Rick Robinson

Oct 19th 2020

The Hunt for Earthlike Exoplanets


Space exploration is not only about seeing the wonders of the universe. Perhaps most of all, it’s a search for life, and an answer to the question of whether we’re alone in the universe.

The search for life beyond Earth continues within our solar system; the search also extends far beyond the solar system, where we aim to discover earthlike exoplanets.

Water, Water Anywhere

Water is the holy grail in the search for extraterrestrial life, says Northrop Grumman’s Robert Lockwood, project manager for NASA’s TESS (Transiting Exoplanet Survey Satellite) mission. Liquid water is essential for terrestrial life; it’s so friendly to complex organic chemistry that it’s regarded as the most likely environment for life elsewhere in the universe.

Within the solar system, we know that Mars once had seas and rivers, and liquid water still occasionally flows on its surface. Meanwhile, Jupiter’s moon Europa has a smooth, icy surface, beneath which, scientists believe, lurks a hidden ocean deeper than any on Earth.

Oceans Beyond the Sun

The search for potential life beyond the solar system is in some ways simpler, but it’s incredibly demanding due to the enormous distances involved. Currently, TESS’s mission is examining candidate stars within about 300 light-years of Earth — close enough to allow for future follow-up examination of the exoplanets that TESS discovers.

What are TESS and other extrasolar survey observations — both space-based and ground-based — looking for? In a nutshell, said EarthSky, they are looking for “Goldilocks” conditions, just right. That means planets that are roughly earth-sized, big enough to hold an atmosphere, but not so big as to be mostly gas or liquid, like Jupiter or even Neptune.

Moreover, the planet must orbit in its parent star’s habitable zone, hot enough that any oceans don’t freeze solid, but not so hot that they boil away.

Seeing Past the Shadows

What makes the search so challenging, Lockwood noted, is that current instruments, even the most powerful, don’t allow astronomers to actually see extrasolar planets. Instead, astronomers must currently suss out planets by observing indirect effects, like the planet’s bulk blocking part of its parent star, slightly dimming the star’s light — the technique used by the TESS mission.

This will change next year, when NASA‘s James Webb Space Telescope is scheduled to launch into orbit. The JWST, as it is known, will be able to take spectroscopic images (which separate light into its individual wavelengths, or spectrum) of the light from the star as it interacts with the planet’s atmosphere. The wavelengths of light absorbed or emitted will allow investigators to search for telltale signs of water vapor in a planetary atmosphere.

Teegarden’s Star and Beyond

One current list of prime candidate exoplanets, from the University of Puerto Rico at Arecibo, identifies 55 leading candidates. One candidate seems to be substantially smaller than Earth, while 20 others are closer to earth-sized. The remaining 34 are classed as “super-Earths” and “mini-Neptunes.”

NASA provided a handy graphic showing the estimated sizes of five of the most earthlike exoplanets. And ScienceAlert reported on the recent discovery of two prime candidates orbiting a dim red dwarf star called Teegarden’s Star. They orbit their parent star every few days, much more frequently than Mercury orbits our Sun (once about every 80 days), but the star is so faint that both planets are in its habitable zone.

And Teegarden’s Star is a mere 12.2 light-years away. This is currently too far for crews to travel to with present-day technology, but spacecraft may be able to reach these worlds with more innovations in space travel. For example, if we learn to build space probes capable of approaching the speed of light, a mission to this pair of worlds will not take much longer than the interplanetary missions that NASA has already undertaken.

JWST observations of planets around Teegarden’s star will mark a giant step in the search for habitable planets beyond Earth.