Chapter 9 of 15
Radio
Magnetised planets shine at radio wavelengths: electrons spiralling down field lines into the auroral zones emit by the cyclotron maser, beamed in a hollow cone, up to the local cyclotron frequency (Zarka 1998; Treumann 2006). A close-in planet can also drive such emission on its star, as Io does on Jupiter (Zarka 2007). No exoplanet's own radio emission has been confirmed. Every claim below is tentative or contested, and says so.
The frequency scale
The cyclotron maser emits at, and just below, f_ce = eB/(2π m_e) ≈ 2.80 MHz per gauss of field at the source, so the highest frequency seen gives the field there. A weak planetary field emits below tens of MHz, where only low-frequency arrays such as LOFAR (10–240 MHz; van Haarlem et al. 2013) can listen; an M dwarf's kilogauss field reaches GHz.
| Quantity | Value | From |
|---|---|---|
| Cyclotron frequency per gauss | 2.799 MHz | eB/(2π m_e) with SI 2019 e and CODATA 2018 m_e [calc] |
| τ Boötis, tentative bursts at 14–21 MHz | ≥ 5.0–7.5 G | least field at the source for that band, if it is cyclotron emission (Turner et al. 2021 band) [calc] |
| YZ Ceti bursts at 2–4 GHz | ≥ 0.71–1.43 kG | least field at the source, on the star (Pineda & Villadsen 2023 band) [calc] |
| LOFAR band | 10–240 MHz | van Haarlem et al. 2013 |
| τ Boo Ab, stored mass and period | 4.5 MJ · 3.31 d | exoplanet.eu |
| YZ Cet b, stored m sin i and period | 0.0022 MJ · 2.02087 d | exoplanet.eu |
The claims, and where they stand
Tentative, not confirmed
τ Boötis b · LOFAR
Circularly polarised bursty emission at 14–21 MHz, about 3σ, with nothing in the off-beams; a slowly varying 21–30 MHz signal was also reported, with the authors' own doubts (Turner et al. 2021). Follow-up LOFAR observations covering 70% of the orbit, including the earlier orbital phases, showed no bursty or slow emission (Turner et al. 2024).
Contested: no planet yet explains it
GJ 1151 · LOFAR
Low-frequency emission from a quiet red dwarf, with the character of a planetary aurora, interpreted as the star interacting with an Earth-size planet on a 1–5 day orbit (Vedantham et al. 2020). Radial velocities then found a planet on a 390-day orbit that is very unlikely to be the source, and no short-period planet (minimum mass below 1.2 Earth masses) (Blanco-Pozo et al. 2023).
Candidate
YZ Ceti · VLA
Coherent bursts at 2–4 GHz, two of them at similar orbital phases of YZ Ceti b, which could plausibly power them; stellar activity is not ruled out, so YZ Ceti is a candidate star–planet-interaction system (Pineda & Villadsen 2023).
Radio found the first planets, another way
In 1992, Wolszczan and Frail found planets around the millisecond pulsar PSR B1257+12 from the regular wobble in its pulses' arrival times (Wolszczan & Frail 1992): the planets tug the pulsar, and the light-travel time changes by milliseconds. That is radio timing, not radio from a planet. Its card on the Galaxy tour ›
| Quantity | Value | From |
|---|---|---|
| PSR B1257+12 b: timing amplitude | 0.00453 ms | (Mp/M_psr)·a/c with exoplanet.eu's mass 7e-05 MJ, a = 0.19 AU, sin i = 1, and an assumed 1.4 M☉ pulsar (its mass is not stored) [calc] |
| PSR B1257+12 c: timing amplitude | 1.59 ms | (Mp/M_psr)·a/c with exoplanet.eu's mass 0.013 MJ, a = 0.36 AU, sin i = 1, and an assumed 1.4 M☉ pulsar (its mass is not stored) [calc] |
| PSR B1257+12 d: timing amplitude | 1.88 ms | (Mp/M_psr)·a/c with exoplanet.eu's mass 0.012 MJ, a = 0.46 AU, sin i = 1, and an assumed 1.4 M☉ pulsar (its mass is not stored) [calc] |
Not an exoplanet detection method
Technosignature searches (SETI)
These look for engineered signals, not for planets or their aurorae, and are kept separate here. Breakthrough Listen observed 1,327 nearby stars at 1.10–3.45 GHz with Green Bank and Parkes and, after excluding terrestrial interference, found zero candidates; the upper limits on a transmitter's power are 2×10¹² W and 9×10¹² W (Price et al. 2020; targets from Isaacson et al. 2017).
SetuHQ does not observe in radio. The scene is an illustration: field lines, cone and orbit are not to scale, and burst times are illustrative; only the cut-off frequency is computed.
Sources
- Treumann 2006, A&ARv 13, 229 (the electron-cyclotron maser)
- Zarka 1998, JGR 103, 20159 (auroral radio emission at the outer planets)
- Zarka 2007, Planet. Space Sci. 55, 598 (star–planet plasma interactions and radio)
- van Haarlem et al. 2013, A&A 556, A2 (LOFAR)
- Turner et al. 2021, A&A 645, A59 (τ Boötis with LOFAR)
- Turner et al. 2024, A&A 688, A66 (LOFAR follow-up of τ Boötis)
- Vedantham et al. 2020, Nature Astronomy 4, 577 (GJ 1151)
- Blanco-Pozo et al. 2023, A&A 671, A50 (GJ 1151 radial velocities)
- Pineda & Villadsen 2023, Nature Astronomy 7, 569 (YZ Ceti)
- Wolszczan & Frail 1992, Nature 355, 145 (PSR B1257+12)
- Isaacson et al. 2017, PASP 129, 054501 (Breakthrough Listen targets)
- Price et al. 2020, AJ 159, 86 (Breakthrough Listen: 1,327 nearby stars)