Free Astronomy Magazine March-April 2023

32 MARCH-APRIL 2023 ASTRO PUBLISHING This is surprising because most of the planets just slightly bigger than Earth that have been studied in de- tail so far all seemed to be rocky worlds like ours. The closest com- parison, say researchers, would be some of the icy moons in the outer Solar System that are also largely composed of water surrounding a rocky core. “Imagine larger versions of Europa or Enceladus, the water-rich moons orbiting Jupiter and Saturn, but brought much closer to their star,” explained Piaulet. “Instead of an icy surface, they would harbour large water-vapour envelopes.” “The secure identification of an ob- ject with the density of the icy moons of the Solar System, but sig- nificantly larger and more massive, clearly demonstrates the great di- versity of exoplanets,” added team member Jose-Manuel Almenara of Grenoble Alpes University in France. “This is expected to be the outcome of a variety of formation and evo- lution processes.“ Researchers caution that the plan- ets may not have oceans like those on Earth directly at the planet’s sur- face. “The temperature in Kepler- 138 d’s atmosphere is likely above the boiling point of water, and we expect a thick dense atmosphere made of steam on this planet. Only under that steam atmosphere could there potentially be liquid water at high pressure, or even water in another phase that occurs at high pressures, called a supercritical fluid,” Piaulet said. The NASA/ESA/CSA James Webb Space Telescope will also facilitate valuable follow-up research. “Now that we have securely identified the ‘water-world’ Kepler-138 d, the James Webb Space Telescope is the key to unveiling the atmospheric composition of such an exotic ob- ject,” shared team member Daria Kubyshkina of the Austrian Acad- T his new Hubblecast episode explores what we can learn through the transits of exoplanets. It also tells us what we have learned from these transits with the help of Hubble and which discoveries we can expect with the NASA/ESA/CSA James Webb Space Telescope. [NASA, ESA, ESO/L. Calçada, M. Kornmesser] emy of Sciences. “It will give us crit- ical information enabling us to compare the composition of the icy moons of the solar system with that of their larger and heavier extraso- lar counterparts.” Recently, another team at the Uni- versity of Montreal found a planet, called TOI-1452 b, that could poten- tially be covered with a liquid-wa- ter ocean, but Webb will be needed to also confirm this. In 2014, data from the NASA Kepler Space Tele- scope allowed astronomers to an- nounce the detection of three planets orbiting Kepler-138, a red dwarf star in the constellation Lyra. This was based on a measurable dip in starlight as each planet momen- tarily passed in front of the star. Benneke and his colleague Diana Dragomir, from the University of New Mexico, came up with the idea of re-observing the planetary sys- tem with the Hubble and Spitzer space telescopes between 2014 and 2016 to catch more transits of Ke- pler-138 d, the third planet in the system, in order to study its atmos- phere. While the earlier Kepler space telescope observations only showed transits of three small plan- ets around Kepler-138, Piaulet and her team were surprised to find that the Hubble and Spitzer obser- vations required the presence of a fourth planet in the system, Kepler- 138 e. 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 habitable zone of its star, a tem- perate region where it 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. The nature of this additional, newly found planet, however, remains an open question because it does not seem to transit its host star. Observ- ing the exoplanet’s transit would have allowed astronomers to deter- mine its size. “As our instruments and techniques become sensitive enough to find and study planets that are farther from their stars, we might start finding a lot more of these water worlds,” Benneke con- cluded. !