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Over the past two and a half decades, astronomers have confirmed the existence of thousands of exoplanets. In recent years, thanks to improvements in instrumentation and methodology, the process has slowly evolved from discovery to characterization. In particular, astronomers hope to obtain spectra of exoplanets of atmospheres that would indicate their chemical composition.
This is not an easy task because direct imaging is very difficult and the only other method is to make observations during transits. However, astronomers from the CARMENES consortium recently reported the discovery of a hot rocky super-Earth orbiting the nearby red dwarf star. While being extremely hot, this planet has retained some of its original atmosphere, which makes it particularly suitable for observations using new generation telescopes.
Part of the problem with characterizing exoplanets is that the ones considered most likely to be habitable are the rocky exoplanets that orbit near their stars. Any light reflected by their atmospheres and surfaces is therefore controlled by the light of their mother star. As such, direct imaging is generally only possible in the case of long orbiting gas giants.
On occasion, astronomers can examine the light passing through a planet’s atmosphere as it passes in front of its parent star (aka. Transits). This also presents challenges, as rocky planets have relatively thin atmospheres compared to gas giants (assuming they have any at all). As a result, many current atmospheric models for rocky planets have yet to be tested.
Astronomers have used both transit photometry and radial velocity measurements – currently the two most effective methods (especially in combination) – to confirm the existence of Gliese 486b. This was done using data from the Transiting Exoplanet Survey Satellite (TESS), the 1.52m Telescopio Carlos Sánchez from the Teide Observatory, the Las Cumbres Observatory Global Telescope Network (LCOGT) and other instruments.
This latest planet discovered by the CARMENES consortium is known as Gliese 486b, a super-Earth that orbits an M-type star (red dwarf) located just 26 light years away. This planet is about 2.8 times the mass of Earth, is similar in composition to Earth and Venus, and orbiting 2.5 million km (1.55 million mi) from its star – about 1, 6% of the distance between the Earth and the Sun – taking 1.5 days in an orbit.
Between its proximity to Earth, its rocky composition, its tight orbit with its mother star, and the fact that it has retained an atmosphere, this exoplanet meets all the requirements for follow-up observations by next-generation telescopes. Since Gliese 486b has a sidereal rotation which is the same as its orbital period (1.5 days), it is tidal locked with its parent star (one side always faces it).
Trifon Trifonov, a planet specialist at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, was the lead author of the research study. As he explained in a recent press release from MPIA:
“The proximity of this exoplanet is exciting because it will be possible to study it in more detail with powerful telescopes such as the future James Webb Space Telescope and future extremely large telescopes.
“We can’t wait for the new telescopes to be available. The results will help us understand how well rocky planets can hold their atmosphere, what they are made of, and how they influence the distribution of energy on the planets.
Even though Gliese 486 is a relatively weak and cold star (relative to the Sun), the amount of radiation it is exposed to is so intense that the planet experiences surface temperatures of up to 700 K (430 ° C; 806 ° F). From all of this, Gliese 486b’s surface is likely to look a lot like Venus, with a hot, dry landscape crisscrossed by rivers of glowing lava.
One big difference, however, is that Gliese 486b appears to have a thin atmosphere (whereas Venus has an incredibly dense one). This is impressive considering that our current planetary models indicate that irradiation tends to strip the planets of their gaseous envelopes. In this case, any atmosphere that the planet still has would be held by the gravity of the super-Earth.
As José A. Caballero of the Centro de Astrobiología (CSIC-INTA, Spain) and co-author of the article concluded:
“The discovery of Gliese 486b was a fluke. One hundred degrees warmer and the entire surface of the planet would be lava. Its atmosphere would consist of vaporized rocks. On the other hand, if Gliese 486b had been a hundred degrees cooler, it would not have been suitable for follow-up observations.
In the future, the CARMENES team hopes to observe Gliese 486b as it makes transits in front of its star (relative to us). It is at this point that small amounts of light will pass through the planet’s thin atmosphere, which will be detectable by observatories like James webb. A second series of spectroscopic measurements will be performed each time the orbit of Gliese 486b takes it behind its star.
At this point, the light reflected from the surface of the planet can be studied to obtain emission spectra. Between these two types of spectroscopic observations, astronomers will be able to field test methods that will allow them to restrict the search for habitable planets. Ground-based observatories such as the Extremely Large Telescope (ELT) and the Giant Magellanic Telescope (GMT) will also allow direct imaging and spectroscopic studies.
The Calar Alto High Resolution Research Consortium for M Dwarfs with Exoearths with Near Infrared and Optical Scale Spectrographs (CARMENES) is made up of more than 200 scientists and engineers from 11 institutions located in Spain and Germany. The study that describes their discovery, “A nearby rocky exoplanet suitable for atmospheric investigation,” was recently published in the journal Science (Vol. 371, No. 6533).
Further reading: MPIA, Science
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