Understanding which chemical elements are present in a star
in what abundances can help astronomers estimate the makeup of planets
that orbit them, which can help predict how similar the planets are to
the Earth. Dr. Diogo Souto of Brazil’s National Observatory and
co-authors have determined detailed chemical abundances of Ross 128,
a cool star just 11 light-years from Earth that’s known to host a
potentially habitable planet 1.35 times more massive than our own. The
team’s work appears in the Astrophysical Journal Letters (arXiv.org preprint).
Ross 128, also known as Proxima Virginis, Gliese 447 and HIP 57548, is a red dwarf star, meaning that it is much smaller, and much cooler, than our Sun.
“Until recently, it was difficult to obtain detailed chemical abundances for this kind of star,” Dr. Souto said.
Using SDSS’s Apache Point Galactic Evolution Experiment (APOGEE) spectrographs, he and his colleagues measured Ross 128’s near-infrared light to derive abundances of carbon, oxygen, magnesium, aluminum, potassium, calcium, titanium, and iron.
“When stars are young, they are surrounded by a disk of rotating gas and dust from which rocky planets accrete,” the astronomers said.
“The star’s chemistry can influence the contents of the disk, as well as the resulting planet’s mineralogy and interior structure. For example, the amount of magnesium, iron, and silicon in a planet will control the mass ratio of its internal core and mantle layers.”
The researchers determined that Ross 128 has iron levels similar to the Sun.
Although they were not able to measure its abundance of silicon, the ratio of iron to magnesium in the star indicates that the core of Ross 128b, an Earth-like exoplanet in the inner edge of the star’s habitable zone, should be larger than Earth’s.
Because they knew Ross 128b’s minimum mass, and stellar abundances, the scientists were also able to estimate a range for the planet’s radius.
“Knowing a planet’s mass and radius is important to understanding what it’s made of, because these two measurements can be used to calculate its bulk density,” they said.
“What’s more, when quantifying planets in this way, astronomers have realized that planets with radii greater than about 1.7 times Earth’s are likely surrounded by a gassy envelope, like Neptune, and those with smaller radii are likely to be more-rocky, as is our own home planet.”
“The estimated radius of Ross 128 b indicates that it should be rocky.”
Lastly, by measuring the temperature of Ross 128 and estimating the radius of the planet, the team was able to determine how much of the host star’s light should be reflecting off the surface of Ross 128b, revealing that the planet likely 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,” Dr. Souto said.
“Although Ross 128b 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.”
_____
Diogo Souto et al. 2018. Stellar and Planetary Characterization of the Ross 128 Exoplanetary System from APOGEE Spectra. ApJL 860, L15; doi: 10.3847/2041-8213/aac896
This
artist’s impression shows Ross 128b, with the red dwarf
star Ross 128
in the background. This system is located in the
constellation of Virgo,
about 11 light-years away. Ross 128b is
20 times closer to the star
than the Earth is to the Sun, and
takes just 9.9 days to circle it. The
planet’s equilibrium
temperature is estimated to be 70 degrees
Fahrenheit (21
degrees Celsius). Image credit: Sci-News.com.
Ross 128, also known as Proxima Virginis, Gliese 447 and HIP 57548, is a red dwarf star, meaning that it is much smaller, and much cooler, than our Sun.
“Until recently, it was difficult to obtain detailed chemical abundances for this kind of star,” Dr. Souto said.
Using SDSS’s Apache Point Galactic Evolution Experiment (APOGEE) spectrographs, he and his colleagues measured Ross 128’s near-infrared light to derive abundances of carbon, oxygen, magnesium, aluminum, potassium, calcium, titanium, and iron.
“When stars are young, they are surrounded by a disk of rotating gas and dust from which rocky planets accrete,” the astronomers said.
“The star’s chemistry can influence the contents of the disk, as well as the resulting planet’s mineralogy and interior structure. For example, the amount of magnesium, iron, and silicon in a planet will control the mass ratio of its internal core and mantle layers.”
The researchers determined that Ross 128 has iron levels similar to the Sun.
Although they were not able to measure its abundance of silicon, the ratio of iron to magnesium in the star indicates that the core of Ross 128b, an Earth-like exoplanet in the inner edge of the star’s habitable zone, should be larger than Earth’s.
Because they knew Ross 128b’s minimum mass, and stellar abundances, the scientists were also able to estimate a range for the planet’s radius.
“Knowing a planet’s mass and radius is important to understanding what it’s made of, because these two measurements can be used to calculate its bulk density,” they said.
“What’s more, when quantifying planets in this way, astronomers have realized that planets with radii greater than about 1.7 times Earth’s are likely surrounded by a gassy envelope, like Neptune, and those with smaller radii are likely to be more-rocky, as is our own home planet.”
“The estimated radius of Ross 128 b indicates that it should be rocky.”
Lastly, by measuring the temperature of Ross 128 and estimating the radius of the planet, the team was able to determine how much of the host star’s light should be reflecting off the surface of Ross 128b, revealing that the planet likely 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,” Dr. Souto said.
“Although Ross 128b 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.”
_____
Diogo Souto et al. 2018. Stellar and Planetary Characterization of the Ross 128 Exoplanetary System from APOGEE Spectra. ApJL 860, L15; doi: 10.3847/2041-8213/aac896
