What exactly is a “normal” solar
system? If we thought we had some idea in the past, we definitely don’t now.
And a new study led by astronomers at Cambridge University has reinforced this
fact. The new study found four gas giant planets, similar to our own Jupiter
and Saturn, orbiting a very young star called CI Tau. And one of the planets
has an extreme orbit that takes it more than a thousand times more distant from
the star than the innermost planet.
CI Tau is about 500 light years away
in a very active part of the galaxy termed a “stellar nursery.” At only 2
million years old, CI Tau is a mere infant in stellar terms. Our own Sun is
about 5 billion years old and has another 5 billion to go, give or take. So CI
Tau is too young to have the type of gas giants that it has. Or so we thought.
Our best model of solar system
formation is called the nebular hypothesis and goes something like this: A
swirling cloud of gas and dust grows and gains density until it collapses into
a star and fusion starts. Around this newly-formed star, the remainder of the
gas and dust keeps swirling as a protoplanetary disc. Over time, this matter
clumps up and forms other planets, moons, and asteroids, and there’s no more
protoplanetary disc. That’s a very brief description of how things go.
ALMA image of the protoplanetary disc around
HL Tauri, another young star similar to CI Tau, but not part of this study.
Astronomers think that the dark circular shapes are the orbital paths of the
planets that are forming around the star. - Image Credit: ALMA
Observatory.
But this process of planet formation
is thought to take a long time, much more than 2 million years. A star like our
Sun can form in as few as 1 million years, but gas planets take between 10 and
100 million years to form, and terrestrial planets take even longer.
“Saturn mass planets are supposed to
form by first accumulating a solid core and then pulling in a layer of gas on
top, but these processes are supposed to be very slow at large distances from
the star. Most models will struggle to make planets of this mass at this
distance.” – Study Lead Author, Professor Cathie Clarke, Cambridge University
Institute of Astronomy.
This isn’t the first time that CI Tau
has caught the eye of astronomers. In 2016, astronomers discovered a hot
Jupiter orbiting the star. This made it the first star of its young age to have
one of these sizzling hot gas giants. A hot Jupiter is a gas giant in the same
mass range as our very own Jupiter, but one that orbits so close to its star
that it’s very hot. Hot Jupiter’s are puzzling because we don’t understand how
they can form ‘in situ,’ or so close to the star. Astronomers think these types
of planets form further away from the star and migrate in. How could this
happen in only 2 million years?
In this new study, a team of
researchers led by Cambridge University have turned their eyes towards CI Tau
again. They used the Atacama Large Millimeter/submillimeter Array (ALMA) to
search the CI Tau system for siblings to the hot Jupiter. What they found just
increased the mystery of the system. They discovered 3 other gas giants
orbiting the star, and one of them follows a far-flung orbit a thousand times
further from the star than the innermost planet, the aforementioned hot
Jupiter. According to the paper, this is “… the most massive ensemble of
exo-planets ever detected at this age.” So what does all this mean?
“It is currently impossible to say
whether the extreme planetary architecture seen in CI Tau is common in hot
Jupiter systems…” – Professor Cathie Clarke, Cambridge University Institute of
Astronomy.
For now, we’re not sure. Around 1% of
stars host hot Jupiters, but most of the known hot Jupiters are hundreds of
times older than CI Tau. Since as far as we know, planets can’t form before the
star does, how can astronomers explain what happened in this system?
“It is currently impossible to say
whether the extreme planetary architecture seen in CI Tau is common in hot
Jupiter systems because the way that these sibling planets were detected –
through their effect on the protoplanetary disc – would not work in older
systems which no longer have a protoplanetary disc,” said Professor Cathie
Clarke from Cambridge’s Institute of Astronomy, the study’s first author.
There are enough questions here to
keep a team of astronomers busy for their entire careers. According to the
researchers, it’s unclear what role, if any, the sibling planets played in
driving the innermost planet into its ultra-close orbit. It’s also unclear if
whatever mechanism might be at play in this system is the same mechanism that
makes hot Jupiters in general. And a further mystery is how the outer two
planets formed at all.
Three of the dishes that make up the Atacama
Large Millimeter/submillimter Array (ALMA). - Image Credit: H. Calderón – ALMA
(ESO/NRAO/NAOJ)
“Planet formation models tend to
focus on being able to make the types of planets that have been observed
already, so new discoveries don’t necessarily fit the models,” said Clarke.
“Saturn mass planets are supposed to form by first accumulating a solid core
and then pulling in a layer of gas on top, but these processes are supposed to
be very slow at large distances from the star. Most models will struggle to
make planets of this mass at this distance.”
This, of course, is what makes
science so compelling and effective. Astronomers observe something and create a
model to explain it. Then they keep observing, and some discoveries reinforce
the model, while some challenge it. So the model keeps getting updated and over
time realistically represents a larger and larger sample of observations.
Astronomers will keep studying this
system to try to unravel some of these mysteries. ALMA has revolutionized the
study of protoplanetary discs, and future work will no doubt lean heavily on
ALMA again. It has the power to image planets forming inside protoplanetary
discs, which are dim, poorly lit places that are very difficult to see into.
Be prepared to be surprised by what
ALMA sees. Again.
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