Measuring the ages of stars is one of the very tough problems that contemporary astronomers are faced with. Up to now only the age of the Sun has been determined with high precision (it is 4.57 billion years, with a precision of 10 million years to each side). An international group of astronomers have determined ages, diameters, densities, masses and distances for 33 stars better than ever before. As an extra, all of these stars have earth-like planets, giving us a clear indication that such planets have formed in our Milky Way Galaxy long before the Earth and are still being formed out there.
The 33 stars have been carefully selected from the more than 1 200 stars with planets around them that have been observed with the highly successful Kepler satellite. The stars have to be sufficiently bright to give a good statistical basis for the results, and they have to show some of the same characteristics similar to the Sun to make them comparable.
Stars pulsate, vibrate and resonate just like sound waves in a musical instrument. The advanced technique of measuring these starry tunes is called asteroseismology - a method quite similar to the one used by geologists to sound out the composition of the interior of the Earth by means of earthquakes.
The NASA-launched Kepler satellite has constantly measured tiny variations in the light from some 145 000 stars over a period of a little more than four years. Analyzing these variations over time gives the periods of the many simultaneous pulsations in each star, and from that the scientists can derive the important basic properties of the individual stars.
The 33 stars selected for the study are not all similar to the Sun, but they behave in much the same way as the Sun does. They are what technically is called “solar-like oscillators”. Víctor Silva Aguirre, lead author of the study, explains: “The term solar-like oscillators means that the stars exhibit pulsations excited by the same mechanism as in the Sun: gas bubbles moving up and down. These bubbles produce sound waves that travel across the interior of stars, bouncing back and forth between the deep interior and the surface producing tiny variations in the stellar brightness.”
The new study gives us values for the selected stars with uprecedented precision. On the average stellar properties are better than the percentages below. If a star e.g. has a calculated age of 5 billion years, the 14% means that it’s true age lies between 4.3 and 5.7 billion years:
- 1.2% (radius),
- 1.7% (density),
- 3.3% (mass),
- 4.4% (distance),
- and 14% (age).
One of the biggest questions in astrophysics is: does life exists beyond earth? To even begin answering this, we need to know how many planets like ours exist out there, and when they formed. However determining ages of stars (and thus of their orbiting planets) is extremely difficult; precise ages are only available for a handful of host stars thanks to asteroseismic observations made with the Kepler satellite.
Our study provides the first sample of homogeneously determined ages for tens of exoplanet host stars with a high level of precision. The stars we studied harbour exoplanets of size comparable to earth (between 0.3 and 15 earth radii), and our results reveal a wide range of ages for these host stars, both younger (down to half the solar age) and older (up to 2.5 times the solar age) than the Sun. This is regardless of the size of the exoplanets in the system or multiplicity, showing that formation of exoplanets similar in size to earth has occurred throughout the history of our Galaxy (and is still taking place!). Actually some of these planets were of the same age as the Earth is now, at the time when the Earth itself formed. This in itself is a remarkable finding.
