Free Astronomy Magazine July-August 2021

49 JULY-AUGUST 2021 ASTRO PUBLISHING N ASA’s Hubble Space Telescope is giving astronomers a rare look at a Jupiter-sized, still- forming planet that is feeding off material surrounding a young star. “We just don’t know very much about how giant planets grow,” said Brendan Bowler of the University of Texas at Austin. “This planetary sys- tem gives us the first opportunity to witness material falling onto a planet. Our results open up a new area for this research.” Though over 4,000 exoplanets have been cataloged so far, only about 15 have been directly imaged to date by telescopes. And the planets are so far away and small, they are simply dots in the best photos. The team’s fresh technique for using Hubble to directly image this planet paves a new route for further exoplanet re- search, especially during a planet’s formative years. This huge exoplanet, designated PDS 70b, orbits the orange dwarf star PDS 70, which is already known to have two actively forming planets inside a huge disk of dust and gas encircling the star. The system is lo- cated 370 light-years from Earth in the constellation Centaurus. “This system is so exciting because we can witness the formation of a planet,” said Yifan Zhou, also of the University of Texas at Austin. “This is the youngest bona fide planet Hub- ble has ever directly imaged.” At a youthful 5 million years, the planet is still gathering material and build- ing up mass. Hubble’s ultraviolet light (UV) sensi- tivity offers a unique look at radia- tion from extremely hot gas falling onto the planet. “Hubble’s observa- tions allowed us to estimate how fast the planet is gaining mass,” added Zhou. The UV observations, which add to the body of research about this planet, allowed the team to directly measure the planet’s mass growth rate for the first time. The remote world has already bulked up to five times the mass of Jupiter over a pe- riod of about 5 million years. The present measured accretion rate has dwindled to the point where, if the rate remained steady for another million years, the planet would only increase by approximately and addi- tional 1/100 th of a Jupiter mass. Zhou and Bowler emphasize that these observations are a single snap- shot in time – more data are re- quired to determine if the rate at which the planet is adding mass is increasing or decreasing. “Our mea- surements suggest that the planet is in the tail end of its formation process.” The youthful PDS 70 system is filled with a primordial gas-and-dust disk that provides fuel to feed the growth