With thousands of discovered planets around stars beyond our own Sun, why haven’t we found the next Earth?
Over the past 30 years, astronomers have gone from zero known extra-solar planets to thousands.
The radial velocity (or stellar wobble) method for finding exoplanets relies on measuring the motion of the parent star, as caused by the gravitational influence of its orbiting planets. (ESO)
Periodic changes in a star’s motion or regular brightness dips give them away.
When a properly-aligned planet passes in front of a star relative to our line-of-sight, the overall brightness dips. When we see the same dip multiple times with a regular period, we can infer the existence of a potential planet. (WILLIAM BORUCKI, KEPLER MISSION PRINCIPAL INVESTIGATOR, NASA / 2010)
Thanks to these techniques, we’ve revealed the masses and radii of worlds nearby and thousands of light years away.
While Kepler has found some Earth-size planets, the vast majority of the ones discovered are larger than Earth, and have very short orbital periods; these are the easiest worlds to detect. (NASA AMES / W. STENZEL; PRINCETON UNIVERSITY / T. MORTON)
Over 200 are Earth-sized, with many residing in the so-called habitable zone around their stars.
The habitable zone is the range of distances from a star where liquid water might pool on the surface of an orbiting planet. If a planet is too close to its parent star, it will be too hot and water would have evaporated. If a planet is too far from a star it is too cold and water is frozen. Stars come in a wide variety of sizes, masses and temperatures. Stars that are smaller, cooler and lower mass than the Sun (M-dwarfs) have their habitable zone much closer to the star than the Sun (G-dwarf). Stars that are larger, hotter and more massive than the Sun (A-dwarfs) have their habitable zone much farther out from the star. (NASA/KEPLER MISSION/DANA BERRY)
Yet with everything we’ve found, there are
no potentially habitable Earth-like worlds around Sun-like stars.
Kepler-186f is one of the smallest, most Earth-sized planets found around a star, with a size just 17% larger than Earth. But it orbits a red dwarf star, meaning it’s not going to have Earth-like conditions. This is true as well for Kepler-438b, one of the other smallest, most Earth-sized planets (just 12% larger than Earth). (NASA AMES/JPL-CALTECH/T. PYLE)
There are three primary reasons for this.
Most of the planets we know of that are comparable to Earth in size have been found around cooler, smaller stars than the Sun. This makes sense with the limits of our instruments; these systems have larger planet-to-star size ratios than our Earth does with respect to the Sun. (NASA / AMES / JPL-CALTECH)
1.) Most of the small planets we know of are found around red dwarf stars.
We’ve classified many worlds outside of our Solar System as being potentially habitable, owing to their distance from their star, their radius and their temperatures. But many of the worlds we’ve found are too large to be rocky, and are found orbiting red dwarf stars, making them quite unlike how Earth is. (NASA AMES / N. BATALHA AND W. STENZEL)
Red dwarfs are the most common, and offer the largest planet-to-star size and mass ratios, making planets easier to detect.
The assumption that worlds just a little bit larger/more massive than Earth would be rocky may be erroneous, and may cause us to eliminate a large fraction of what were previously classified as potentially habitable worlds. (PHL @ UPR ARECIBO)
2.) Larger planets are easier to find; most are too large to be rocky without a giant gas envelope.
Illustration of the planet-finding space telescope, Kepler, from NASA. Kepler has found thousands of planets around stars in the Milky Way, teaching us about the mass, radius, and distribution of worlds beyond our Solar System. But its primary mission lasted only three years, meaning planets with long periods (on the order of years) could not be robustly detected. (NASA AMES/ W STENZEL)
3.) We didn’t observe them for long enough to detect planets with ~1 year orbital periods.
Today, we know of over 3,500 confirmed exoplanets, with more than 2,500 of those found in the Kepler data. These planets range in size from larger than Jupiter to smaller than Earth. Yet because of the limitations on the size of Kepler and the duration of the mission, there have been zero Earth-sized planets found around Sun-like stars that fall into Earth-like orbits. (NASA/AMES RESEARCH CENTER/JESSIE DOTSON AND WENDY STENZEL; MISSING EARTH-LIKE WORLDS BY E. SIEGEL)
If our own Solar System were at the distance of most stars, we wouldn’t have discovered Earth.
It will take longer-duration missions with excellent light-gathering power and sensitivity to reveal the first Earth-like world around a Sun-like star. There are plans in both NASA’s and ESA’s timelines for such missions. (NASA AND PARTNERS)
It’s the next generation of planet-finders, like James Webb and PLATO, that will hopefully deliver our first true Earth-like world.
Mostly Mute Monday tells the scientific story of an astronomical object or phenomenon in images, visuals, and no more than 200 words. Talk less; smile more. Ethan Siegel is the author of
Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.