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takyon [soylentnews.org] writes:

When astronomers were trying to determine the mass of a "super-Earth" exoplanet orbiting the red dwarf K2-18, they found a second one [universetoday.com]:

When K2-18b was first discovered in 2015 [caltech.edu], it was found to be orbiting within the star's habitable zone (aka. "Goldilocks Zone [universetoday.com]"). The team responsible for the discovery also determined that given its distance from its star, K2-18b's surface received similar amounts of radiation as Earth. However, the initial estimates of the planet's size left astronomers uncertain as to whether the planet was a Super-Earth or a mini-Neptune.

For this reason, Cloutier and his team sought to characterize the planet's mass, a necessary step towards determining it's atmospheric properties and bulk composition. To this end, they obtained radial velocity measurements of K2-18 using the HARPS spectrograph. These measurements allowed them to place mass constraints on previously-discovered exoplanet, but also revealed something extra.

[...] Essentially, their radial velocity measurements revealed that K2-18b has a mass of about 8.0 ± 1.9 Earth masses and a bulk density of 3.3 ± 1.2 g/cm³. This is consistent with a terrestrial (aka. rocky) planet with a significant gaseous [envelope] and a water mass fraction that is equal to or less than 50%. In other words, it is either a Super-Earth with a small gaseous atmosphere (like Earths) or "water world" with a thick layer of ice on top.

They also found evidence for a second "warm" Super Earth named K2-18c, which has a mass of 7.5 ± 1.3 Earth masses, an orbital period of 9 days, and a semi-major axis roughly 2.4 times smaller than K2-18b. After re-examining the original light curves obtained from K2-18, they concluded that K2-18c was not detected because it has an orbit that does not lie on the same plane.

K2-18b [wikipedia.org] was discovered by the Kepler Space Observatory in 2015.

Also at the University of Toronto Scarborough [utoronto.ca].

Characterization of the K2-18 multi-planetary system with HARPS: A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit [arxiv.org]

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