A Planet Almost Exactly Earth's Size Has Been Found 72 Light-Years Away

We've just found an exoplanet almost exactly the same size as Earth orbiting a tiny star not very far away at all.

It's called K2-415b, and its similarities (and differences) to our own home world might shed some light on how Earth-like planets form and evolve in different ways, in systems very different from our own.

"Small planets around M dwarfs are a good laboratory to explore the atmospheric diversity of rocky planets and the conditions at which a habitable terrestrial planet can exist," writes an international team of astronomers led by Teruyuki Hirano of the Astrobiology Center in Japan.

"Being one of the lowest mass stars known to host an Earth-sized transiting planet, K2-415 will be an interesting target for further follow-up observations, including additional radial velocity monitoring and transit spectroscopy."

The Milky Way galaxy is a big place, with lots of interesting worlds in it, but so far it has proven evasive on one of the biggest questions humanity has ever asked: why are we here? And not just why, but how, and why this planet, and is there anywhere else out there where life could potentially happen?

Since Earth is the only place in the Universe where we know for a fact life has emerged, one of the tools that could help deliver answers is a population of exoplanets that are similar to Earth. Similar in size, composition, temperature, mass; perhaps even planetary system architecture.

The best population of exoplanets to start this research is small, Earth-sized worlds orbiting small stars, relatively nearby, in such a way that they transit, or pass between us and the star. That's because they're the best candidates for characterizing an atmosphere.

As the exoplanet passes in front of the star, a fraction of the star's light will pass through an atmosphere, with some wavelengths on the spectrum being absorbed or amplified by elements in the atmosphere.

Around smaller, dimmer, cooler stars like red dwarfs, the habitable temperature zone is much closer to the star than it is around a star like the Sun. This means that the orbital period is shorter, so many transits can be recorded and stacked to amplify the spectrum data. And, obviously, closer stars will appear brighter, which will make such observations easier.

Small exoplanets, however, are harder to find than large ones. Within 100 light-years of the Solar System, just 14 exoplanets smaller than 1.25 times the radius of Earth have been found orbiting red dwarf stars – including all 7 worlds in the TRAPPIST-1 system.

This is an instance where there's no such thing as too many data points, and Hirano and his colleagues seem to have found a doozy. The exoplanet K2-415b is 1.015 times the radius of Earth, orbiting one of the smallest red dwarf stars found hosting an Earth-sized world. The star, K2-415, is just 16 percent of the mass of the Sun.

The exoplanet was first spotted in data from the now-retired Kepler planet-hunting telescope in 2017, and it also appeared in data from Kepler's successor, TESS.

The researchers followed up, taking infrared observations to see if they could detect a faint 'wobble' in the star's motion, as it is ever-so-slightly tugged about on the spot by the gravity of the exoplanet.

This wealth of data revealed the presence of a world, as well as its characteristics. The amount of starlight blocked when the exoplanet transits can be used to calculate the planetary radius. The amount of wobble gives its mass.

Those two parameters can be combined to calculate the exoplanet's density. And, of course, the periodicity of the transits reveals the exoplanet's orbital period.

This is where K2-415b starts to seriously differ from Earth. Although the exoplanet is about Earth-sized, its mass is much higher, around three times that of Earth. This means that K2-415b is denser than Earth, too.

And it's much, much closer to its star: It has an orbital period of just four days. It's true that the habitable zone of a red dwarf star can be much closer than the Sun's habitable zone, with orbits measurable in days rather than months, but that's a little too close for comfort, even for a red dwarf.

However, only a little. K2-415b sits just inside the rim of K2-415's habitable zone. That could mean that it still has an atmosphere to probe. In the Solar System, Venus is just inside the habitable zone, and its atmosphere is a dense and intriguing horror show.

It's also possible that K2-415 is a multi-planet system; this raises the possibility of a currently undetected exoplanet in the star's habitable zone.

So we're unlikely to find signs of life on K2-415b. But the system represents an excellent target for exoplanet atmospheric characterization, and follow-up surveys looking for hidden, potentially life-harboring worlds.

The research has been accepted for publication in The Astronomical Journal, and is available on arXiv.

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