A planet almost exactly the size of Earth has been found 72 light-years away

We’ve just found an exoplanet almost exactly the same size as Earth orbiting a small star not too far away.

It’s called K2-415b, and its similarities (and differences) to our own homeworld could shed some light on how Earth-like planets form and evolve in different ways, in systems vastly different from our own.

“The small planets around M dwarfs are a good laboratory for exploring the atmospheric diversity of rocky planets and the conditions under which a habitable terrestrial planet can exist.” writes an international team of astronomers led by Teruyuki Hirano of the Japan Center for Astrobiology.

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

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

The Milky Way galaxy is a big place, with many interesting worlds, but so far it has proven elusive on one of the most important questions humanity has ever asked: why are we here? And not just why, but how, and why this planet, and is there somewhere else where life could happen?

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

The best population of exoplanets to begin this investigation with are small, Earth-sized worlds that orbit small, relatively close stars in such a way that they transit or pass between us and the star. That’s because they are the best candidates to characterize an atmosphere.

As the exoplanet passes in front of the star, a fraction of the light from the star will pass through an atmosphere, and some wavelengths in the spectrum will be absorbed or amplified by elements in the atmosphere.

Around smaller, dimmer, and 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 the closest stars will appear brighter, making such observations easier.

However, small exoplanets are harder to find than large ones. Within 100 light-years of the Solar System, only 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 a case where there aren’t too many data points, and Hirano and his colleagues seem to have found a solution. The exoplanet K2-415b is 1,015 times the radius of Earth and orbits one of the smallest red dwarf stars found to host an Earth-sized world. The star, K2-415, is only 16 percent the mass of the Sun.

The exoplanet was first detected 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 slight ‘wobble’ in the star’s motion, as the exoplanet’s gravity nudges it very slightly in place.

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 density of the exoplanet. And, of course, the periodicity of the transits reveals the orbital period of the exoplanet.

This is where K2-415b begins to seriously differ from Earth. Although the exoplanet is about the size of Earth, its mass is much greater, around three times that of Earth. This means that K2-415b is also denser than Earth.

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 habitable zone of the Sun, with orbits measurable in days rather than months, but that’s too close to be comfortable, even for a red dwarf.

However, just a little. K2-415b lies just inside the edge of K2-415’s habitable zone. That could mean it still has atmosphere to plumb. In the Solar System, Venus is just inside the habitable zone, and its atmosphere is a dense and intriguing horror show.

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