Hubble has confirmed the size of the Earth-like exoplanet LTT1445Ac, providing new insights into its composition and further study of its atmosphere. Many planets around nearby stars are not directly visible. Instead, they are discovered when they temporarily pass in front of their parent star, an event called a transit. During the transit, they block a little of the light from the star, essentially casting a shadow for Earth observers.
NASA's Hubble Telescope has made precise measurements of LTT1445Ac, an Earth-sized exoplanet in a nearby three-star system. The rocky planet, originally discovered by TESS, is similar in size to Earth but much warmer. This research improves our understanding of exoplanets and their atmospheres. Image source: ESA/Hubble (M. Kornmesser and L.L. Christensen)
Astronomers learn a lot from these transient events. They can measure the planet's orbital period, study its atmosphere, and estimate its size. The tricky thing is that the planet may just brush the edge of the star during the transit, so the measurement of its diameter is not accurate.
NASA's Transiting Exoplanet Survey Satellite (TESS) discovered such an exoplanet for the first time, LTT1445Ac, which is only 22 light-years away from its star. But TESS does not have the required optical resolution to determine the precise diameter of a planet by refining its trajectory on the star's surface.
At this time, Hubble used its keen vision to accurately measure that the diameter of the planet was only 1.07 times the diameter of the Earth. This makes it a cousin of Earth in size. But the similarities end there. LTT1445Ac is too close to its red dwarf star to be habitable. Its surface temperature is about 500 degrees Fahrenheit - about the same temperature as inside a pizza oven.
This artist's concept rendering of the nearby exoplanet LTT1445Ac is about the same size as Earth. The planet orbits a red dwarf star. The star is a triplet system, with two closely orbiting red dwarfs visible above and to the right. The dark spot in front of the bright reddish sphere in the center of the image is the planet LTT1445Ac, which is crossing the star's surface. The planet's surface temperature is about 500 degrees Fahrenheit. The lower left foreground is LTT1445Ab, another planet in the system. The view is from 22 light-years away looking back at the Sun, the bright spot on the lower right. Some background stars are part of the Baltic constellations. Image source: NASA, ESA, Leah Hustak (STScI)
NASA's Hubble Space Telescope has measured the size of the closest Earth-sized exoplanet to pass by the surface of a nearby star. This arrangement, known as a "transit," opens the door for follow-up studies to understand what kind of atmosphere this rocky world might have.
The dwarf planet, named LTT1445Ac, was first discovered in 2022 by NASA's Transiting Exoplanet Survey Satellite (TESS). However, because TESS does not have the required optical resolution, the geometry of the planet relative to its star's orbital plane as seen from Earth is uncertain. This means the detection may have been a so-called flyby, in which the planet passes only a small part of its parent star's disk. This would produce an inaccurate lower bound on planet diameter.
"It's possible that this system had an unlucky geometry, and if that was the case, we wouldn't be able to measure the correct size," said Emily Pass of the Harvard Center for Astrophysics and the Smithsonian Center for Astrophysics in Cambridge, Massachusetts. "But, with Hubble's capabilities, we determined its diameter."
Hubble observations show that the planet made a normal complete crossing of the star's disk, and its true size is only 1.07 times the diameter of the Earth. This means the planet is a rocky world, like Earth, with roughly the same surface gravity. But its surface temperature is about 500 degrees Fahrenheit, too hot for life as we know it.
This image compares two scenarios in which an Earth-sized exoplanet passes in front of its host star. The bottom track shows the planet just passing by the star. Studying this transiting light can lead to an inaccurate estimate of the planet's size, making it appear smaller than it actually is. The path above shows the optimal geometry, where the planet passes through the entire disk of the star. The precision of the Hubble Space Telescope can distinguish between these two cases, allowing precise measurements of the planet's diameter. Source: NASA, ESA, Elizabeth Wheatley (STScI)
The planet orbits the star LTT1445A, part of a three-star system of three red dwarfs located 22 light-years away in the constellation Eridan. According to reports, there are two more planets on this star that are larger than LTT1445Ac. Two other dwarf stars, LTT1445B and C, which are closely adjacent and about 3 billion miles away from LTT1445A, were also resolved by Hubble. The alignment of these three stars and the fringe orbits of the BC pair suggest that everything in this galaxy is coplanar, including the known planets.
"Transiting planets are exciting because we can analyze the characteristics of their atmospheres through spectroscopy using not only Hubble but also the James Webb Space Telescope. Our measurements are important because they suggest to us that this is likely a very nearby terrestrial planet," Paz said. "We look forward to follow-up observations that will allow us to better understand the diversity of planets around other stars."
The research was published in the Astronomical Journal.