Search for life in space: Can surface gravity measurements be the answer?
Surface gravity is the intensity of the force that pulls everything on the surface of a star or celestial body towards the centre.
The team led by Thomas Kallinger of the University of Vienna has used data from the Kepler space telescope – which is searching for other worlds like the Earth – to show that variations in the brightness of distant stars can give more accurate measurements of surface gravity.
The new measurement method, which makes it possible to study even the faintest of stars, will be used in future ESA and NASA missions in order to better determine the size and habitability of planets orbiting distant stars. The study is reported in the journal ‘Science Advances’.
Thousands of planets have discovered around distant stars but most of them were found using the so-called transit method in which the size of the planet is always only measured relative to that of the star.
In order to determine whether a newly discovered planet is more like Earth or more similar to a gas giant planet like Jupiter, it is necessary to measure the gravitational acceleration of the star.
The previous method used to measure the gravitational acceleration required the detailed analysis of stellar oscillations which was only applicable to few bright stars applicable where the signal clearly stood out from the background noise.
The University of Vienna’s team have developed a method which can measure the surface gravity of a star with high accuracy, to within a few percentage points. The ability to measure minimum brightness changes is a key factor. Although the lights of the stars in the night sky appear constant, they are subject to measurable changes. Acoustic oscillations inside the star changes the amount the emitted light, as well as convective motions – the rising hot gas bubbles and their drop after they have cooled on the surface which can also cause changes in brightness. Both phenomena are directly influenced by the surface gravity of the star. They can be used to determine the measurement of acceleration due to gravity, which arises from the mass and the radius of the star.
The new measurement method, which is called by the researchers as the ‘Autocorrelation Function Time Scale Technique’, now makes it possible, even for faint stars with strong noisy measurements to determine an accurate value for the acceleration due to gravity. The method is based on the time scale of the variations in brightness of the gravitational acceleration is inversely proportional, allowing an accuracy in the calculation of about four percent. The previous method, to determine the gravity acceleration at faint stars, only yielded an accuracy of about 25 percent.
The new technique will be able to tell scientists how big and bright a star is and if a planet around the star is the right size and temperature to have water oceans which could indicate life.
Precise stellar surface gravities from the time scales of convectively driven brightness variations: Thomas Kallinger, Saskia Hekker, Rafael A. Garcia, Daniel Huber, Jaymie M. Matthews in Science Advanes. DOI: 10.1126/sciadv.1500654
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