The planetary transit method is one of the most important indirect methods for detecting an exoplanet. It had revealed the existence of the exoplanet LTT 1445Ac just 22 light-years from the Sun, but doubts remained about its radius. The Hubble Telescope has just confirmed that its size is comparable to Earth’s, making it a target for studying the composition of a possible atmosphere.
Whether exoplanets, the first galaxies or the planets of the solar system, the James Webb Space Telescope seems to have pushed the Hubble Telescope into the shadows. And yet it is still active and used by astrophysicists to make interesting discoveries, as evidenced by a recent publication that can be read open access on arXiv.
It is an exoplanet that was discovered by the transit method only about 22 light-years from the SunSun in the constellation Eridanus in the southern hemisphere and owes its name to a river in Greek mythology. It was first performed by Ptolemy Ptolemy, then by William Herschel, who cataloged it in 1783.
The exoplanet is called LTT 1445Ac and was discovered by NASA’s Transiting Exoplanet Survey Satellite (TessTess) in 2022. The problem: At the time, we didn’t know exactly whether the transit took place in a simple way or only at the edge of the star’s disk. We therefore had considerable uncertainty about its radius. We could estimate its mass quite correctly by the method of radial velocities, as in all cases of planetary transit, but without a well-determined radius nor a well-established density, and so the mystery hung over whether we had discovered a rocky planet or not.
Methods for discovering exoplanets have changed significantly since the 1990s and can be divided into two main categories: direct methods and indirect methods. The three main methods are the direct imaging method, the indirect transit method and the indirect radial velocity method. Discover exoplanets in our nine-part web series. © A playlist proposed by the CEA and the University of Paris-Saclay as part of the European H2020 research project Exoplanets-A
A diameter 1.07 times the diameter of the Earth
Fortunately, the Hubble Telescope was called to help and was therefore mobilized to study the planetary transit of LTT 1445Ac around LTT 1445A, which is part of a triple system of red dwarf stars. As a reminder, two other exoplanets were discovered around LTT 1445A and LTT 1445B and C and are located about 3 billion kilometers from LTT 1445A.
Hubble’s precise observations show that LTT 1445Ac actually has a diameter 1.07 times that of Earth, as the observed transit is completely classical and non-grazing. This makes LTT 1445Ac the closest Earth-sized exoplanet discovered through a transit.
However, it is not an exoEarth because it completes its orbit at a distance so close to its M dwarf that its average surface temperature is at least 260 °C, and perhaps much more if it has an atmosphere that represents a Venus hell.
Astrophysicist Emily Pass of the Harvard and Smithsonian Center for Astrophysics in Cambridge, Massachusetts, said of the discovery she made: “Transiting planets are exciting because we can spectroscopically observe their atmospheres not only with Hubble, but also with the James Webb Space Telescope “Webb Space Telescope. Our measurements are important because they tell us that it is likely a very close telluric planet. We look forward to further observations that will allow us to better understand the diversity of planets around other stars. »