Free Astronomy Magazine January-February 2024

37 JANUARY-FEBRUARY 2024 ASTRO PUBLISHING Cambridge, Massachusetts. Hubble observations show that the planet makes a normal transit fully across the star’s disk, yielding a true size of only 1.07 times Earth’s diameter. This means the planet is a rocky world, like Earth, with approximately the same surface gravity. But at a sur- face temperature of roughly 260 de- grees Celsius, it is too hot for life as we know it. The planet orbits the star LTT 1445A, which is part of a triple system of three red dwarf stars that is 22 light-years away in the constellation Eridanus. The star has two other re- ported planets that are larger than LTT 1445Ac. A tight pair of two other dwarf stars, LTT 1445B and C, lies about 4.7 billion kilometres away from LTT 1445A, also resolved by Hubble. The alignment of the three stars and the edge-on orbit of the BC pair suggests that everything in the system is coplanar, including the known planets. “Transiting planets are exciting since we can characterise their atmos- pheres with spectroscopy, not only with Hubble but also with the James Webb Space Telescope. Our meas- urement is important because it tells us that this is likely a very nearby ter- restrial planet. We are looking for- ward to follow-on observations that will allow us to better understand the diversity of planets around other stars,” said Pass. “Hubble remains a key player in our characterisation of exoplanets” , added Professor Laura Kreidberg of the Max Planck Institute for Astron- omy in Heidelberg (who was not part of this study). “There are pre- cious few terrestrial planets that are close enough for us to learn about their atmospheres — at just 22 light years away, LTT 1445Ac is right next door in galactic terms, so it’s one of the best planets in the sky to follow up and learn about its atmospheric properties.” small portion of the parent star’s disk. This would yield an inaccurate lower limit of the planet’s diameter. “There was a chance that this system has an unlucky geometry and if that’s the case, we wouldn’t meas- ure the right size. But with Hubble’s capabilities we nailed its diameter,” said Emily Pass of the Harvard-Smith- sonian Center for Astrophysics in T his diagram compares two scenarios for how an Earth-sized exoplanet is pass- ing in front of its host star. The bottom path shows the planet just grazing the star. Studying the light from such a transit could lead to an inaccurate estimate of the planet’s size, making it seem smaller than it really is. The top path shows the optimum geometry, where the planet transits the full disk of the star. Hubble Space Telescope’s accuracy can distinguish between these two scenarios, yielding a precise measurement of the planet’s diameter. [NASA, ESA, E. Wheatley (STScI)] !