A previously-unknown Earth-sized planet, along with a larger companion, have been discovered orbiting a nearby star by astronomers using the Transiting Exoplanet Survey Satellite (TESS). This is the first time that TESS has found a planet the same size as our home world. Joining this planet, its companion world is slightly smaller than the planet Neptune.
The HD 21749 system, found just 53 light years from Earth, is now known to be home to at least two planets, including this newly-discovered Earth-sized world and its sibling, a warm sub-Neptune. The Earth-sized HD 21749c takes roughly eight days to orbit its sun, while its much-larger partner, HD 21749b, takes 36 days to journey once around the star.
“It’s so exciting that TESS, which launched just about a year ago, is already a game-changer in the planet-hunting business. The spacecraft surveys the sky and we collaborate with the TESS follow-up community to flag potentially interesting targets for additional observations using ground-based telescopes and instruments,” said Johanna Teske of the Carnegie Institution for Science.
Tools of the Trade
The Planet Finder Spectrograph (PFS), part of the Magellan II telescope at Carnegie’s Las Campanas Observatory in Chile, was utilized to further investigation of these newly-discovered worlds.
“There was quite some detective work involved, and the right people were there at the right time. But we were lucky, and we caught the signals, and they were really clear,” said Diana Dragomir of MIT’s Kavli Institute for Astrophysics and Space Research.
Launched in 2017, the TESS space telescope utilizes four cameras on a mission to explore 500,000 of the brightest stars in the sky. The orbiting observatory is expected to discover 3,000 worlds orbiting other systems, potentially doubling the number of known exoplanets in the Milky Way Galaxy. Astronomers estimate that around 500 of these worlds will be about the same size as Earth.
Time for a New Direction
As a planet orbits its star, the planet tugs, slightly, on its sun, creating a wobble. The timing of this wobble is equal to the time it takes for the world to make one trip around the star. Astronomers used this radial velocity method to determine the mass of the larger of these two worlds. Exoplanets with longer orbits create slower wobbles, making them more difficult to measure. The 36-day orbit of HD 21749b is the longest yet found by TESS, and researchers believe most planets found by this new planet-hunter will travel around their stars in periods of 10 days or less.
“The future of exoplanet science is bright, as Transiting Exoplanet Survey Satellite (TESS) once again demonstrates with the discovery of its longest-period confirmed planet to date… The HD 21749 system is a prime target for comparative studies of planetary composition and architecture in multi-planet systems,” researchers write in an article announcing the results of the study, published in The Astrophysical Journal Letters.
Measurements indicate HD 21749b has roughly 23 times the mass of the Earth, packed into a world with a diameter 2.7 times greater than our home world. This gaseous world shows no signs, so far, of possessing a significant rocky structure.
The discovery of new worlds by TESS will soon be followed up by observations from the upcoming James Webb Telescope, due for launch in 2021. That orbiting observatory will have the ability to analyze the composition of exoplanets and their atmospheres, providing evidence of which worlds may be capable of supporting life.