Astronomers using NASA's Hubble Space Telescope have found the building blocks of solid planets that are capable of having substantial amounts of water. This rocky debris, currently orbiting a white dwarf star called GD 61, is considered a relic of a planetary system that survived the burnout of its parent star. The finding suggests that the star system — located about 150 light-years away and at the end of its life — had the potential to contain Earth-like exoplanets, the researchers say.
"These water-rich building blocks, and the terrestrial planets they assemble, may in fact be common. A system cannot create things as big as asteroids and avoid building planets, and GD 61 had the ingredients to deliver lots of water to their surfaces," according to Jay Farihi of the University of Cambridge, United Kingdom. Though it's hard to predict exactly what types of planets there might have been, Farihi emphasized that, "Our results demonstrate that there was definitely potential for habitable planets in this exoplanetary system. The system almost certainly had (and possibly still has) planets, and it had the ingredients to deliver lots of water to their surfaces."
The new research findings are reported today in the journal Science.
Observations made with Hubble's Cosmic Origins Spectrograph (COS) allowed the team, led by Farihi, to do a robust chemical analysis of the debris falling into GD 61. The discovery complements other leading astronomical observations that measure the size and density of planets, but not their actual composition, say researchers.
"The only feasible way to see what a distant planet is made of is to take it apart, and nature does this for us using the strong gravitational tidal forces of white dwarf stars," said Farihi. "This technique allows us to look at the chemistry that builds rocky planets, and is a completely independent method from other types of exoplanet observations."
The white dwarf GD 61 is a relic of a star that once burned hotter and brighter than our Sun. The star exhausted its fuel in just 1.5 billion years. (Our Sun will last roughly ten times as long.)
NASA's Far Ultraviolet Space Explorer (FUSE) first found an abundance of oxygen in the dwarf's atmosphere in 2008. Eventually astronomers realized that this was the telltale signature of material falling into the star and polluting its atmosphere. White dwarfs typically have pure hydrogen or pure helium atmospheres. The "polluted white dwarf" scenario was bolstered by NASA Spitzer Space Telescope observations in 2011, which showed that the star has a tightly orbiting disk containing debris that falls onto the star and contaminates the otherwise pristine atmosphere.
The only way to obtain a more precise measurement of the amount of oxygen in the debris around GD 61 requires observations in the ultraviolet, which can only be carried out above Earth's atmosphere. The team used COS aboard Hubble to obtain the required data. The COS observations were then analyzed by Detlev Koester of the University of Kiel, in Germany, using a computer model of the white dwarf atmosphere to derive the elemental abundances.
Combing their results with a previous study that used the W. M. Keck Observatory on the summit of Mauna Kea, Hawaii, the team also detected magnesium, silicon, and iron, which, together with oxygen, are the main components of rocks. By counting the number of these elements relative to oxygen the researchers were able to predict how much oxygen should be in the atmosphere of the white dwarf. They found significantly more oxygen than should have been carried by rocky minerals alone. "The oxygen excess can be carried by either water or carbon mono- or dioxide. In this star there is virtually no carbon, indicating there must have been substantial water," said Boris Gnsicke of the University of Warwick, in Coventry, United Kingdom. He added that the small amount of carbon seen in the white dwarf rules out comets as the source of water. Comets are rich in both water and carbon compounds.
In their Hubble survey the team observed nearly 100 white dwarfs. Analysis is still ongoing, but the team estimates that at least 20 percent of the dwarfs show ongoing accretion of planetary debris, and it could possibly be as high as 50 percent.
How do the asteroids fall into the stellar remnant? The best model at present is based on how Jupiter perturbs members of our main asteroid belt. The Kirkwood Gaps in the asteroid belt represent areas where asteroids lose energy to Jupiter and sometimes fall into the Sun.
Infrared observations using the Spitzer telescope show that Sun-like stars that are similar to the parent star of GD 61 have inner debris belts analogous to our main asteroid belt. And, interestingly, these systems appear to have a gap just outside their inner belts that may be caused by one or more planets, say the investigators. "It looks like a pattern of a planet next to an asteroid belt whose members get thrown into the star may be a common feature of solar systems," said Farihi.
Earth is essentially a "dry" planet, with only 0.02 percent of its mass as surface water. So oceans came long after it had formed, most likely when water-rich asteroids in the solar system crashed into our planet.
The new discovery shows that the same water "delivery system" could have occurred in this distant, dying star's solar system — as this latest evidence points to it containing a similar type of water-rich asteroid that would have first brought water to Earth.
Six billion years from now an alien astronomer measuring similar abundances in the atmosphere of our burned-out Sun may reach the same conclusion that terrestrial planets once circled our parent star. Though the progenitor star was different from our Sun, nevertheless, "it's a look into our future," said Gänsicke.
Space Telescope Science Institute, Baltimore, Md.
University of Cambridge
Institute of Astronomy
Cambridge CB3 0HA
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