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Current and upcoming telescopes which find exoplanets and take spectra of their atmospheres seem to be optical or infrared. What about radio telescopes applied to exoplanets? They are larger and more easy to use with interferometry. Why aren't radio frequencies as useful as optical/IR for exoplanet studies?

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Radio telescopes are frequently used to observe the births of stars and their planetary systems. The longer wavelengths are able to penetrate from beneath the envelope of gas and dust that shrouds any attempts to view these events at optical or infrared wavelengths. Unfortunately, the smallest angular resolution of a telescope goes as $\lambda/D$, where $D$ is the telescope diameter and $\lambda$ the observation wavelength. As radio waves are $10^5$ to $10^{6}$ times longer than optical waves, then one needs enormous telescopes to get similar resolving power. This is attempted/accomplished by constructing arrays that can act as a single aperture.

Whilst interferometric techniques to gain spatial resolution are certainly possible, if one wants to directly image exoplanets then optical/IR is still the way to go because this is where most of the intrinsic light from an exoplanet is found. It is possible that planets orbiting stars may emit radio waves in their own right, through charged particles accelerated in their magnetospheres. Jupiter, for instance, is a strong source of radio waves in our solar system, but would be very difficult to spot at the distances of exoplanets. Some searches have been attempted, but nothing yet found (e.g. George & Stevens 2007).

Of course transit observations do not require superb angular resolution, but they do require lots of signal and precision. Levels of precision that are routinely achieved by optical telescopes (especially in space - e.g. Kepler) are not possible with radio telescopes, and the parent stars themselves are weak, variable radio sources, which makes studying any transit signature very difficult. In fact the main use of transits has been the other way around - to search for intrinsic planetary emission by looking for a drop in flux when the planet is eclipsed by the star (e.g. Lecavelier Des Etangs et al. 2011). Radio emission might be enhanced in hot Jupiters if the exoplanetary magnetic field interacts with the stellar magnetic field. This paper does claim to have detected a weak exoplanetary signal from a hot Jupiter, identified by its disappearance when eclipsed by the parent star. However, the result was not repeatable.

The SETI programme targeted a set of Kepler stars with evidence for a planet (or planet candidate) in the "habitable zone". The initial searches for narrow-band radio emission (at 1.1-1.9 Hz) appears to have concentrated on those systems containing multiple planets.

The (null) results for $\sim 100$ system were published by Siemion et al. (2013). The observations were not sensitive enough to detect the sort of "radio chatter" emitted from our planet, but might have detected some sort of deliberate signalling beacon.

A slightly more imaginative approach is to observe Kepler multiple planet systems when two planets are in conjunction as seen from the Earth. There is then the chance of seeing communications from one planet to the other. (e.g. http://adsabs.harvard.edu/abs/2014ebi..conf..5.2K ).

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  • $\begingroup$ The Spektr-R interferometer is already an enormous radio telescope! Precisely resolution seems to be the great advantage of radio over optical. But angular resolution should not be relevant for observing an exoplanet while it is transiting. A miniscule (1/1000000?) of the star's light passes through the exoplanet's atmosphere. Could radio telescopes make a useful spectra of that? $\endgroup$ – LocalFluff Jan 11 '15 at 12:24
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    $\begingroup$ @LocalFluff Angular resolution is used to image exoplanets. This is precisely what people are trying to do with big telescopes in the optical/IR. You can observe transits with a 10-inch optical telescope. To measure transits you just need lots of signal and precision. You do not get that with an interferometer. I'll think of an edit. $\endgroup$ – Rob Jeffries Jan 11 '15 at 12:38
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Some radio telescopes have been used to observe young stars and protoplanetary disks, if that counts.

  • Atacama Large Millimeter Array (ALMA): Though it's used for a variety of things, ALMA was used in late 2014 to observe a young star, HL Tau. Among the data gathered was information about HL Tau's circumstellar disk. It found a series of gaps in the disk. According to ALMA Deputy Director Stuartt Corder,

    These features are almost certainly the result of young planet-like bodies that are being formed in the disk. This is surprising since HL Tau is no more than a million years old and such young stars are not expected to have large planetary bodies capable of producing the structures we see in this image.

    ALMA was used because it could see through gas and dust surrounding the star.

    The telescope was also used in December to find evidence of "Pluto-size objects" around a different star, HD 107146. Only dust grains in the circumstellar disk were detected, but their behavior was considered consistent with the existence of Pluto-sized objects nearby. A paper was published on the findings, entitled "ALMA observations of the debris disk around the young Solar Analog HD 107146."

    Here's one of ALMA's pictures, of the first star, and a picture of the second star.

  • VLA: The VLA, too, has been used to look at young stars and protoplanetary disks. Astronomers looked at star-forming region LDN 1551 and made observations of what appear to be protoplanetary disks around two young stars; the abstract from the resulting paper can be found here. In another instance, the VLA was used to observe protoplanetary disk NGC 2071; the abstract from the resulting paper can be found here.

  • Green Bank Observatory: Within the last decade or so, there have been a handful of observations of young stars and protostars, though exoplanets do not appear to have been studied.

I would include the images for both telescopes, but it appears they are copyrighted. To my knowledge, though, these two telescopes have not been used for observing full-fledged exoplanets. I'm also investigating other major radio telescopes to see if they've been involved.

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