14 | New Scientist | 18 April 2020ASTRONOMERS have discovered
what seems to be the core of a
once-mighty gas giant, devoid
of the thick shroud of gas that
used to surround it. This huge but
decrepit world, the first we have
seen of its kind, could help us learn
about how planets are formed.
David Armstrong at the
University of Warwick, UK, and
his colleagues found this strange
planet, called TOI-849b, using
NASA’s Transiting Exoplanet
Survey Satellite. This watches
stars for periodic dips in their
light as planets pass in front of
them. The team then confirmed it
using several other observatories.
Although TOI-849b is smaller
than Neptune, it is 40 times as
massive as Earth and has roughly
the same density. That means it
is probably a rocky world, not a
gaseous one like Neptune – and
the largest rocky world we have
ever seen. Its surface could contain
almost 150 copies of Earth’s
surface (arxiv.org/abs/2003.10314).
“We don’t see any other planets
that are this size and this density,”
says Armstrong. Usually, a rocky
world this big would be expectedto build up a thick atmosphere and
become a gas giant like Jupiter.
We don’t know why it didn’t.
One idea is that it could have
been starved of gas as it formed, so
that it could build a core but not an
atmosphere, but it isn’t clear what
would cause that. Perhaps more
likely is the idea that it formed like
a regular gas giant but somehow
lost its gas, either because it got
too close to its star and theatmosphere was ripped away or
through a catastrophic collision
with another giant planet.
The location of the planet is also
unexpected. It orbits its star once
every 18.4 hours, an orbit so close
that the planet’s temperature is
about 1500°C. Usually, Neptune-
sized planets so close to their stars
either get ripped apart or partially
vapourised by radiation, leading
astronomers to nickname the
area in which TOI-849b orbits
as the “hot Neptunian desert”.
“This planet is really bizarre
compared with the planets in oursolar system, but also compared
with the other 4000 planets
we know of,” says Carole Haswell
at the Open University, UK.
“Because it’s unique, it has
potential to challenge our ideas
of how planet formation works
because it doesn’t follow the
evolutionary path of the vast
majority of planets we know of.”
If TOI-849b did form like a
normal gas giant before some
event tore away its atmosphere,
it could also help us learn about
planets closer to home. “It’s very,
very hard to study the cores of
planets,” says Armstrong. “Even
in the solar system, we don’t
know that much about Jupiter’s
core, or Saturn or Neptune or
Uranus, because there’s all that
atmosphere in the way.”
This planet may have a thin
atmosphere that forms as heat
from the star vaporises rocks and
dust on the planet. If so, the next
generation of telescopes may
be able to use that atmosphere
to learn about the chemical
composition of TOI-849b, which
could help us understand giant
ES planets more generally. ❚
ONews
A huge, dense , rocky
exoplanet challenges our
ideas of planet formationSpace junkRockets armed with
talcum powder could
stop space junkCLOUD-EMITTING rockets could
prevent satellites from colliding by
altering their trajectories, avoiding
potential disasters that would see
Earth’s orbits littered with junk.
There are 2000 active satellites
in orbit today, along with 3000
dead satellites and many more
pieces of dangerous smaller debris.
Often satellites must dodge out
of the way of this debris – or other
satellites – but defunct satellitescan’t adjust their movement.
Darren McKnight at US
technology firm Centauri and
his colleagues think they have a
solution: using suborbital sounding
rockets – which are normally used
to launch experiments briefly into
space – to launch a cloud of particles
in front of potentially dangerous
debris and change its trajectory.
“If you could avoid the collision,
you can make space operations safe
without the very expensive issue
of removing an object from orbit,”
says McKnight. “Instead, you
would just nudge it out of the way
if it got too close to another object.”The rocket would carry a small
capsule capable of emitting a cloud
of particles. This could be as simple
as 100 grams of talcum powder,
says McKnight. When the offending
object collides with the cloud, it
would lose enough momentum
to change its trajectory and avoid
a potentially disastrous collision
(Acta Astronautica, doi.org/drgd).
The timing would need to be
exact because the capsule wouldhave to release its particles about
10 seconds in front of the target
object, at altitudes of hundreds of
kilometres. But sounding rockets
are relatively cheap at a few million
dollars and can be launched from
many locations around the world.
McKnight and his colleagues
haven’t yet launched a rocket to
test the idea, but they hope their
suggestion will prompt further
discussion on how to prevent
space junk disasters. “I’m concerned
there will have to be some bad event
before people get motivated to
spend money on this,” he says. ❚
Jonathan O’CallaghanExoplanetsLeah CraneRevealing the heart of Jupiter
The largest rocky exoplanet ever seen might be the core of a former gas giant
“ You can make space safe
without the very expensive
issue of removing
an object from orbit”