A radio survey of the sky has detected activity from a variety of nearby red dwarfs. In some cases, astronomers suspect, these radio signals indicate the presence of previously undetected planets around these low-mass stars. If they’re right, it adds a new method for finding our nearest planetary neighbors beyond the Solar System. Eventually, it could also help reveal whether conditions around these stars are suited to planets holding onto their atmospheres.
In 2020 astronomers announced the first evidence of an exoplanet (planet outside our Solar System) detected via radio waves. The finding concerned a previously obscure red dwarf known as GJ1151, but even the team responsible acknowledged there might be other explanations for the signal they were picking up.
Now the same team have published a pair of papers in Nature Astronomy and Astrophysical Journal Letters providing more detail on GJ1151 and adding three more planetary suspects around nearby low-mass stars. The team used the Low Frequency Array (LOFAR) telescope to scan an eighth of the northern sky and found radio emissions apparently coming from 19 red dwarfs between 12 and 150 light-years away.
“This discovery is an important step for radio astronomy and could potentially lead to the discovery of planets throughout the galaxy,” said Dr Benjamin Pope of the University of Queensland in a statement.
The Nature paper notes most of these are active stars whose emissions are easy to explain. Four, however, are quieter and would not be expected to produce signals like this unless they have a planet moving through their magnetic field.
The authors think the situation is similar to that of Jupiter and Io. Near the beginnings of radio astronomy, Jupiter was accidentally discovered to be emitting circularly polarized radio waves, now attributed to volcanic eruptions on Io enhancing Jupiter's aurora. Planets in close orbits around red dwarfs are in many ways more similar to Jupiter and Io, Pope told IFLScience, than to the Sun and Mercury, inspiring the search for radio signals revealing their presence.
The sky is full of radio sources, however, particularly black holes, and until the construction of LOFAR, the most powerful radio telescope ever built in the low-frequency part of the radio spectrum, detection was almost impossible. Leiden University's Dr Joseph Callingham and colleagues; “put a pair of polarized sunglasses on LOFAR” Pope told IFLScience, allowing it to focus on circularly polarized emissions similar to Jupiter's.
In the Astrophysical Journal paper, Pope and colleagues report their follow-up visual studies of the relevant stars, seeking planet-induced wobbles. They ruled out any planets substantially larger than the Earth but left the possibility of Earth-, or Mercury-sized planets very much open. Pope told IFLScience even small planets could cause the detected emissions.
The planets in question probably have orbits of two to five days, Pope added around such cool stars these could still place them in the habitable zone.
Proxima Centauri, the nearest red dwarf, is too far south for LOFAR to study but is perfectly placed for study with the Square Kilometer Array, which will be ten times more powerful than LOFAR when it comes online.
Pope hopes the work will eventually tell us about the plasma environment in which these planets move, revealing their chances of sustaining atmospheres in the face of stellar activity.
The survey also revealed one star, WX Uma, that has a powerful dipolar magnetic field with a radio source at least 5 stellar radii from the star. Pope told IFLScience that the team “[d]on’t think it is a serious candidate for a planet, but hesitate to offer any firm explanation.” Something strange is causing the unusual field shape, and efforts are underway to explain it.