A new survey of 369 sun-like stars has confirmed what earlier studies have shown, that the Sun is remarkable inactive compared with similar stars.
A comprehensive catalogue containing the rotation periods of thousands of stars has been available only for the last few years. It is based on measurement data from NASA’s Kepler Space Telescope, which recorded the brightness fluctuations of approximately 150000 main sequence stars (i.e. those that are in the middle of their lifetimes) from 2009 to 2013. The researchers scoured this huge sample and selected those stars that rotate once around their own axis within 20 to 30 days. The Sun needs about 24.5 days for this. The researchers were able to further narrow down this sample by using data from the European Gaia Space Telescope. In the end, 369 stars remained, which also resemble the Sun in other fundamental properties.
The exact analysis of the brightness variations of these stars from 2009 to 2013 reveals a clear picture. While between active and inactive phases solar irradiance fluctuated on average by just 0.07 percent, the other stars showed much larger variation. Their fluctuations were typically about five times as strong. “We were very surprised that most of the Sun-like stars are so much more active than the Sun,” says Dr. Alexander Shapiro of MPS.
It is possible that this inactivity might be because the Sun just happens to be going through a quiet phase, but that is becoming increasingly less likely as the surveys find more and more sun-like stars, and none as inactive as the Sun.
If the Sun is this unusual, we must ask if this inactivity is a fundamental requirement for life to form. Active stars provide a more inhospitable environment. If inactive stars like the Sun are very rare, however, that suggests that life itself in the universe could be very rare as well.