The Kepler space telescope has discovered its 1000th exoplanet, including one that’s been described as the most Earth-like to date. Meanwhile astronomers have confirmed the theory that the speed at which a star spins shows its age.

Nasa announced this week that Kepler has discovered another eight exoplanets (a planet from outside our solar system), taking its all time total to 1,004. That’s more than all other space telescopes put together.

Of the new batch, three have qualified for NASA’s Kepler Hall of Fame for being “Earth-like”, bringing the total to five. To qualify for this category, an exoplanet must be less than twice the size of Earth and be in the habitable zone, meaning it’s within the range of distance from its sun where liquid water could theoretically exist.

Based on the observed diameter and mass, NASA infers that two of these new Earth-like planets are made of rock. One of them, Kepler 438b, appears to be a closer match for Earth when compared to the previous “best twin” Kepler 186f.

While 186f is very similar in size to Earth and 438b is 12 percent bigger, 438b is a much closer match for temperature, receiving around 40 percent more heat (from its sun) than Earth does.

There’s certainly room for even closer matches however. All eight of the “Earth-like” exoplanets discovered to date orbit suns considerably smaller than our own.

Meanwhile a study published in Nature shows astronomers can be confident about a method used to estimate the age of a star. That’s always been tricky as many of a stars properties, such as brightness, mass, size and temperature, tend to vary very little during its life (with the spectacular exception of its birth and death.)

For several decades, astronomers have pondered the theory that cool stars (those of a similar size to our Sun and smaller) spin at a decreasing speed as they age, a pattern likened to the movement of a spinning top.

The astronomers tested this theory using data from Kepler, which proved useful in two ways:

• the high quality of images from Kepler make it easier to see and track sunspots, which are the only practical way to measure how long a star takes to spin; and
• Kepler’s data covers stars known to be of a wider range of ages than previous studies.

The research shows that using a star’s spin speed is enough to accurately infer its age within 10 percent of the true figure.

[Image credit: NASA]