The method of planetary transit is one of the primary indirect techniques for detecting exoplanets. It revealed the existence of the exoplanet LTT 1445Ac, situated only 22 light-years away from the Sun. However, uncertainty persisted regarding its radius. The Hubble Space Telescope has recently confirmed that its size is comparable to that of Earth, making it a target for studying the potential composition of its atmosphere.
Whether dealing with exoplanets, the early galaxies, or the planets of the Solar System, the James Webb Space Telescope seems to have overshadowed the Hubble Telescope. Nevertheless, the Hubble remains active and is utilized by astrophysicists to make interesting discoveries, as demonstrated by a recent publication accessible on arXiv.
This publication focuses on an exoplanet discovered through the transit method, approximately 22 light-years from the Sun, in the Eridanus constellation in the southern hemisphere, named after a river in Greek mythology. Ptolemy was the first to record it, and William Herschel later cataloged it in 1783.
NASA’s Transiting Exoplanet Survey Satellite (TESS) discovered the exoplanet, known as LTT 1445Ac, in 2022. However, at that time, the certainty of whether the transit occurred simply or at the star’s disk’s edge was unclear. Consequently, there was notable uncertainty regarding its radius. While its mass could be estimated relatively accurately through the radial velocity method typical in planetary transit cases, the lack of a well-defined radius led to an undetermined density. Thus, the mystery persisted as to whether the detected object was a rocky planet or not.
A Diameter 1.07 Times That of the Earth
Fortunately, the Hubble Space Telescope could be called upon for assistance and was thus mobilized to study the planetary transit of LTT 1445Ac around LTT 1445A, which is part of a triple system of red dwarf stars. For context, two other exoplanets have been detected around LTT 1445A, and LTT 1445B and C are located approximately 3 billion kilometers from LTT 1445A.
Precise observations from the Hubble reveal that LTT 1445Ac actually has a diameter 1.07 times that of Earth, as the observed transit is entirely conventional and not grazing. This makes LTT 1445Ac the closest Earth-sized exoplanet discovered by transit.
However, it is not an exoEarth because it orbits at such a close distance to its M dwarf that its average surface temperature is at least 260 °C, possibly even higher if it possesses an atmosphere, turning it into a Venusian hell.
Astrophysicist Emily Pass from the Center for Astrophysics – Harvard and Smithsonian in Cambridge (Massachusetts), commented on this discovery in which she participated, 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.“
Featured Image: An artist’s concept of the exoplanet LTT 1445Ac. NASA / ESA / Leah Hustak, STScI.