Just a year ago, the scientific community was excited by the discovery of an exoplanet called Ross 128 b, which is only 11 light-years from Earth. Now, the new work of a team led by Diogo Souto of the Brazilian National Observatory and including Johanna Teske of Carnegie Science DTM, has for the first time determined the detailed chemical abundances of the planet’s host star, Ross 128, the Carnegie Institute said in a statement.
Understanding what elements are present in a star and in what abundance, can help researchers estimate the composition of the exoplanets that orbit it, which can help predict how similar the planets are to Earth. “Until recently it was difficult to obtain detailed chemical abundances for this type of star,” said lead author Souto, who developed a technique for making these measurements last year.
The research institute notes that, like the host star of exoplanet Ross 128, about 70 percent of all stars in the Milky Way are red dwarfs, which are much colder and smaller than our Sun. Based on the results of major planetary search research, astronomers estimate that many of these red dwarf stars harbor at least one exoplanet.
Several planetary systems around red dwarfs have made headlines in recent years, including Proxima b, a planet that orbits the closest star to our own Sun, Proxima Centauri and the seven planets of TRAPPIST-1, which is not much larger than the Jupiter of our Solar System.
Using the APOGEE spectroscopic instrument from Sloan Digital Sky Survey, the team measured the near infrared light of the star to derive abundances of carbon, oxygen, magnesium, aluminum, potassium, calcium, titanium and iron, the statement said. “APOGEE’s ability to measure near infrared light, where Ross 128 is brighter, was key to this study,” said Teske, adding that it allowed them to address some fundamental questions about the Ross Earth’s similarity 128 b.
When stars are young they are surrounded by a disk of rotating gas and dust from which the rocky planets form. The chemistry of the star can influence the content of the disk, as well as the mineralogy and the inner structure of the resulting planet. For example, the amount of magnesium, iron and silicon on a planet will control the mass ratio of its inner core and mantle layers.
The team determined that Ross 128 has iron levels similar to those of our Sun. Although they were not able to measure their silicon abundance, the iron to magnesium ratio in the star indicates that the core of their planet, Ross 128 b, should be bigger than Earth.
Since they knew the minimum mass of Ross 128b and the stellar abundances, the team could also estimate a range for the radius of the planet, which is not possible to measure directly due to the orientation of the planet’s orbit around the star.
Knowing the mass and radius of a planet is important to understand what it is made of, since these two measurements can be used to calculate its apparent density. In addition, by quantifying the planets in this way, astronomers have realized that planets with radii greater than 1.7 times those on Earth are probably surrounded by a gaseous envelope, such as Neptune, and those with smaller radii are more rocky, as is our own home planet. The estimated radius of Ross 128 b indicates that it must be rocky.
Finally, by measuring the temperature of Ross 128 and estimating the radius of the planet, the team was able to determine what part of the host star’s light should be reflected on the surface of Ross 128 b, revealing that in our second closest rocky neighbor there is probably a temperate climate.
“It’s exciting what we can learn about another planet by determining what the light of its host star tells us about the chemistry of the system,” said Souto. “Although Ross 128 b is not the Earth’s twin, and there is still much we do not know about its potential geological activity, we could reinforce the argument that it is a temperate planet that could have liquid water on its surface,” he concluded.