The Hubble Space Telescope has measured the diameter of the closest transiting exoplanet to us, discovering that it’s a rocky planet very similar in size to Earth. Unfortunately, nonetheless, the world’s surface is much too hot to support liquid water or life as we comprehend it.
The planet, referred to as LTT 1445Ac, was first discovered in 2022 by NASA’s Transiting Exoplanet Survey Satellite (TESS). LTT 1445Ac orbits a cool M-dwarf star that’s just 1 / 4 of the mass of our sun; the star itself is present in a triple system of M-dwarfs positioned 22.5 light years away within the direction of the constellation of Eridanus, the River. LTT 1445Ac is joined by two other planets within the triplet, LTT 1445Ab and LTT1445Ad, each of which orbit farther out.
Although there are many exoplanets which might be closer to us, LTT 1445Ac is the closest that truly transits, or moves in front of, its star. When a planet makes such a transit, it blocks a few of its star’s light — the larger the planet, the more light it blocks.
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TESS discovered LTT 1445Ac by measuring this dip in light, but there was an issue. Scientists are very fortunate once they get to watch a transit, as these events can only be noticed in those cases where our line of sight is well-aligned with the planet’s orbital plane around its star. Even a slight misalignment can deem a planet’s transit unseen. Within the case of LTT 1445Ac, TESS was unable to differentiate whether the alignment meant the whole lot of the planet was transiting its star, or if it was only a “grazing transit”. A grazing transit would mean the planet merely appeared to clip the sting of the star, exhibiting only a partial transit. If this were the case, it could suggest a misleading value for the scale of the planet.
“There was a likelihood that this method has an unlucky geometry and if that is the case, we wouldn’t measure the proper size,” Emily Pass of the Harvard–Smithsonian Center for Astrophysics, who led the Hubble observations, said in a statement. “But with Hubble’s capabilities, we nailed it.”
The Hubble Space Telescope was able to verify that the entire of the planet indeed transits the star, and that the diameter of the planet is nearly 1.07 times larger than Earth.
Previously, the ESPRESSO (Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations) instrument on the European Southern Observatory’s Very Large Telescope in Chile had measured the radial velocity of the system. Radial velocity is a measure of how much a star appears to “wobble” because it moves around a shared center of mass with an orbiting planet. The more massive the planet, the larger the wobble; ESPRESSO measured the mass of LTT 1445Ac to be 1.37 times the mass of Earth.
Knowing its radius and its mass, it was then a straightforward matter to calculate LTT 1445Ac’s density, which got here out to five.9 grams per cubic centimeter. Earth, by comparison, has a mean density of 5.51 grams per cubic centimeter. All in all, LTT 1445Ac is a bit of larger, a bit of denser and somewhat more massive than our planet, with a surface gravity 1.37 times greater than on Earth (a surface gravitational acceleration of 13.4 meters per second squared in comparison with Earth’s 9.8 meters per second squared).
Alas, orbiting its star at a distance of three.98 million kilometers (2.47 million miles/0.02659AU) every 3.12 days, LTT 1445Ac is simply too hot for liquid water, with a surface temperature of roughly 260 degrees Celsius (500 degrees Fahrenheit). Life as we comprehend it cannot survive without liquid water, so LTT 1445Ac’s scorching hot surface might be lifeless (unless a type of biology unbeknownst to us manages to thrive in such unrelenting heat.)
Planet b within the system, a super-Earth also seen to transit, appears to be inhospitable as well, though the omens could be more positive for planet d. D is a rocky planet throughout the star’s habitable zone, but unfortunately it doesn’t transit, and subsequently astronomers are unable to review its atmosphere to determine whether it hosts habitable conditions.
Astronomers study exoplanetary atmospheres during transits using a method called transmission spectroscopy. As a planet moves in front of its star as seen from our vantage point, its atmosphere (if it has one) can absorb a few of the starlight, with different molecules absorbing at different wavelengths, revealing their presence.
It’s currently a mystery as as to whether LTT 1445Ac has an environment, but being the closest known transiting planet (alongside its sibling planet b) makes it a main goal for spectroscopic follow-up studies with each Hubble and the James Webb Space Telescope (JWST). The following closest known transiting exoplanet is Gliese 486b, positioned 26.4 light years away. The JWST observed 486b’s atmosphere earlier in 2023, detecting possible signs of water vapor.
TESS was designed to search out transiting exoplanets across the brightest stars within the sky, which also are likely to be among the many closest. Their proximity to us makes spectroscopic follow-up studies of their atmospheres way easier.
If LTT 1445Ac is anything to go by, TESS is succeeding in its mission.
The invention was reported in September in The Astronomical Journal.