The age factor

ASTRONOMY

The age factor

FIERY ENCOUNTER This artist’s impression shows a close-up of the extrasolar planet XO-1b passing in front of a sun-like star 600 light years from the earth. Photo courtesy: NASA

The last decade has seen great interest in locating planets that orbit stars other than the sun and the last three years have been very fruitful in finding them. And if any such planets seem to have features that could harbour life, then knowing how old the planets are would help identify the ones that have been around long enough for finding signs of life on them to be more likely. At the 218th meeting (May 23) of the American Astronomical, Astronomer Soren Meibom of the Harvard-Smithsonian Centre for Astrophysics announced his team’s success in estimating accurately the age of individual stars, which, in turn, would be the age of a planetary system.

Discovering exo-planets

As planets are typically small, compared to the mother-star, and as they have no light of their own, planets of distant stars cannot be detected from the earth, because of the glare of their luminous parents. The first instance of detection, in 1995, of a giant planet orbiting a star 51 light years away in the constellation Pegasus, was indirect – using the slight movement of the mother star to balance that of the planet as it goes round, to deduce the presence of the planet. The movement of the parent star affects the frequency of the light emitted by the star, like the speed of an approaching railway engine makes its whistle sound shriller, and then drop in pitch as the engine passes and starts moving away. The back and forth movement of the star causes a similar change of frequency of light, which can be detected on the earth. The mass and distance of the planet can then be worked out from the timing and extent of the change.

This method, naturally, works best with massive planets, moving quite fast in close orbits and the first exo-planets detected were all of this type, clearly too large and too hot, not in any way ‘earth-like’. But a new method was soon devised - to observe the slight drop in the intensity of light from the star at the time the planet passes between the star and the earth. The Kepler project is an orbiting observation platform launched in 2009 specifically to find exo-planets using this method, accurately and sensitively, free from distortion by the earth’s atmosphere. In February 2011, the Kepler team announced that between May and September 2009, they had found 1,235 planet candidates, circling 997 host stars, more than twice the number known till then.

This tally included 68 planetary candidates of earth-like size and 54 planetary candidates in the habitable zone (neither too hot nor too cold) of their star. A feature of the Kepler data is that the orbits of planets detected around a star need to be in the same plane, so that they all block starlight going to in the direction of the earth. This limits the number of planets that can be detected, but the Kepler team estimates that 19 per cent of all stars have multiple planets and six per cent of stars host earth-size planets.

Now, some of these earth-sized candidates would also be in the habitable zone and then it becomes a question of great interest to see if they contain signs of life. But for this quest, it is useful first to know how old the planetary system is, as the probability is greater in older systems.

Age of planet systems

In the case of stars in a cluster of stars, which could be considered to have evolved together, it is found that two parameters – the brightness and the spectral colour of the stars, show a relationship. The parameters change as the stars age, but they change uniformly for all the stars in the cluster and the relation corresponds to the age of the cluster. Thanks to the known age of some clusters, the age of any cluster can hence be estimated. But this method, which works for clusters, is not reliable for individual stars – for which the Harvard-Smithsonian Centre team report success of a new method.

The origin of stars lies in the collapse of molecules of gas, spread over thousands of light years, by the effect of mutual gravity. As the gas crashes in and gets more dense, it gets hot and energetic and with the tremendous pressures and temperatures, there is nuclear fusion, or the formation of new elements from simpler ones. Fusion also releases huge energy and the star gets hotter still and begins to expand.

The expansion, in turn, causes cooling and compression starts again, for another cycle of element formation, and so on, till finally the star settles into a steady burning of nuclear fires for millions of years.

In the process of collecting together the sparse gas over vast distances, any slight net rotational movement of the mass of gas would get concentrated and enhanced in the compressed star, in the same way an acrobat spinning with her arms outstretched can suddenly spin faster just by drawing her arms in. Stars are thus invariably in a state of a fairly rapid spin, just like the earth and planets also have a period of rotation. But unlike planets which have little influence, except gravity, on other bodies in space, stars are fiercely hot and transfer energy to their surroundings. The result is that stars gradually lose energy and begin to spin slower and slower.

Gyrochronology

The Harvard-Smithsonian Centre team measured the spin rates of stars in clusters of known ages. Determining the rate of spin of stars was done with the help of the Kepler orbiting observatory, in the same way as detecting exo-planets. To start with, the team looked for changes in the brightness caused by dark spots on the surface of a star. 

Every time the dark spot appeared in the part of the star facing the telescope, it caused a slight dip in the total light emitted from the star. Once the spot went out of view, the brightness went back to the original value. By watching for drop and restoration of brightness, in this manner, the Kepler data is able to exactly provide the period of rotation of a star.

Over four years, the Harvard-Smithsonian team isolated and studied about 7,000 stars, first identifying the stars as part of a cluster and then measuring their speed of rotation. The data collected showed a strong relationship between the speed and the age of the stars - which can then be used to estimate the age of a new star, with the help of its speed of rotation. The team is in the process of refining the method, by testing the relationship in older star clusters, where the rate of spin is slower and there are fewer dark spots to use as markers.

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