Exoplanet hunters have a number of different ways to detect worlds around other stars in the Galaxy.
This includes observing how the star is slightly pulled by the gravity of the planet it orbits (known as ‘radial velocity’).
Or observing its star’s light partially blocked by a transiting exoplanet (known as the ‘transit method’).
But how else can astronomers indirectly infer the existence of an invisible planet around a star?
Using Jupiter and Io
View of Jupiter’s moon Io captured by the Galileo spacecraft. Credit: NASA/JPL/University of Arizona
One idea that has been floating around for some time is the possibility of detecting exoplanets via radio waves emitted through magnetic interactions between stars and planets.
The underlying process is known on Jupiter and its moon Io.
As Io orbits around Jupiter and through its strong magnetic field, charged particles from the moon are swept towards the planet in what are known as Alfvén waves.
This in turn accelerates the flow of electrons that emit cone-shaped radio waves.
Astronomers propose that the same physical process could be used to detect planets close to their stars.
Small red M-dwarfs would be good candidates to examine because such stars generally have strong magnetic fields.
The illustration shows Jupiter’s magnetic field. Credit: NASA Goddard Space Flight Center
Practice it
Encouragingly, a number of M dwarfs have been found to emit bright radio emissions that are characteristic of star-planet magnetic interactions – although other mechanisms have not been ruled out – but none of the stars are known to be close by. on planets.
The trick of this planet hunting method is that radio waves are emitted in the form of rays that sweep through space as the planet orbits.
So you need to listen with a radio telescope at the right time to catch the emissions.
For Io and Jupiter, within the plane of our Solar System, the orbital, rotational, and magnetic axes are all parallel to each other, and perpendicular to our line of sight from Earth.
This neat configuration means we detect bursts of radio emission when Io is at orbital phases 0.25 and 0.75.
This means that when the Moon moves directly towards or away from us in its orbit around Jupiter, its radio emission cone is tilted towards the Earth.
However, for exoplanetary systems, the orbital, rotational and magnetic axes in the star-planet system will most likely not be aligned.
An artist’s impression of a Jupiter-like exoplanet orbiting a white dwarf. Credit: WM Keck Observatory/Adam Makarenko
Which exoplanet?
Robert Kavanagh and Harish Vedantham at the Netherlands Institute for Radio Astronomy have investigated when such radio emissions are most likely to be detected, and what survey strategies are best for picking them up from different planetary systems.
Their computer modeling shows that the planetary systems that are easiest to detect using this radio method are those oriented almost facing Earth.
This is useful because many other planet detection techniques, such as transit or radial velocity, are most sensitive to planetary systems located directly in front of our line of sight.
In fact, as Kavanagh and Vedantham point out, this may be the main reason why none of the nearby planets discovered around M dwarfs are known to produce radio emissions indicative of this.
Lewis Dartnell is reading Hunting for Exoplanets via Star-Planet Magnetic Interactions: Geometric Considerations for Radio Emissions by Robert D Kavanagh and Harish K Vedantham Read online at: arxiv.org/abs/2307.02555
2023-10-20 11:11:46
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