Hubble measured the size of the nearest Earth-sized exoplanet

This alignment, called a transit, opens the door to follow-on studies.


The NASA/ESA Hubble Space Telescope has measured the size of the nearest Earth-sized exoplanet- LTT 1445Ac. The exoplanet passes across the face of a neighboring star.

The planet was discovered by NASA’s Transiting Exoplanet Survey Satellite- in 2022. Nevertheless, because TESS lacks the necessary optical resolution, the geometry of the planet’s orbital plane concerning its star as viewed from Earth is unknown. This suggests that the detection might have been a so-called grazing transit, in which the planet merely passes across a little area of the disk of the parent star. This would result in a lower-than-true estimate of the planet’s diameter.

The planet revolves around the star LTT 1445A, which is 22 light-years away in the constellation Eridanus and a member of a triple system of three red dwarf stars. Two more planets that are larger than LTT 1445Ac have been reported for this star.

LTT 1445A, which Hubble has also resolved, is located approximately 4.8 billion miles apart from a close pair of additional dwarf stars, LTT 1445B and C. All objects in the system, including the planets known to exist, appear to be coplanar based on the alignment of the three stars and the edge-on orbit of the BC pair.

According to Hubble’s observations, the planet completes regular transit over the star’s disk, giving it an actual size of only 1.07 times the diameter of Earth. This indicates that the planet has a surface gravity similar to Earth’s and is a rocky world. However, at about 260 degrees Celsius on the surface, it is too hot for life as we know it.

Emily Pass of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, USA, said, “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.”

“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 this is likely a very nearby terrestrial planet. We are looking forward to follow-on observations that will allow us better to understand the diversity of planets around other stars.”

Professor Laura Kreidberg of the Max Planck Institute for Astronomy in Heidelberg, Germany (who was not part of this study) said“Hubble remains a key player in our characterization of exoplanets. There are precious 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.”

Journal Reference:

  1. Emily Pass, Jennifer Winters, et al. HST/WFC3 Light Curve Supports a Terrestrial Composition for the Closest Exoplanet to Transit an M Dwarf. arXiv:2307.02970v2
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