Discovery of a transiting “exo-Venus” 40 light years away
Combining data obtained from space and ground-based telescopes, an international research team has discovered an exoplanet, a priori temperate and nearly Earth-sized, very similar to Venus. It is the closest "exo-Venus" detected to date by the transit method i.e., by observing the periodic mini-eclipses (transits) of its star caused by the passage of the planet in front of it.
The new world, called Gliese 12 b, has a diameter slightly smaller than our planet, comparable to the one of Venus (about 12,000 km). The spectra from CARMENES, among others, have made it possible to estimate the planet's mass at about 1.5 times that of the Earth, implying it is a rather dense, rocky planet. The surface equilibrium temperature of the exoplanet would be about 42°C. The final temperature, however, will depend on whether this planet has been able to retain an atmosphere (and its composition) from the time of its formation to the present. If Gliese 12 b also had a very dense atmosphere, temperatures could reach hundreds of degrees in case of a strong greenhouse effect, as in the case of Venus, capable of melting lead (about 450°C).
Possible aspect of the Gliese 12 b planet (right), orbiting the red dwarf star Gliese 12 (left). Credits:NASA/JPL-Caltech/R. Hurt (Caltech-IPAC)
The exoplanet orbits in less than 13 days around its host star, Gliese 12, a cool red dwarf located 39 light-years away in the Pisces constellation. This star has only 27% the size of the Sun and 60% of its surface temperature. The distance separating Gliese 12 from the exoplanet is only 7% of the Earth-Sun distance, so it receives from its star 1.6 times more energy than our planet, explaining its warmer equilibrium temperature (the one of a summer in Andalusia!) than that of the Earth, only -18ºC, although the average surface temperature is 15ºC thanks to our atmosphere.
"Although we don't know yet if Gliese 12 b has an atmosphere, we have been thinking of it as an exo-Venus, with a size and energy received from its star similar to those of our planetary neighbor in the Solar System," says Masayuki Kuzuhara, associate professor of the project at the Tokyo Astrobiology Center who co-leads the research team who has just published its results in The Astrophysical Journal Letters.
“This is a unique candidate for new atmospheric studies that could help to unravel some aspects of the evolution of our own Solar System", explains Enric Pallé, an IAC researcher who also participated in the discovery."Although the Earth and Venus are very similar planets in terms of size and mass, their histories have been very different. The Earth is still habitable, but Venus is not due to its total loss of water; the atmosphere of Gliese 12 b could tell us a lot about how the atmospheric evolution and habitability conditions of terrestrial planets change as they evolve", he adds.
An important factor in retaining an atmosphere is the stormy character of its star. Red dwarfs tend to be magnetically active, resulting in frequent and powerful eruptions of X-rays and ultraviolet radiation. However, Kuzuhara et al.'s analysis concludes that Gliese 12 shows no signs of extreme behavior, making this system an ideal candidate for studying its atmosphere with the James Webb Space Telescope.
Thanks to current technology, the transit method can be used to analyze the chemical composition of exoplanetary atmospheres. By studying the unique pattern of chemical fingerprints generated when starlight passes through the planet's gaseous layer, the molecules present can be identified and their composition better understood.
"To date, we only know of a handful of transiting planets which are close enough to us and that meet the necessary criteria for this type of study; Gliese 12 b is an exceptional candidate which will help us to better understand the diversity of atmospheres around temperate planets similar to Earth", concludes Pallé.
This discovery has counted with an important Spanish participation, especially in the measurements performed with the CARMENES spectrograph (Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs), at Calar Alto Observatory (CAHA), and in the observations performed with the MuSCAT2 instrument (Multicolour Simultaneous Camera for studying Atmospheres of Transiting exoplanets), installed at the Carlos Sánchez Telescope (TCS), at Teide Observatory.
Artist’s view comparing the Earth with different possible interpretations of Gliese 12 b, from no atmosphere (with the planet surface visible in maroon color) up to a Venus-like dense atmosphere (white-light yellow color, to the right). Credits: NASA/JPL-Caltech/R. Hurt (Caltech-IPAC)
NOTA: Shishir Dholakia (astrophysicist at University of Southern Queensland, Australia) et al. have just published their results on Gliese 12 b, in parallel but independently in the Monthly Notices of the Royal Astronomical Society journal.
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