A New Exoplanet in the System

In 2014, data from NASA’s Kepler Space Telescope allowed astronomers to announce the detection of three planets orbiting Kepler-138. This was based on a     1    dip in starlight as the planet momentarily passed in front of their star, a transit.

Benneke and his colleague Diana Dragomir, from the University of New Mexico, came up with the idea of re-observing the planetary     2    with the Hubble and Spitzer space telescopes between 2014 and 2016 to catch more transits of Kepler-138d, the third planet in the system, in order to study its atmosphere.

While earlier NASA Kepler space telescope observations only showed transits of three small planets around Kepler-138, Piaulet and her team were     3    to find that the Hubble and Spitzer observations suggested the presence of a fourth planet in the system, Kepler-138e.

This newly found planet is small and farther from its star than the three others, taking 38 days to complete an orbit.The planet is in the     4    zone of its star, a temperate region where a planet receives just the right amount of heat from its cool star to be neither too hot nor too cold to allow the presence of liquid water. Observing the exoplanet’s transit would have allowed astronomers to     5    its size.

With Kepler-138e, the masses of the    6    known planets were measured again via the transit timing-variation method, which consists of     7    small variations in the precise moments of the planets’ transits in front of their star caused by the gravitational pull of other nearby planets.

The researchers had another surprise: they found that the two water worlds Kepler-138c and d are twin planets, with     8    the same size and mass, while they were previously thought to be greatly different. The closer-in planet,Kepler-138b, on the other hand, is     9     to be a small Mars-mass planet, one of the smallest exoplanets known to date.

“As our instruments and techniques become     10     enough to find and study planets that are farther from their stars, we might start finding a lot more water worlds like Kepler-138 c and d,” Benneke concluded.