Asking how much you would weigh on the surface of a star sounds pointless – no scales would save you from frying. However, knowing the surface gravity, which determines weight, on distant suns is an essential component of understanding planets in orbit around those stars, including their prospects for harboring life. A newly published technique will greatly expand the number of stars where we can do this.
"If you don't know the star, you don't know the planet," study coauthor and University of British Columbia Professor Jaymie Matthews said in a statement.
The one thing we know about the planets we have discovered around other stars is their orbital period. Combined with the gravitational pull of the parent star we can calculate their average distance, an essential piece of information to work out whether their temperature is suitable for life.
Moreover, for those planets discovered when they pass in front of their star, Matthews noted: "The size of an exoplanet is measured relative to the size of its parent star. If you find a planet around a star that you think is Sun-like but is actually a giant, you may have fooled yourself into thinking you've found a habitable Earth-sized world.” Surface gravity can be used to find the star's radius.
Several techniques already exist for estimating the surface gravity of stars, most of which involve finding the mass and radius. However, many stars we are now finding planets around are too distant or too variable for these methods to be reliable. Matthews and the University of Vienna's Dr. Thomas Kallinger have published a different route that could be more accurate, called the autocorrelation function timescale technique, in Science Advances.
“When you cook soup on a stovetop, the soup rises to carry heat from the bottom of the pot to the surface, where some heat is lost to the air. The liquid then sinks to pick up more heat and the cycle starts again,” said Kallinger. Stars experience a similar convection process.
“If you turn up the heat, convection in the soup can become so violent that it will make the pot vibrate,” Kallinger added. And again, something similar occurs with stars.
There is a logarithmic relationship between the surface gravity of a star and the period over which these vibrations occur. Matthews and Kallinger have demonstrated a technique for filtering variations in stellar light to measure this period for a group of test stars, matching their already known surface gravity with great precision. Such periods can also be measured for stars beyond the reach of current methods; the authors anticipate errors of just four percent.
“The European Gaia mission will greatly extend the results of the earlier Hipparcos mission, in measuring stellar parallaxes, and hence distances, to stars across the Milky Way Galaxy,” Matthews told IFLScience. These distances will enable us to establish stars' intrinsic brightness. Adding in the surface gravity will give us the stellar mass. With these factors combined, we will be well positioned to determine whether planets are Earth-like in both size and temperature.
There is a strong relationship between the surface gravity of a star and the period of its vibrations. Consequently, measuring the timing of vibration can allow us to determine the star's gravity, which in turn can reveal its mass and radius. Credit: Jaymie Matthews and Thomas Kallinger