Hubble measures size of nearest Earth-sized exoplanet

The Hubble Space Telescope, operated by NASA, recently determined the size of LTT 1445Ac, the closest Earth-sized exoplanet that undergoes a transit across its neighboring star. This alignment, known as a transit, provides an opportunity for subsequent investigations into the potential atmosphere of this rocky world.

Originally discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS) in 2022, uncertainties existed about the planet’s orbital plane relative to its star due to TESS’s limited optical resolution. This raised the possibility of a grazing transit, where the planet only partially crosses the star’s disk, leading to an inaccurate lower limit of its diameter.

Emily Pass from the Center for Astrophysics | Harvard & Smithsonian in Cambridge, Massachusetts, explained, “There was a chance that this system has an unlucky geometry, and if that’s the case, we wouldn’t measure the right size. But with Hubble’s capabilities, we nailed its diameter.” The findings were detailed in a paper recently published in The Astronomical Journal.

One of our 188 spatial-scan images of the LTT 1445ABC system. Stars appear as rectangles instead of circles in spatial scans, as the light is smeared in the direction of the scan. This technique allows high-SNR observations of bright sources to be obtained without saturating any pixels. Our 380 × 20 px apertures are indicated in white. Credit: The Astronomical Journal (2023). DOI: 10.3847/1538-3881/acf561

Hubble’s observations have confirmed that LTT 1445Ac, the planet in question, undergoes a standard transit, crossing the entire disk of its host star. This revelation establishes the planet’s size at only 1.07 times that of Earth, classifying it as a rocky world with a comparable surface gravity. However, with a scorching surface temperature of around 500 degrees Fahrenheit, conditions on this planet are inhospitable for life as we know it.

Situated in the constellation Eridanus, 22 light-years away, LTT 1445Ac orbits the star LTT 1445A, which is part of a triple system consisting of three red dwarf stars. Notably, this star system includes two larger planets than LTT 1445Ac. Additionally, the Hubble Space Telescope resolved the presence of two other dwarf stars, LTT 1445B and C, positioned approximately 3 billion miles away from LTT 1445A. The alignment of these three stars and the edge-on orbit of the BC pair suggest a shared plane of motion, including the confirmed planets.

Emily Pass expressed enthusiasm, stating, “Transiting planets are exciting since we can characterize their atmospheres with spectroscopy, not only with Hubble but also with the James Webb Space Telescope. Our measurement is important because it tells us that this is likely a very nearby terrestrial planet. We are looking forward to follow-on observations that will allow us to better understand the diversity of planets around other stars.”

Source: Harvard-Smithsonian Center for Astrophysics

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