Nearby rocky exoplanet could be habitable

Chemical composition of host star can inform scientists about the nature of orbiting planets.
By Laurel Kornfeld | Nov 28, 2018
A rocky exoplanet 11 light years away may not be an Earth twin but likely has a temperate climate that could sustain liquid water on its surface.

Known as Ross 128 b, the planet was discovered last fall by researchers who used the Sloan Digital Sky Survey's (SDSS) APOGEE spectroscopic instrument. It orbits a red dwarf star, Ross 128, that is smaller, cooler, and dimmer than our Sun.

Scientists estimate 70 percent of stars in the galaxy are red dwarfs, most of which likely have at least one orbiting planet.

Now, a group of researchers led by Diogo Souto of Brazil's Observatorio Nacional and by Johanna Teske of the Carnegie Institution for Science measured the chemical composition the star using APOGEE in the near-infrared and found signs of carbon, oxygen, magnesium, aluminum, potassium, calcium, titanium, and iron.

Knowing which elements are present in a star helps researchers predict the type of exoplanets that might be orbiting that star and whether any of those planets could be habitable.

"Until recently, it was difficult to obtain detailed chemical abundances for this type of star," explained Souto, who last year pioneered a new technique to obtain this information.

"The ability of APOGEE to measure near-infrared light, where Ross 128 is brightest, was key for this study. It allowed us to address some fundamental questions about Ross 128 b's 'Earth-like-ness,'" Teske explained.

A star's chemical content influences the protoplanetary disk of gas and dust that rotates around it in its early days. Planets form within these disks, and a star's chemistry therefore influences its interior structure, mineraology, mass, and layering.

Ross 128 was found to have an iron level similar to that of our Sun. Based on its ratio of iron to magnesium, the researchers believe Ross 128 b has a larger core than the Earth has.

Once they had a good estimate of the planet's mass and chemical abundance, the scientists were able to come up with a range for its radius.

Knowing a planet's mass and radius enables scientists to calculate its bulk density.

Planets with 1.7 or more Earth radii are not considered habitable, as they are likely surrounded by a layer of gas, meaning they resemble our solar system's ice giants.

In contrast, planets with lower radii, as Ross 128 b appears to have, are expected to be more rocky than gaseous.

The researchers also measured the exoplanet's temperature and estimated how much of the star's light reflects off its surface, which indicated it has a temperate climate.

"It's exciting what we can learn about another planet by determining what the light from its host star tells us about the system's chemistry," Souto noted. "Although Ross 128 b is not Earth's twin, and there is still much we don't know about its potential geologic activity, we were able to strengthen the argument that it's a temperate planet that could potentially have liquid water on its surface."

A paper detailing these findings has been published in The Astrophysical Journal Letters.



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