Watery, rocky planets may be common in the Milky Way

Apr 13, 2010
An artist's impression of a massive asteroid belt in orbit around a star. Credit: NASA-JPL / Caltech / T. Pyle (SSC)

(PhysOrg.com) -- An international team of astronomers have discovered compelling evidence that rocky planets are commonplace in our Galaxy. Leicester University scientist and lead researcher Dr Jay Farihi surveyed white dwarfs, the compact remnants of stars that were once like our Sun, and found that many show signs of contamination by heavier elements and possibly even water, improving the prospects for extraterrestrial life. On Tuesday 13th April Dr Farihi presented his results at the RAS National Astronomy Meeting (NAM 2010) in Glasgow.

White dwarf stars are the endpoint of for the vast majority (>90%) of all stars in the , including our Sun. Because they should have essentially pure hydrogen or pure helium atmospheres, if heavier elements (in astronomy described as ‘metals’, examples including calcium, magnesium and iron) are found then these must be external pollutants. For decades, it was believed that the , the tenuous gas between the stars, was the source of metals in these polluted .

Farihi and his team used data from the Sloan Digital Sky Survey (SDSS), a project that aims to survey the sky in infrared light, imaging more than 100 million objects and following up 1 million of these by obtaining their spectrum (dispersing the light by colour).

By examining the positions, motions and spectra of the white dwarfs identified in the SDSS, Farihi and his team show that this is no longer a viable theory. Instead, rocky planetary debris is almost certainly the culprit in most or all cases.

The new work indicates that at least 3% and perhaps as much as 20% of all white dwarfs are contaminated in this way, with the debris most likely in the form of rocky minor planets with a total mass of about that of a 140 km diameter asteroid.

This implies that a similar proportion of stars like our Sun, as well as stars that are a little more massive like Vega and Fomalhaut, build terrestrial planetary systems. Astronomers are thus playing the role of celestial archaeologists by studying the 'ruins' of and or their building blocks.

The scientists also measured the composition of the contaminating planetary debris through its chemical signature which stands out in the otherwise pure atmosphere of the white dwarfs.

Excitingly, it appears a significant fraction of these stars are polluted with material that contained water, with important implications for the frequency of habitable planets around other stars. If internal water is present in a substantial fraction of asteroids around other stars, like those that contaminated the white dwarfs, it is conceivable that at least simple life may be common throughout the Galaxy.

Dr Farihi comments: “In our own Solar System with at least one watery, habitable planet, the asteroid belt - the leftover building blocks of the terrestrial planets - is several percent water by mass. From our study of white dwarfs, it appears there are basic similarities found among asteroid-like objects around other ; hence it is likely a fraction of these white dwarfs once harbored watery planets, and possibly life.”

Explore further: Young binary star system may form planets with weird and wild orbits

More information: SDSS home page www.sdss.org

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2.2 / 5 (6) Apr 13, 2010
The surface of a planet (or star) may be unlike the composition of its interior.

Chemical gradients in planet Earth mimic those across the Solar System:

a.) Compounds made of lightweight elements (H, C, N, O) are abundant in the outer part.

b.) Iron (Fe) and Nickel (Ni) are abundant near the center.

Water covers about 67% of the Earth's surface, and Hydrogen (H) covers 91% of the Sun's surface. But iron (Fe) is the most abundant in the interior of both.

With kind regards,
Oliver K. Manuel

4 / 5 (1) Apr 13, 2010
There is also a strong possibility of rocky and icy debris in interstellar space. Many comets must have been ejected from solar systems.
not rated yet Apr 14, 2010
The study doesn't say what orbits the planets would be in, so theoretically the planets could all be outside the goldilocks zone where water is liquid.

There are many compelling reasons for why Earth-like planets is common, but you can't know for sure they are there, until you observe them directly.
not rated yet Apr 14, 2010
Agreeing with Alien, here. Because the stars are dead, it's impossible to determine prehistoric orbits of planets that are no longer there. The important thing here is that this is further evidence that water is not unique to our planet, and (by a HUGE leap in imagination) that life may not be unique to here.
not rated yet Apr 14, 2010
Oliver what you said kinda goes with out saying -- everyone especially an astronomer knows that the atmosphere is going to be different than the surface and interior of the body looked at -- but we don't have a tool to look at those and likely never will unless we can get a satelite in orbit around it. And the interior of the sun is NOT mostly iron if it was it would be going nova by now. But it is more abundant in the interior.
not rated yet Apr 14, 2010
the delta here is that we have a firm measure of "rockiness" for other star systems, specifically very old stars, but no reason to imagine that these are any different to younger stars - they all form out of the same stuff.
its not so safe to assume a degree of general rockiness and wetness based on the solar system - a sample of one.
the pure speed with which early life appeared on earth - within a few million years from the instant it was viable - suggests that other systems will have it.
the same argument holds whether it was independently forged in situ, forged on the massive surface area of space dust, or carried "in vivo" from another place.
not rated yet Apr 14, 2010
People here, and the author, seem to be missing part of the picture. White dwarfs are the results of supernova explosions which destroy most planets, and especially inner planets. The metals (astronomer talk for elements heavier than Helium) that are detected now are from way outside the habitable zone. Some of it may have encountered drag from the gases blasted off when the supernova occurred. Other parts may have been long period comets, out in the equivalent of the Oort cloud.

In any case, the metals detected are a very small fraction of whatever was in orbit around the star before it went bang. Also, the water signature in particular indicates that the debris that is being detected has not yet reached the surface of the white dwarf. Once any piece of debris has been vaporized, light pressure will keep it near the surface for millennia. It should be possible to use Doppler data from the spectrum to determine the height of the water/OH- ions above the star.
not rated yet Apr 14, 2010
All I can say is that not having access to faster than light travel is painful. There is so much out there to see and no way to get to it. We need to get working on our warp drives so we can just go see for ourselves. (yes, I'm aware of the huge technological hurdles).