An artist’s impression of a compact white dwarf star surrounded by a disk of debris pulled in after it exhausted its nuclear fuel. Studying that material in nearby systems shows the composition of Earth and its sister planets is not unusual. Image: NASA/JPL-Caltech
An analysis of 18 planetary systems up to 456 light years away shows the elements that went into their assembly are similar to what is found in Earth’s neighbourhood, indicating their composition is “broadly similar to that of the Earth,” researchers say.
“It is difficult to examine these remote bodies directly,” Siyi Xu of the Gemini Observatory in Hawaii said at the Goldschmidt geochemistry conference in Boston. “Because of the huge distances involved, their nearby star tends to drown out any electromagnetic signal, such as light or radio waves. So we needed to look at other methods.”
The solution was to study white dwarf stars, which represent the end state for stars similar to the Sun, compact bodies that have blown off their outer atmosphere as they ran out of nuclear fuel. As such stars cool, they pull in material from planets, asteroids and comets that remain in orbit, “forming a dust disk, a little like the rings of Saturn,” Siyi Xu said.
“As this material approaches the star, it changes how we see the star,” the researcher said. “This change is measurable because it influences the star’s spectroscopic signal, and allows us to identify the type and even the quantity of material surrounding the white dwarf. These measurements can be extremely sensitive, allowing bodies as small as an asteroid to be detected.”
Using the Keck Observatory in Hawaii and the Hubble Space Telescope, the researchers were able to detect calcium, magnesium and silicon in most of the targeted stars. They also may have found water.
“It’s likely that there will be a lot of water in some of these worlds,” Siyi Xu said during a conference in Boston. “For example, we’ve previously identified one star system, 170 light years away in the constellation Boötes, which was rich in carbon, nitrogen and water, giving a composition similar to that of Halley’s Comet. In general though, their composition looks very similar to bulk Earth.”
That would imply the same building blocks that went into making Earth a habitable planet likely are commonplace and “we can probably expect to find Earth-like planets elsewhere in our galaxy.”
Sara Seager, Professor of Planetary Science at Massachusetts Institute of Technology, was not involved in the research, but she said the results were exciting.
“It’s astonishing to me that the best way to study exoplanet interiors is by planets ripped apart and absorbed by their white dwarf host star,” said said in an independent comment on the research. “It is great to see progress in this research area and to have solid evidence that planets with Earth-like compositions are common – fuelling our confidence that an Earth-like planet around a very nearby normal star is out there waiting to be found.”