That's because scientists have picked up 'coherent'
radio signals from a planet 12 light years away from Earth, which suggest it
has a magnetic field.
Magnetic fields are essential for a habitable
planet, as they protect any life it harbours from bombardment by cosmic radiation
and charged particles.
Researchers from the US National Science Foundation
(NSF) say the radio signal came from a rocky planet called YZ Ceti b, which
orbits the small red dwarf star YZ Ceti.
It likely originated from an interaction between the
planet's magnetic field and the star it orbits, similar to the aurora borealis
– or Northern Lights – here on Earth.
'The search for potentially habitable or
life-bearing worlds in other solar systems depends in part on being able to
determine if rocky, Earth-like exoplanets actually have magnetic fields,' says
Joe Pesce, NSF's programme director for the National Radio Astronomy
Observatory.
'This research shows not only that this particular
rocky exoplanet likely has a magnetic field but provides a promising method to
find more.'
The Earth's magnetic field is a layer of electrical
charge that surrounds it and stretches out into space. It is largely generated
by the superheated, swirling liquid iron that makes up our planet's outer core,
1,900 miles (3,000 km) below our feet.
As heat escapes from the inner core, the iron moves
around in convection currents, and the motion generates powerful electrical
currents.
The rotation of Earth on its axis causes these currents to form a magnetic field.
As well as allowing compasses to work, the magnetic field deflects charged particles fired from the sun known as 'solar wind', as well as cosmic radiation from outer space.
Without this protective layer, these particles would
likely strip away the Ozone layer, our only line of defence against harmful UV
radiation.
Therefore, a magnetic field is thought to be one of the essential ingredients to make a planet habitable, as it can prevent its atmosphere from being worn away.
'Whether a planet survives with an atmosphere or not
can depend on whether the planet has a strong magnetic field or not,' said
Sebastian Pineda, an astrophysicist at the University of Colorado.
So when scientists detected a repeating radio signal
emanating from YZ Ceti b with the Karl G. Jansky Very Large Array telescope, it
raised hopes that it could sustain life.
The fact that it could be detected so far away
indicates it is very strong, suggesting that the magnetic field on the planet
is too.
'This is telling us new information about the
environment around stars,' said Pineda.
Earth's magnetic field can attract some of the
charged particles from the sun, causing them to collide with the atoms in the
upper atmosphere, like oxygen and nitrogen.
When they do this, some of the energy in the
collisions is transformed into the green-blue light, known as the Northern
Lights, or aurora borealis.
This is the only visual representation of the
magnetic field we can experience, but otherwise it is invisible.
In the new study, published in Nature Astronomy, the authors describe the signals as 'auroral radio emissions'.
That's because they think the radio waves are the
result of interactions similar to those of aurora borealis.
When charged particles burst away from the YZ Ceti,
some of them rebound off the magnetic field of YZ Ceti b to interact with the
star's magnetic field instead.
This produces an aurora on the star itself, which
results in the radio waves detected on Earth.
'There should also be aurora on the planet if it has
its own atmosphere,' said Jackie Villadsen, an astronomer at Bucknell
University.
The fact that the star and planet are close together
- YZ Ceti b completes a full orbit in only two days - means that these
interactions, and resulting radio waves, occur fairly frequently.
Villadsen said: 'These planets are way too close to
their stars to be somewhere you could live, but because they are so close the
planet is kind of ploughing through a bunch of stuff coming off the star.
'If the planet has a magnetic field and it ploughs
through enough star stuff, it will cause the star to emit bright radio waves.'
This gives the researchers a good opportunity to
conclude whether magnetic fields on distant planets are actually detectable
from Earth.
As they are invisible they are notoriously difficult
to identify, so it is similarly difficult to conclude that the planets they
extend around are habitable.
If it does have a magnetic field, YZ Ceti b is a particularly
strong candidate for a habitable exoplanet - or planet outside or solar system
- because it is rocky and a similar size to Earth.
Despite getting a result that 'no one has seen
happen before', the team says that they are still awaiting a 'really strong
confirmation of radio waves caused by a planet'.
Pineda said: 'There are a lot of new radio
facilities coming online and planned for the future.
'Once we show that this is really happening, we'll
be able to do it more systematically. We're at the beginning of it.'
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