In the quest to understand distant worlds, many planets orbiting neighboring stars remain unseen by direct observation. Instead, astronomers detect them through transits, moments when these planets pass in front of their parent stars, causing a subtle dimming of the starlight observable from Earth. The information gleaned from these transits is extensive, encompassing details such as the planet’s orbital period, atmospheric composition, and estimated size. However, challenges arise when the planet’s trajectory during transit only grazes the star’s edge, leading to imprecise diameter measurements.
Enter LTT 1445Ac, an exoplanet located a mere 22 light-years away, initially identified by NASA’s Transiting Exoplanet Survey Satellite (TESS). While TESS lacked the optical resolution to precisely determine the planet’s diameter, the Hubble Space Telescope stepped in with its sharp vision, revealing that LTT 1445Ac is approximately 1.07 times the size of Earth. Despite this cousin-like size resemblance, the proximity of LTT 1445Ac to its red dwarf sun renders it inhospitable, with scorching surface temperatures reaching around 500 degrees Fahrenheit—reminiscent of the intense heat within a pizza oven.
Observations of LTT 1445Ac by the Hubble Space Telescope.
NASA’s Hubble Space Telescope has undertaken a crucial measurement, determining the size of the closest Earth-sized exoplanet known to pass in front of a neighboring star, a celestial event known as a transit. This alignment provides an invaluable opportunity for subsequent investigations into the nature of the rocky world’s atmosphere, if it has one.
Initially identified by NASA’s Transiting Exoplanet Survey Satellite (TESS) in 2022, the tiny exoplanet, LTT 1445Ac, posed a challenge due to uncertainties in the geometry of its orbital plane relative to its star when observed from Earth. TESS lacked the necessary optical resolution, potentially resulting in a “grazing transit,” where the planet only grazes a small part of the parent star’s surface, leading to an imprecise lower limit on the planet’s diameter.
Emily Pass from the Center for Astrophysics | Harvard & Smithsonian in Cambridge, Massachusetts, emphasized the critical role of Hubble’s capabilities in overcoming potential uncertainties in the system’s geometry, stating, “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 observations by Hubble revealed that the exoplanet, LTT 1445Ac, undergoes a standard transit across the entire disk of its parent star, providing an accurate size measurement at only 1.07 times Earth’s diameter. Despite its rocky composition, akin to Earth, and comparable surface gravity, the scorching surface temperature of approximately 500 degrees Fahrenheit renders it inhospitable for life as we currently understand it.
Situated in the constellation Eridanus, the Earth-sized exoplanet LTT 1445Ac orbits the star LTT 1445A, forming part of a triple system comprising three red dwarf stars located 22 light-years away. Notably, LTT 1445A hosts two additional planets, both larger than LTT 1445Ac. The Hubble Space Telescope also resolved a tight pair of dwarf stars, LTT 1445B and C, positioned approximately 3 billion miles from LTT 1445A. The alignment of these three stars, along with the edge-on orbit of the BC pair, implies a shared orbital plane for all components, including the known planets.
Emily Pass highlights the significance of transiting planets, expressing excitement about the prospects for characterizing their atmospheres through spectroscopy, leveraging instruments like the James Webb Space Telescope in addition to Hubble. The precise measurement obtained is crucial in identifying LTT 1445Ac as a likely nearby terrestrial planet. Anticipating subsequent observations, Pass emphasizes their importance in enhancing our understanding of planetary diversity around distant stars. This research is set to be published in The Astronomical Journal.