The Hubble Space Telescope has successfully measured the diameter of the closest transiting exoplanet to Earth, revealing that it is a rocky planet similar in size to our own. Unfortunately, the surface of this planet, named LTT 1445Ac, is too hot to support liquid water or sustain life as we know it.
NASA’s Transiting Exoplanet Survey Satellite (TESS) first discovered LTT 1445Ac in 2022. The planet orbits a cool M-dwarf star, which is only a quarter of the mass of our sun. This star is part of a triple system of M-dwarfs located 22.5 light years away in the constellation of Eridanus, the River. LTT 1445Ac is accompanied by two other planets, LTT 1445Ab and LTT 1445Ad, which orbit farther out.
While there are closer exoplanets to Earth, LTT 1445Ac is the closest one that actually transits in front of its star. When a planet transits, it blocks some of the star’s light, and the size of the planet determines the amount of light it blocks.
TESS discovered LTT 1445Ac by observing this dip in light. However, there was uncertainty about whether the planet’s orbit was aligned in a way that allowed for a full transit or just a partial one. Emily Pass of the Harvard–Smithsonian Center for Astrophysics, who led the Hubble observations, stated that there was a chance the system had an unfavorable geometry, which would have resulted in inaccurate measurements. Fortunately, the Hubble Space Telescope confirmed that the entire planet does transit the star, and its diameter is approximately 1.07 times that of Earth.
Previously, the ESPRESSO instrument on the European Southern Observatory’s Very Large Telescope in Chile determined the radial velocity of the LTT 1445Ac system. Radial velocity measures the extent to which a star appears to wobble as it orbits around a shared center of mass with a planet. Based on ESPRESSO’s measurements, LTT 1445Ac was found to have a mass 1.37 times that of Earth.
Combining the measurements of radius and mass, scientists were able to calculate the density of LTT 1445Ac, which is approximately 5.9 grams per cubic centimeter. In comparison, Earth has an average density of 5.51 grams per cubic centimeter. Despite being slightly larger, denser, and more massive than Earth, LTT 1445Ac has a surface gravity only 1.37 times greater than that of our planet.
Unfortunately, LTT 1445Ac orbits its star at a distance of 3.98 million kilometers (2.47 million miles) every 3.12 days, resulting in a scorching surface temperature of around 260 degrees Celsius (500 degrees Fahrenheit). With such extreme heat, liquid water, which is essential for life as we know it, cannot exist on this planet. Therefore, it is unlikely that LTT 1445Ac supports any form of life unless there is a previously unknown biological adaptation to such extreme conditions.
Another planet in the LTT 1445 system, planet b, is also inhospitable. However, there may be more hope for planet d, which is a rocky planet within the star’s habitable zone. Unfortunately, planet d does not transit, making it challenging for astronomers to study its atmosphere and determine if it can support habitable conditions.
Astronomers use a technique called transmission spectroscopy to study exoplanetary atmospheres during transits. As a planet passes in front of its star, its atmosphere can absorb some of the starlight, revealing the presence of different molecules at specific wavelengths.
It remains unknown whether LTT 1445Ac has an atmosphere, but being the closest known transiting planet, it is a prime target for spectroscopic follow-up studies using both the Hubble Space Telescope and the upcoming James Webb Space Telescope (JWST). The next closest transiting exoplanet is Gliese 486b, located 26.4 light years away. The JWST recently observed 486b’s atmosphere and detected possible signs of water vapor.
TESS was specifically designed to discover transiting exoplanets around the brightest stars in the sky, which are often the closest ones. Their proximity makes it easier to conduct spectroscopic studies of their atmospheres.
The successful measurement of LTT 1445Ac’s diameter by the Hubble Space Telescope showcases the success of TESS in achieving its mission. The discovery was reported in September in The Astronomical Journal.
