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SYFY WIRE Bad Astronomy

Confirmed! A tiny nearby exoplanet with only 40% of Earth's mass

By Phil Plait

A planetary system just 35 light years from Earth hosts four and possibly five planets. This includes one (if it exists) that's squarely in its star's habitable zone, and another that's the lightest-weight planet ever found using the radial velocity method: It has only 40% of Earth's mass. That's pretty cool.

The host star is a red dwarf called L98-59, which is substantially smaller, cooler, and dimmer than the Sun, shining only 1% as bright as our own star. Three planets were already known to orbit it, discovered using the transit method: Watching the star dim by a fraction as a planet passes directly in front of it. This only works for systems that are almost exactly edge-on as seen from Earth.

Artwork comparing the sizes of Mars and Earth to the three planets orbiting the red dwarf L 98-59. We don’t know what the planets actually look like, but we do know their sizes and masses. Credit: NASA Goddard Space Flight Center

Those planets were announced in 2019. Since then, a team of astronomers reobserved the star using various telescopes to better characterize the planets. The transit method gives you the size of the planet (the bigger the planet the more the star dims, since more of its face is blocked), but we really need the mass to understand the planets better; with the mass you get the density (mass / volume) which tells you roughly what the planet is made of.

That takes the radial velocity method: As the planet orbits the star, its gravity tugs on the star. While the planet makes a big circle around the center of mass of the system, the star makes a smaller circle around it. That means that for half the orbit the star is headed toward us and half away from us, and we can measure that using the Doppler shift.

This is extremely difficult to do, since the star's velocity is very small, but it's possible by breaking its light up into a spectrum and carefully measuring the result. Further complicating things is that if there are several planets there are multiple shifts on top of each other.

However, the results are worth it. They were able to easily see the previously discovered planets (named L98-59b, c, and d, in order outward from the star), but they also found strong evidence of a fourth planet, called L98-59e, and hints of a fifth.


That last one still needs to be confirmed, but if it's real they find it has a mass of at least 2.5 times Earth's, and takes a mere 23 Earth days to orbit the star (the length of its year). If it's only slightly larger than Earth than it may be a rocky world. It orbits roughly 15 million kilometers from the star, only 1/10th the distance of the Earth from the Sun, but the star is so dim it receives about the same amount of warmth as Earth does, so its temperature may actually be close to ours. That's pretty interesting! Unfortunately it doesn't transit so its size is unknown.

Again, if it exists, this would make it a pretty good candidate for it being like Earth. On the other hand it might be like Venus if it has a thick atmosphere, so let's not get ahead of ourselves. But still.


The team also got much better measurements of the inner three planets, and the innermost one, L98-59b, they find to have a diameter of only 0.85 times Earth, and a mass only 0.4 times ours. That gives it a density of about 65% of Earth's. Given its size, that means it's rocky and may not have as big an iron core. It's about 3.3 million kilometers from the star, so it's hot, likely around 350°C (670°F). Ouch.

But this is still important! Measuring the mass of a planet using the radial velocity method is easier the more massive a planet is; a beefy one tugs harder on its star. One with just 0.4 Earth masses (or about half that of Venus) is extremely difficult to measure, and this is the lowest mass planet ever confirmed this way!

That's good, because a planet like Earth orbiting farther out from a star like the Sun imparts a lower velocity than one closer in, making them hard to find as well. The results for L98-59b show that it's possible to push these measurements quite a ways.

Only a half-dozen planetary systems have been found where we've been able to get the diameters and masses of at least two planets, so this is helpful. Also, finding multiple planets with such different characteristics around one star is advantageous for people who study comparative planetology. We can assume these planets all formed at the same time around this star, which removes one uncertainty when comparing them to each other.

Also, I just love these miniature systems orbiting red dwarfs, like TRAPPIST-1. They're so different than our own, with all rocky planets squeezed so close in to their host star. Yet they may be the most common kind of system in the Universe! If we want to find life Out There, despite our prejudices, dim bulb red dwarfs may be the best places to look.