In our endless search to understand the Universe and our place within it, precious little blips in data can hint at entire new worlds.
Dips in the light levels of a star can betray the presence
of orbiting planets - and now astronomers have taken the first steps towards
using peeps of radio emission to reveal new exoplanetary mysteries.
"Observing planetary auroral radio emission is the most
promising method to detect exoplanetary magnetic fields," explained
Cornell University astronomer Jake Turner and colleagues in their new paper,
"the knowledge of which will provide valuable insights into the planet's
interior structure, atmospheric escape, and habitability."
When stellar wind - charged particles streaming from the
host star - hits a planet's magnetic field, its change in speed can be detected
as striking variations in radio emissions, statistically described as 'bursty'.
Earth's own magnetic field trills and squeaks like alien
birds as it channels solar winds. We've also heard similar cries from other
planets in our Solar System.
Of course, to detect a whisper of such radio signals coming from an exoplanet, we first need a way to look beyond all the noise from Earth and elsewhere.
A few years ago, the team developed the BOREALIS pipeline
program to do just that. They tested it on Jupiter and then calculated what
Jupiter's radio emissions would look like if it were much farther away.
There have already been some tentative detections of new
planets using these radio emissions, including early this year when astronomers
linked radio wave activity to interactions between star GJ 1151's magnetic
field and a potential Earth-sized planet. But these have all yet to be
confirmed by follow-up radio observations.
So Turner's team decided to test the technique they
developed, using Netherland's Low Frequency Array Radiotelescope (LOFAR) to
look at three systems with known exoplanets: 55 Cancri, Upsilon Andromedae, and
Tau Boötis.
Only the Tau Boötis system, 51 light years away, exhibited
the peeps in radio data that fit the researchers' predictions from their tests
with Jupiter. It came in the form of 14-21 MHz bursty emissions and is within
roughly three standard deviations of certainty (3.2 sigma).
In 1996, a hot-Jupiter exoplanet was discovered on a
3.3128-day orbit around the scorching young F-type star and the smaller red
dwarf that make up the Tau Boötis binary system.
"We make the case for an emission by the planet
itself," said Turner. "From the strength and polarisation of the
radio signal and the planet's magnetic field, it is compatible with theoretical
predictions."
If their measurements are correct, they suggest the planet's
surface magnetic field strength ranges from around 5 to 11 gauss (Jupiter
ranges from 4 to 13 gauss, for comparison, and measurements of its magnetic
field have revealed the planet has a core of metallic hydrogen). The observed
magnetic field emission strength also fits previous predictions.
"The magnetic field of Earth-like exoplanets may
contribute to their possible habitability," Turner explained, "by
shielding their own atmospheres from solar wind and cosmic rays, and protecting
the planet from atmospheric loss."
The signal they detected is weak and still needs to be verified by other low-frequency telescopes before researchers can confirm the true origin of the detected radio emissions.
"We cannot rule out stellar flares as the source of the
emissions," the researchers cautioned, but emissions from the planet
remain a possibility.
If other telescopes like LOFAR-LBA and NenuFAR can
corroborate these findings, such radio emission detections from exoplanets will
open up an exciting new field of research, providing us with a potential way to
peer further into distant, alien worlds.
This research was published in Astronomy & Astrophysics.