sweden, has gone right down to the level of atomic nuclei
and molecules to try and work out how cosmic dust particles stick
together in pebbles and then turn into baby planets, known as
planetesimals.
‘Planet formation takes place when these
dust particles collide, and they grow to larger and larger sizes,’ he
said. ‘This growth takes us then from micrometres, all the way up to
10,000 kilometres or so.’
One clue to how this dust forms into
pebbles can be found on Earth in meteorites – pieces of asteroids that
are leftovers from the formation of the solar system.
‘There’s a mystery in there,’ Prof.
Johansen said. ‘If you look inside an asteroid, you do find
millimetre-sized pebbles, which is fine. But the problem with those
pebbles is they are not what we expect them to be. We would expect them
to be fluffy dust aggregates, a bit like if you have a sandbox after it
rains, and you can pick up a piece of dried out sand that is very
fragile,’ he said.
Instead, the pebbles are spherical and
hard, like they have been heated and cooled – similar to objects that
have been struck by lightning.
‘Lightning takes place as thunderclouds
discharge their electric charge to the ground,’ said Prof. Johansen.
‘This discharge is very similar to the shock you experience from the
static electricity when you put on a jumper.’
‘If we see a certain composition of the
system … that might allow us to see that there might be habitable
planets in those systems.’
Dr Bertram Bitsch, Max Planck Institute for Astronomy, Heidelberg, Germany
Prof. Johansen theorised that there must
be a mechanism during planet formation that creates positively and
negatively charged particles, and he and his team investigated what that
was.
‘While a thundercloud obtains a charge
difference between its top and bottom by falling hail particles, we
found that in the protoplanetary disc the decay of a radioactive element
called Aluminium-26 is very efficient at charging dust clouds,’ he
said.
Chemical composition
The finding was part of a project called PLANETESYS,
which is using computer simulations to replicate the physical processes
taking place when planets form – all the way from dust to a planetary
system. It includes details about the chemical composition of each
pebble.
One thing Prof. Johansen can examine from
looking at this chemical composition is how planets accrete water – a
vital ingredient for life.
‘An obvious question is, “How much water
does a planet get?” We can begin to speculate about if it’s normal to
get the Earth’s amount of water, if it’s a lot of water or a little bit.
But maybe you can also get too much water, which may be good for life
but not good for civilisations,’ he said.
Dr Bertram Bitsch from the Max Planck
Institute for Astronomy in Heidelberg, Germany, says that understanding
more about how planets arise would help identify potentially habitable
planets elsewhere in the universe.
‘If you understand more (about) the
formation process of how we can make a system like the solar system,
then we can maybe make predictions (about) how often these systems would
exist and how common it would be to find Earth-like planets (orbiting)
other stars.’
‘Then, if we see a certain composition of
the system … that might allow us to see that there might be habitable
planets in those systems.’
Recipe
Dr Bitsch thinks he might know the recipe
for how solar systems end up with Earth-like planets. With a careful
blending of conditions, from where baby planets form, to their chemical
composition and gravitational interactions, he can try to model the
conditions to generate solar systems with habitable planets.
But figuring out the right recipe requires
working backwards after running many simulations with complex
supercomputing power, which he’s doing in a project called PAMDORA which runs until 2022.
‘I want to use computer simulations …
where we look at the gravitational interactions between multiple bodies
to model the stages from planetesimals all the way to fully formed
planetary systems with terrestrial planets, super Earths, and gas
giants,’ he said.
In his simulations, Dr Bitsch looks at the
how pebbles in the swirling discs form into moon-size planetary
embryos, which then develop into fully formed planets.
Altering the different mechanisms at work can influence what types of planets a solar system may end up with.
‘There are many different pathways that
can happen, and many different parameters that can influence the outcome
of the simulations,’ he said. ‘For example, how big are the pebbles,
how many would there be, and where would your initial planetesimals form
that would then start to form proto-planets?’
To see which variables matter most, he
runs hundreds of computer simulations that last weeks at a time and can
simulate tens of millions of years to model the highly chaotic meetings
of multiple objects.
For Earth-like planets, one key factor is
how close baby planets form to their home star, since the difference in
temperature can determine if planets accrete water directly during the
gas-disc stage or from a late water delivery from asteroids or comets,
like for our own Earth.
‘One thing that is already in the code is
looking at the composition of super-Earths. For example, are they rocky
or dominated by water-ice?’ Dr Bitsch said.
Super-Earths, which are planets like the
Earth, but maybe two to ten times more massive, don’t exist in our solar
system, but are relatively common among other stars.
‘Lots of super-Earths have been found and
detected, and the question is what are they made of? This can give us
the answer to where they have been formed.’
The research in this
article was funded by the European Research Council. If you liked this
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