Circle versus rectangle: Finding Earth-like planets

The Earth supports the only known life in the universe, all of it depending heavily on the presence of liquid water to facilitate chemical reactions. While single-celled life has existed almost as long as Earth itself, it took roughly three billion years for multicellular life to form. Human life has existed for less than one-10 thousandth of the age of Earth.

All of this suggests that life might be common on planets that support liquid water, but it might be uncommon to find life that studies the universe and seeks to travel through space. To find extraterrestrial life, it might be necessary for us to travel to it.

However, the vastness of space, coupled with the impossibility of travelling or communicating faster than the speed of light, places practical limits on how far we can roam.

Only the closest stars to the sun could possibly be visited in a human lifetime, even by a space probe. In addition, only stars similar in size and temperature to the sun are long-lived enough, and have stable enough atmospheres for multicellular life to have time to form.

For this reason, the most valuable stars to study are the 60 or so sun-like stars that are closer to us than approximately 30 light-years. The most promising planets orbiting these stars would have sizes and temperatures similar to Earth, so solid ground and liquid water can exist.

An Earth-like planet, a needle in the haystack

Observing an Earth-like exoplanet separately from the star it is orbiting around is a major challenge. Even in the best possible scenario, the star is a million times brighter than the planet; if the two objects are blurred together, there is no hope of detecting the planet.

Optics theory says that the best resolution one can get in telescope images depends on the size of the telescope and the wavelength of the observed light.

Planets with liquid water give off the most light at wavelengths around 10 microns (the width of a thin human hair and 20 times the typical wavelength of visible light). At this wavelength, a telescope needs to collect light over a distance of at least 20 meters to have enough resolution to separate Earth from the sun at a distance of 30 light-years.

Additionally, the telescope must be in space, because looking through Earth’s atmosphere would blur the image too much. However, the James Webb Space Telescope (JWST)—is only 6.5 meters in diameter, and that telescope was extremely difficult to launch.

Because deploying a 20-meter space telescope seems out-of-reach with current technology, scientists have explored several alternative approaches.

One involves launching multiple, smaller telescopes that maintain extremely accurate distances between them, so that the whole set acts as one telescope with a large diameter. But, maintaining the required spacecraft position accuracy (which must be precisely calibrated to the size of a typical molecule) is also currently infeasible.

Other proposals use shorter wavelength light, so that a smaller telescope can be used. However, in visible light, a sun-like star is more than 10 billion times brighter than Earth. It is beyond current capability to block out enough starlight to be able to see the planet in this case, even if in principle the image has a high enough resolution.

One idea for blocking the starlight involves flying a spacecraft called a ‘starshade’ that is tens of meters across, at a distance of tens of thousands of miles in front of the space telescope, so that it exactly blocks the light from the star while the light from a companion planet is not blocked.

However, this plan requires that two spacecraft be launched (a telescope and a starshade). Furthermore, pointing the telescope at different stars would entail moving the starshade thousands of miles, using up prohibitively large quantities of fuel.

A rectangular perspective

A new study ¹ proposes a more feasible alternative.

They 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 JWST, operating at roughly the same infrared (10 micron) wavelength as JWST, with a mirror that is a one by 20-meter rectangle instead of a circle 6.5 meters in diameter.

With a mirror of this shape and size, they can separate a star from an exoplanet in the direction that the telescope mirror is 20 meters long. To find exoplanets at any position around a star, the mirror can be rotated so its long axis will sometimes align with the star and planet.

The team shows 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. While the design will need further engineering and optimization before its capabilities are assured, there are no obvious requirements that need intense technological development, as is the case for other leading ideas.

If there is about one Earth-like planet orbiting the average sun-like star, then we would find around 30 promising planets. Follow-up study of these planets could identify those with atmospheres that suggest the presence of life, for example, oxygen, which was formed through photosynthesis.

For the most promising candidate, we could dispatch a probe that would eventually beam back images of the planet’s surface, says the astrophysicists. The rectangular telescope could provide a straightforward path towards identifying our sister planet, Earth 2.0.