A study of 18 exoplanet systems located up to 456 light years away revealed most planets to be composed of the same elements that make up the Earth in largely the same proportions.
The findings reveal Earth’s chemical composition to be “normal” compared with those of many exoplanets, suggesting Earth-like planets are common in the galaxy.
Because exoplanets are so far away and their electromagnetic signals are usually drowned out by light from their parent stars, a team of researchers studied the atmospheres of white dwarf stars, stellar remnants of Sun-like stars that have exhausted their hydrogen and are now very small and dense.
As they cool, white dwarfs pull in material from the planets, asteroids, and comets that orbited them when they were normal stars, which change the white dwarfs’ spectroscopic signals, enabling scientists to identify the materials that made up these objects. This material forms a dust disk surrounding each white dwarf.
The researchers used spectrographs on the Hubble Space Telescope (HST) and on the Keck telescope in Hawaii, the largest ground-based optical and infrared telescope, to observe the white dwarfs.
“In this study, we have focused on the sample of white dwarfs with dust disks,” explained Siyi Xu of the Gemini Observatory in Hawaii. “We have been able to measure calcium, magnesium, and silicon content in most of these stars, and a few more elements in some stars. We may also have found water in one of the systems, but we have not yet quantified it: it’s likely that there will be a lot of water in some of these worlds.”
Xu, who presented the study’s findings to the Geochemical Society‘s 2018 Goldschmidt Conference, held in Boston this month, noted the researchers found one star system rich in carbon, nitrogen, and water, with a composition similar to that of Halley’s Comet, in the constellation Bootes 170 light years away.
“This would mean the chemical elements, the building blocks of Earth, are common in other planetary systems. From what we can see, in terms of the presence and proportion of these elements, we’re normal, pretty normal. And that means we can probably expect to find Earth-like planets elsewhere in our galaxy.”
The research team hopes to obtain more precise measurements of white dwarfs and the objects orbiting them with data from the Gaia satellite, which has categorized 1.7 billion stars.