To resolve nearby Earth-like exoplanets, a new telescope design that is rectangular rather than circular may be necessary, according to a new study that explores what the next great space telescope might look like.
“We show that it is possible to find nearby, Earth-like planets orbiting sun-like stars with a telescope that is about the same size as the James Webb Space Telescope[(JWST], operating at roughly the same infrared wavelength as JWST, with a mirror that is a one by 20 meter [65.6 by 3.3 foot] rectangle instead of a circle 6.5 meters [21.3 feet] in diameter,” Heidi Newberg, who is a professor of astrophysics at Rensselaer Polytechnic Institute in New York, wrote in an editorial about the concept.
Top of the National Academies’ Astronomy and Astrophysics Decadal Survey is a new space telescope that is capable of imaging Earth-size planets in the habitable zone of sun-like stars. Although no design has been settled upon yet, a round-ish mirror with a minimum aperture of 26 feet (eight meters) has been mooted. This is five feet (1.5 meters larger than the current largest orbiting observatory, the JWST.
Yet Newberg believes there is another way.
If the aim is to image a planet with an atmosphere laden with water vapor, then the telescope would need to be optimized to detect light with a wavelength of 10 microns (10 millionths of a meter, equivalent to the thickness of a human hair), which is the infrared wavelength at which water vapor emits.
The JWST’s Mid-Infrared Instrument (MIRI) can observe at this wavelength, and indeed it has detected water vapor in the atmosphere of hot, massive exoplanets. Observing in infrared also provides a contrast boost: a planet would be a billion times fainter than its star in visible light, but in the best case scenario it would be “only” a million times fainter at 10 microns — still extremely faint, but feasibly within range of a next-generation space telescope.
However, the JWST’s 21.3-foot (6.5-meter) segmented mirror is too small to resolve an Earth-size, water-rich planet in the habitable zone of a sun-like star. The angular resolution of a telescope is determined by the observed wavelength divided by the telescope diameter and multiplied by 1.22 (called this the Rayleigh criterion). To resolve an Earth-size planet at 10 microns at a distance of about 30 light-years would require a telescope aperture approaching 20 meters (65.6 feet). But such a telescope would be prohibitively expensive and an engineering nightmare as it would need to be folded up several times to fit inside the faring of whichever rocket might launch it.
Alternatively, many small telescopes could be launched into space to work as an optical interferometer, combining the light of all the telescopes to give the resolution of a larger aperture. However, this would need extremely precise alignment between the smaller telescopes, a technological challenge that would be expensive and perhaps not even possible with current technology.
Newberg’s team, however, realized that a large rectangular telescope mirror would be much more efficient than a huge circular one, and because of its relative simplicity it would be much less costly than an interferometer. A telescope mirror that is a strip with dimensions of 65.6 feet by 3.3 feet (20 meters by 1 meter) would have a smaller area than a circular mirror of the same width, and therefore be much less expensive. The idea would then be to align the telescope lengthways with the orientation of the target exoplanet relative to its star. If the planet is in another orientation, the rectangular telescope can then be rotated.
Remarkably, such a telescope would actually have a slightly smaller collecting area (65.6 square feet, or 20 square meters) than the JWST (83.3 square feet, or 25.4 square meters). The difference is that all its collecting area would be in the orientation that is needed to image a planet, with nothing wasted.
There are 69 approximately sun-like stars (spectral classes K, G and F), not to mention almost 300 of the coolest stars, M dwarfs, all within 32.6 light-years (10 parsecs) of our solar system that a new telescope could target.
“We show that this design can, in principle, find half of all existing Earth-like planets orbiting sun-like stars within 30 light years in less than three years,” writes Newberg. “If there is about one Earth-like planet orbiting the average sun-like star, then we would find around 30 promising planets.”
A paper describing the new telescope concept was published on Sep. 1 in the journal Frontiers in Astronomy and Space Sciences.