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A Jupiter-sized object at 10 pc subtends an angle of 0.0001 arcseconds (100 micro-arcsec) at the Earth. The Event Horizon Telescope interferometry network is capable of a (demonstrated) angular resolution of about 20 micro-arcseconds.

Why haven't we seen a picture of a giant exoplanet at microwave wavelengths yet? Is it just a sensitivity issue? If so, it's an interesting contrast to the situation at optical/near-IR wavelengths where such a planet would be bright enough but could not be resolved.

For reference, the image below is Jupiter as seen by the Juno spacecraft at microwave wavelengths - showing that there is certainly imaging structure to be seen.

enter image description here

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I think the radio emissions from the nearby host star would drown out any signal from the planet.

To use our Sun and Jupiter as an example, at 10 millimeter wavelength, the flux density of the Sun for a collector on Earth is about 10^7 janskys. The flux of Jupiter at 1 AU would be about 100 janskys. This means Jupiter's RF signature power at 10 millimeter wavelength is about 5 orders of magnitude less than the Sun's at about the same distance.

A similar effect is observed when we lose communications satellite downlink signal during solar transits. The Sun is such a powerful radio emission source that it causes electromagnetic interference, making it impossible to resolve the satellite signal.

I wondered if we could point a radio antenna at a planet without also being exposed to the RF energy of the nearby host star, so I looked up ALMA's antenna pattern. "The FWHM of the ALMA primary beam is 19" at 300 GHz for a 12-m antenna" according to ALMA's website. So even if a radio telescope was pointed at the planet, and the corresponding star was an enormous 19 arcseconds away, the interfering signal power from the star would only be halved.

The Sun is the most powerful natural radio source from the perspective of the Earth, and when I was working for the National Radio Astronomy Observatory, I was taught not to observe anywhere near the Sun unless I was observing the Sun, itself. The VLA publishes recommended standoffs from the Sun while observing.

enter image description here

Not to say it is impossible to detect radio waves from an exoplanet, but I think it would take a very clever radio astronomer to extract the radio emissions from such a high noise floor imposed by the nearby star. If one could find a "rogue" planet without a host star, I imagine we could image it with a radio telescope. However, it might not be emitting much power without interacting with a star.

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A 5 pixel by 5 pixel image would barely supply enough resolution to enable the observers to be able to say "we photographed a planet 10 parsecs away!" But that's about all they could say.

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  • $\begingroup$ The EHT image of the M87 ring is about 50 microarcsec across. $\endgroup$
    – ProfRob
    Commented Feb 14, 2023 at 21:59
  • $\begingroup$ @ProfRob And with that they were able to say they had observed a black hole accretion ring (and they definitely did do so). Despite the extreme coolness of that image, that was about all they could say. $\endgroup$
    – David Hammen
    Commented Feb 14, 2023 at 22:24
  • $\begingroup$ So is your answer that nobody has bothered because the planets would only be resolved into a 5x5 image? $\endgroup$
    – ProfRob
    Commented Feb 14, 2023 at 23:14

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