Astronomers identify signature of Earth-eating stars

What if we could determine if a given star is likely to host a planetary system like our own by breaking down its light into a single high-resolution spectrum and analyzing it? 

A spectrum taken of the Sun is shown above. 

The dark bands result from specific chemical elements in the star’s outer layer, like hydrogen or iron, absorbing specific frequencies of light. 

By carefully measuring the width of each dark band, astronomers can determine just how much hydrogen, iron, calcium and other elements are present in a distant star. 

The new model suggests that a G-class star with levels of refractory elements like aluminum, silicon and iron significantly higher than those in the Sun may not have any Earth-like planets because it has swallowed them. 

Credit: N.A.Sharp, NOAO /NSO /Kitt Peak FTS /AURA /NSF

Some Sun-like stars are 'Earth-eaters.' During their development they ingest large amounts of the rocky material from which 'terrestrial' planets like Earth, Mars and Venus are made.

Trey Mack, a graduate student in astronomy at Vanderbilt University, has developed a model that estimates the effect that such a diet has on a star's chemical composition and has used it to analyze a pair of twin stars that both have their own planets.

The results of the study were published online May 7 in the Astrophysical Journal.

"Trey has shown that we can actually model the chemical signature of a star in detail, element by element, and determine how that signature is changed by the ingestion of Earth-like planets," said Vanderbilt Professor of Astronomy Keivan Stassun, who supervised the study.

"We can actually see the signature predicted by our model, in detail, element by element."

This ability will add substantially to astronomers' understanding of the process of planet formation as well as assist in the ongoing search for Earth-like exoplanets, according to the astronomers.

First, some background: Stars consist of more than 98 percent hydrogen and helium. All the other elements make up less than 2 percent of their mass.

Astronomers have arbitrarily defined all the elements heavier than hydrogen and helium as metals and have coined the term "metallicity" to refer to the ratio of the relative abundance of iron to hydrogen in a star's chemical makeup.


Since the mid-1990's, when astronomers developed the capability to detect extrasolar planets in large numbers, there have been several studies that attempt to link star metallicity with planet formation.

In one such study, researchers at Los Alamos National Laboratory argued that stars with high metallicity are more likely to develop planetary systems than those with low metallicity.

Another study concluded that hot Jupiter-sized planets are found predominantly circling stars with high metallicity while smaller planets are found circling stars with a wide range of metal content.

Building on the work of coauthor Simon Schuler of the University of Tampa, Mack took this type of analysis a step further by looking at the abundance of 15 specific elements relative to that of the Sun.

He was particularly interested in elements like aluminum, silicon, calcium and iron that have melting points higher than 1,200 degrees Fahrenheit (600 degrees Celsius) because these are the refractory materials that serve as building blocks for Earth-like planets.

More information: Paper: iopscience.iop.org/0004-637X/787/2/98/ On Arxiv: arxiv.org/abs/1404.1967

Consciousness Signature: Awareness Warnings during surgery

Anaesthesia wearing off mid-operation is not just the stuff of nightmares – it occurs in around 0.2 per cent of surgeries globally. Keeping tabs on the brain during surgery could stop this.

Indirect signs of consciousness such as changes in heart rate, blood pressure and muscle tone can be tracked during surgery but more reliable indicators are not currently available.

"It's slightly frightening that millions are given anaesthesia every year and the anaesthetist has no way of knowing with certainty that the patient is unconscious," says Tony Absalom at the University of Groningen in the Netherlands.

To look for a more concrete signature that could be monitored during surgery, Emery Brown at the Massachusetts Institute of Technology and his team attached an EEG cap with 64 electrodes to the heads of 10 adults.

They used this to measure changes in brain activity across multiple brain regions as unconsciousness was induced using a general anaesthetic.

As the volunteers lost and regained consciousness they were asked to press a button whenever they heard a click or a spoken word, allowing brain activity to be matched to different stages of wakefulness.

By constructing a montage of the activity from different parts of the brain, the team identified recognisable patterns that corresponded to different levels of consciousness, allowing them to tell when someone was waking up.

Mysterious sleep
EEG monitoring during surgery is carried out in about 2 per cent of hospitals in the UK, but only three or four electrodes are used, in a strip across the forehead.

This looks at just one brain region, however, and so can only give you a probability of unconsciousness, not a conclusive answer, says Absalom.

"You don't want something that says a patient is probably asleep. You want to know 'Are they or aren't they?'"

Using more electrodes, as Brown has done, should shore this up. Although applying more electrodes means the patient's head must be shaved and a conductive gel applied.

"The mechanisms underlying anaesthesia remain a mystery," says Ram Adapa at the University of Cambridge. But these results support the idea that anaesthesia affects the communication and synchronisation between brain regions, he says. "It is yet another piece of the jigsaw puzzle."

However, Adapa notes that it may be a while before the technique can be used in a clinical setting. EEG measurements are very sensitive to mechanical and electrical interference, he says, "and the operating room environment is unfortunately burdened by both."

Journal reference: 
Electroencephalogram signatures of loss and recovery of consciousness from propofol PNAS, 10.1073/pnas.1221180110

Saturn's moons Mimas and Tethys bear 'Pac-Man' signatures

Saturn's moons Mimas (left) and Tethys are now both known to exhibit the curious temperature patterns.

Astronomers have seen that the temperature of Saturn's moon Tethys has hotter regions uncannily like the 1980s arcade game character Pac-Man.

A similar feature was spotted in 2010 on Mimas, another Saturnian moon.

A report in Icarus suggests the effect is due to high-energy electrons bombarding the sides of the moons that face their direction of orbital travel.

That compacts the surfaces to a hard, icy texture that does not heat or cool as rapidly as the unaffected surface.

Thermal images of both moons were obtained by the Cassini-Huygens mission, launched in 1997 to study the Saturn system in detail.

The temperatures seen by the spacecraft are distinctly chilly - the warmest parts of Tethys were at - 183C, but inside the "mouth" of the Pac-Man shape it was 15C cooler still.

At the time of the finding of the first Pac-Man shape on Mimas, scientists were unsure what might be the cause, theorising that differing surface textures probably played a role.

The existence of another such shape nearby has cemented the idea that fast-moving electrons are responsible.

"Finding a second Pac-Man in the Saturn system tells us that the processes creating these 'Pac-Men' are more widespread than previously thought," said Carly Howett, of the Southwest Research Institute in Texas and lead author of the study.

"The Saturn system - and even the Jupiter system - could turn out to be a veritable arcade of these characters," she said.