The field of exoplanet research continues to grow by leaps and bounds. Thanks to missions like the Kepler Space Telescope, over four-thousand planets have been discovered beyond our Solar System, with more being confirmed all the time. Thanks to these discoveries and all that we’ve learned from them, the focus has begun to transition from the process of discovery to characterization.
For instance, a group of astronomers was able to image the surface of a planet orbiting a red dwarf star for the first time. Using data from the NASA Spitzer Space Telescope, the team was able to provide a rare glimpse at the conditions on the planet’s surface. And while those conditions were rather inhospitable – akin to something like Hades, but with less air to breathe – this represents a major breakthrough in the study of exoplanets.
As they indicated in their study, which recently appeared in the journal Nature, the planet they observed (LHS 3844b) is a terrestrial (aka. rocky) body that orbits a cool M-type (red dwarf) star located 48.6 light-years from Earth. This planet was originally discovered by the Transiting Exoplanet Survey Satellite (TESS) in 2018, is 1.3 times the radius of Earth, and orbits its star with a period of 11 days.
True to its name, TESS detected the planet using the Transit Method, where periodic dips in the star’s luminosity are indications that a planet is passing in front of it (aka. transiting) relative to the observer. During follow-up observations using data from Spitzer‘s Infrared Array Camera (IRAC), the team was able to detect light for the surface of LHS 3844b.
Ordinarily, this is a difficult prospect because light reflected from the planet’s surface is drowned out by the much brighter light coming from the star. However, since the planet orbits so closely to its star, it is likely to be “tidally locked” – where one side is permanently facing towards the star. As a result, the dayside experiences temperatures of about 770 °C (1440 °F) and radiates a lot of infrared light.
This observation was the first time Spitzer data was able to provide information about the atmosphere of a terrestrial planet around an M-type dwarf. This is especially encouraging since M-type dwarfs are the most common type of star in the Universe, accounting for 75% of the stars in the Milky Way alone. They are also the most long-lived, capable of remaining in their main sequence for up to 10 trillion years.
Unfortunately, the results were less than encouraging as far as the search for “potentially-habitable” planets goes. Based on the planet’s orbit and the data obtained by Spitzer, the planet has little to no atmosphere and is likely to be covered in cooled volcanic material. This is the same material that comprises the dark lunar regions known as mare, which means that this planet is probably similar to Mercury or Earth’s Moon.
This was inferred by using LHS 3844b’s surface albedo (i.e. its reflectiveness), which was quite dark. Renyu Hu, an exoplanet scientist at NASA’s Jet Propulsion Laboratory and a co-author on the study, concluded with his colleagues that this was likely the result of the surface being covered with basalt, a kind of volcanic rock.
“We know that the mare of the Moon are formed by ancient volcanism,” said Hu, “and we postulate that this might be what has happened on this planet.”
Another less-than-encouraging find was the negligible heat transfer that takes place between the planet’s dayside and nightside. The team learned of this by measuring the temperature difference between the two sides of the planet. In this respect, LHS 3844b is once again comparable to Mercury and the Moon – two bodies that have virtually no atmosphere and experienced massive temperature variations between the dayside and nightside.
As Laura Kreidberg, a researcher at the Harvard and Smithsonian Center for Astrophysics (CfA) and lead author of the new study – explained, the absence of an atmosphere was the most likely explanation for the extreme variation they saw. “The temperature contrast on this planet is about as big as it can possibly be,” she said. “That matches beautifully with our model of a bare rock with no atmosphere.”
Still, the implications of this study are quite profound. Aside from this being the first time that astronomers have been able to image the surface of a rocky planet orbiting a red dwarf star (a major accomplishment in its own right), it could also shed light on how planetary atmospheres are lost over time. This is of extreme importance when it comes to the search for potentially habitable planets.
Consider Mars, otherwise known as “Earth’s Twin”. Whereas Earth has managed to retain its atmosphere and (as a result) liquid water on its surface, Mars lost its atmosphere over the course of billions of years, to the point that it had roughly 0.5% of Earth’s atmospheric pressure. This is attributed to Mars losing its magnetic field shortly after the planet formed and cooled.
Because of this, Mars’ surface underwent drastic climate change where all its surface water was lost. Studying rocky exoplanets that have lost their atmospheres – particularly those that orbit the most common star in the Universe – could therefore help astronomers learn more about a key indicator of habitability. As Kreidberg explained:
“We’ve got lots of theories about how planetary atmospheres fare around M dwarfs, but we haven’t been able to study them empirically. Now, with LHS 3844b, we have a terrestrial planet outside our solar system where for the first time we can determine observationally that an atmosphere is not present.”
Compared to our Sun (a G-type yellow dwarf star), M-type red dwarfs emit less overall light, but high levels of ultraviolet radiation. Not only can this be harmful to life in high doses, but it can also erode a planet’s atmosphere. What’s more, red dwarfs are particularly violent in their youth and produce a lot of flares, which results in bursts of radiation and particles that can strip away a planet’s atmosphere.
Granted, this latest study doesn’t exactly inspire a rosy outlook for rocky planets that orbit M-type stars. And since there is research that indicates that red dwarf systems could be the most likely place to find rocky planets that orbit within the star’s habitable zone (HZ), it also doesn’t bode well for habitability studies. But as Kreidberg said, these findings are by no means universal:
“I’m still hopeful that other planets around M dwarfs could keep their atmospheres. The terrestrial planets in our solar system are enormously diverse, and I expect the same will be true for exoplanet systems.”
In the meantime, astronomers are enthused by the results of this study because of what it means for exoplanet studies. In the coming years, the launch of the James Webb Space Telescope – which has considerably more advanced IR imaging capabilities – will allow for direct imaging studies of more in the way of rocky planets that orbit red dwarf stars.
These include Proxima b, the closest planet beyond our Solar System, and the seven-planet system of TRAPPIST-1. Already, Spizter has used its IRAC instrument to gather data on the TRAPPIST-1 system, which revealed that some of them likely contain water ice. In addition, multiple ground-based telescopes will be coming online in the next decade that will allow for direct imaging studies of nearby exoplanets.
Just in time too, since NASA plans to terminate the Spitzer/IRAC operations by February of 2020, as a cost-savings measure. Much like Hubble and Kepler, Spitzer has helped to point the way towards future discoveries!
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