Meet Kepler-138b, a Mars-sized planet with the smallest measured mass

You’ve heard of hot Jupiters, mini-Neptunes and super-Earths -- but how about an almost-Mars? Scientists using data from NASA’s Kepler spacecraft have pinned down the mass of an exoplanet that’s slightly lighter than the Red Planet -- making it the smallest such world to have its density measured.

The planet Kepler-138b, described in the journal Nature, could shed light on the kinds of small, rocky worlds that might be found around distant stars -- and whether these planetary systems are similar, or very different, from our own inner solar system.

“The detection of such a small exoplanet in a tight orbit could help to clarify how we fit into the big picture,” Gregory Laughlin of UC Santa Cruz, who was not involved in the study, wrote in a commentary.

The red dwarf star Kepler-138 and its planets lie some 200 light-years away in the direction of the constellation Lyra. Three planets, Kepler-138b, Kepler-138c and Kepler-138d, circle their star tightly, with respective orbits of roughly 10, 14 and 23 days. (Because they’re so close in, these planets, particular Kepler-138b, are searingly hot and unsuitable for life.)

Kepler-138b was first detected using data from NASA’s Kepler spacecraft, which from 2009 to 2013 stared at a patch of sky containing more than 150,000 stars, waiting for the repeated dips in starlight that signaled that a planet was regularly transiting in front of its star.

By measuring the amount of blocked starlight, scientists can determine the size of the planet -- but not its mass, and thus not its density. Without the mass and density, they can’t tell if a planet is rocky like Earth or gassy like Jupiter.

In some cases, astronomers can determine a planet’s mass by looking at the motions of its home star -- the planet’s tiny gravitational pull can cause the star to wobble, which squeezes and stretches the starlight in a predictable way.

The problem is, this only really works for significantly massive planets, which have enough weight to give their star a detectable wobble.

“Characterizing rocky planets is particularly difficult, because they are generally smaller and less massive than gaseous planets,” the authors wrote. “Therefore, few exoplanets near the size of Earth have had their masses measured.”

But even if Kepler-138b can’t push its massive star around, it can certainly give its fellow planets a shove (and vice versa). Each planet’s gravity tugs the neighboring planets slightly off-kilter from their proper orbit timing. (Imagine running around a track, with the runner in the next lane occasionally yanking on your jersey. It might add a few seconds to your lap time.)

Using those slight deviations in the each planet’s transits, the scientists were able to determine the mass and density of each world. Kepler-138b is 0.066 Earth masses -- its mass and density are somewhat lower than those of Mars (which weighs in at roughly a tenth of Earth’s mass).

“Kepler-138 b is by far the smallest exoplanet, both by radius and mass, to have a density measurement,” the authors wrote. “Thus it opens up a new regime to physical study. It is likely to become the prototype for a class of small close-in planets that could be common.”

Meanwhile, Kepler-138c and Kepler-138d are both slightly larger than Earth, but their densities are very different: Kepler-138c’s is similar to Earth’s, while Kepler-138d is less than half as dense.

It seems Kepler-138d could have a greater share of lighter materials, including water and hydrogen, the authors said. If that’s true, then it’s possible that Kepler-138d didn’t form so close into its red dwarf star, but farther out where water could survive without being boiled off.

“A planet made of rock and water would be more stable against mass loss, and would imply that the planet formed at a greater distance from the star and migrated,” the study authors wrote.

Those sorts of migrations reveal much about the evolution of a planetary system. (For example, in our own backyard, Jupiter is thought to have made major migrations early in the solar system’s history, leaving a trail of chaos in its wake.)

The findings, Laughlin said, could help scientists better understand the kinds of complex dynamics that have formed other planetary systems -- and how they compare to our own.

“It is imperative to improve our understanding of how our system’s architecture and evolution fit into the overall census -- the authors’ study is a step towards that goal,” he wrote.

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