For the First Time, a Mars-Sized Exoplanet Reveals All

The more I think about this, the more it rates a “Holy Haleakala!”: For the first time, astronomers have found not only the size but also the mass of an exoplanet smaller than Earth.

So: Holy Haleakala!

The planet orbits the star Kepler-138 (also called 2MASS J19213157+4317347, if you’re particular about such things). It’s a red dwarf star, meaning it’s less massive and cooler than the Sun. It’s about 200 light-years away.

There are three exoplanets orbiting Kepler-138, which are given the designations Kepler-138b, c, and d. All three were discovered in observations made by the Kepler observatory using the transit method; we see the planet’s orbits edge-on, so as they orbit their star they periodically block a little bit of its light. That dip is small, but measurable.

The length of time it takes for the planet to cross the star’s face tells us the size and shape of the orbit. The amount of light blocked tells us the size of the planet, since a bigger planet blocks more light. The inner planet has a diameter of about 0.52 times Earth’s, or about 6,600 kilometers—almost exactly the same size as Mars! The other two are both about 1.2 times the diameter of Earth, or about 15,000 km.

In general it’s much tougher to find the planets’ masses. But in this case we have a leg up: The planets orbit very close in to their star on very tight orbits, so the spacing between their orbits is small. When an inner planet passes an outer one, they tug on each other gravitationally, speeding up the inner one and slowing down the outer one as they approach, then they reverse as they pull apart.

This has a subtle but measurable affect on the timing of the transits. By carefully measuring the exact times of the transits of the planets, their gravitational affects on each other can be found, and from that their masses can be worked out.

The planets’ masses were found to be 0.066, 2, and 0.65 times Earth’s mass. Why is this important? Because if we know a planet’s size and mass, we can calculate the density, and that gives us a hint of what the planet’s made of!

The inner planet has a density of about 2.6 grams per cubic centimeter. That means it’s probably rocky, and doesn’t have a lot of heavy metals like iron or nickel like Earth does (our density is about 5.5 g/cc, and for comparison water has a density of 1 g/cc). So, while it’s roughly the same size as Mars, it has roughly 60 percent the mass. The second planet out has a density of 6.2 g/cc—similar to though a bit higher than Earth's—and the outer planet is 2.1 g/cc.

Mind you, all three orbit the star very closely. The inner planet is only 11 million kilometers away! Even though it’s a red dwarf star, that means the inner planet is still pretty hot. Even the outer of the three is closer to the star than Mercury is to the Sun. All of them would be pretty unpleasant places to visit (if I’ve done my math right, even the outer planet would likely be hotter than the boiling point of water).

So Earthlike they ain’t. In fact, all three seem to me to be pretty different from each other, which is interesting. In our solar system the inner three planets (Mercury, Venus, and Earth) have similar densities. In fact, Venus and Earth have similar compositions as well, though Venus is, um, somewhat different than us in detail. I’d guess our inner solar system and the Kepler-138’s formed differently somehow.

These observations are a fantastic achievement! Think about this: Just by staring at a star for a long time, and measuring its light carefully, we can figure that it has planets, how many it has (or a minimum, at least; it might have more we don’t see), what their orbits are, how big the planets are, what their masses and densities are, and even take a stab at what they’re made of.

And all this, from a distance of over 2 quadrillion kilometers! That’s amazing.

Science! I love this stuff.