LTT 1445Ac, a terrestrial planet in the triple star system LTT 1445, is only 1.07 times the size of Earth, according to a paper to be published in the Astronomical Journal.
An artist’s concept of the exoplanet LTT 1445Ac. Image credit: NASA / ESA / Leah Hustak, STScI.
LTT 1445 is a hierarchical trio of M dwarfs located 22.5 light-years away in the constellation of Eridanus.
Also known as TIC 98796344, TOI 455, L 730-18 or BD-17 588, the system consists of the brighter star LTT 1445A and the fainter visual binary LTT 1445BC.
LTT 1445A hosts at least three exoplanets: LTT 1445Ab, LTT 1445Ac and LTT 1445Ad.
“Photometry from NASA’s Transiting Exoplanet Survey Satellite (TESS) revealed two small transiting planets in the LTT 1445 system: planet b and planet c,” said Dr. Emily Pass from the Harvard & Smithsonian’s Center for Astrophysics and her colleagues.
“High-precision radial velocity follow-up of LTT 1445A from the ESPRESSO spectrograph on ESO’s Very Large Telescope also yielded an additional candidate planet d, which is likely non-transiting.”
“LTT 1445A is the nearest M dwarf with a known transiting planet,” they added.
“Planets b and c therefore offer some of the most favorable conditions to characterize the atmospheres of terrestrial exoplanets.”
An artist’s impression of LTT 1445Ab in the triple-star system LTT 1445. Image credit: Sci.News.
Previous studies of LTT 1445Ac concluded that the light curve from TESS was consistent with both grazing and non-grazing geometries.
As a result, the radius and hence density of the planet remained unknown.
“There was a chance that this system has an unlucky geometry and if that’s the case, we wouldn’t measure the right size,” Dr. Pass said.
“But with the capabilities of the NASA/ESA Hubble Space Telescope we nailed its diameter.”
This image of the LTT 1445 system was obtained by the NASA/ESA Hubble Space Telescope in 2003. Image credit: NASA / ESA / Hubble / Sci.News / Winters et al.
In their research, the astronomers observed the LTT 1445 system for six spacecraft orbits of Hubble, including one partial transit of LTT 1445Ac.
The 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 surface temperature of roughly 260 degrees Celsius (500 degrees Fahrenheit), it is too hot for life as we know it.
The alignment of the three stars and the edge-on orbit of the LTT 1445BC pair suggests that everything in the system is co-planar, including the known planets.
“Transiting planets are exciting since we can characterize their atmospheres with spectroscopy, not only with Hubble but also with the NASA/ESA/CSA James Webb Space Telescope,” Dr. Pass said.
“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.”
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Emily K. Pass et al. 2023. HST/WFC3 Light Curve Supports a Terrestrial Composition for the Closest Exoplanet to Transit an M Dwarf. AJ, in press; arXiv: 2307.02970
