Planets around other stars are discovered using the transit method where the planet blocks a portion of the star's light. Utilizing JWST's mid-infrared camera, the atmospheres of those distant worlds can be observed by measuring the influence on starlight as it passes through or by the planet. LHS 3844 is a red-dwarf solar system about 48 light-years away and hosts a rocky planet 1.3 times the size of Earth. By observing the planet's day-side just before and after passing its host star, astronomers were able to get a rare glimpse at the planet's surface, giving new insights into exoplanetary geology.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
In our quest to understand exoplanets, much of the work has revolved around transits. That is when a planet goes in front of its host star and blocks a small fraction of that light. Typically, that's been used to determine sizes of exoplanets, which is the easiest thing to measure, though still quite difficult. That small drop in light really represents the ratio of the planet's area to that of the star. For an Earth‑like planet around a Sun‑like star, that ratio is under 9 parts in 100,000, or that planet would block less than 9,000ths of a percent of the star's light. So very difficult, though now astronomers are doing it with some regularity. And we have to add to that the complexity of knowing the exact size of the star, since all stars except our Sun are too distant to be seen as other than points; measuring their sizes requires indirect techniques. During transits, we don't actually see the planet, we only see how much light it blocks, which is a bit disappointing, since of course we want to know what those planets are like. Which leads to an even more difficult measurement to make called transit spectroscopy.
During a transit, some small quantity of starlight passes through the planet's atmosphere and that light is affected by the contents of that atmosphere. If we optimistically assume we can see through Jupiter's atmosphere to a depth of 3,000 miles, then to detect it via transit spectroscopy requires accuracy better than 2 parts in 10 million. For Earth, which has roughly 90 miles of substantial atmosphere, that becomes 4 parts in 10 billion with a B. But there are things that can be done to improve that, such as looking for exoplanets around smaller stars, which is what most astronomers are doing.
Rather than looking at transits, another technique is to look at the day side of the planet before what we call a secondary transit. During a transit, the night side of the exoplanet is facing us, but just before and after the planet goes behind the star – the secondary transit – we are viewing the day side. In particular, if we look in the infrared, we can compare the temperature of the day side to the night side and this has been done for several hot Jupiter exoplanets to determine how the winds are circulating the heat.
That technique has now been used to examine the surface composition of a super‑Earth using JWST. The results are published in Nature Astronomy by Dr. Sebastian Ziba of Harvard Smithsonian and collaborators. They used JWST's MIRI, which is a mid‑infrared detector, to look at the planet LHS 3844B, also called Kuaꞌkua. That planet is 30% larger than Earth and orbits its star in only 11 hours. The star LHS 3844 is a red dwarf, only 15% the mass of our Sun and most importantly, 19% the size and half the temperature, making it far easier to see the planet.
The LHS system is 48 light‑years away in the constellation Indus and if that doesn't sound familiar, it's because it's in the southern hemisphere where we do not see it here in Springfield. With only an 11 hour orbit, Kua'kua is tidally locked to its star and so the day side, Dr. Ziba and collaborators saw, is always the day side. Their observations indicate that day side is 1,340 degrees Fahrenheit, which is above the melting point of lead, aluminum, cadmium, magnesium and a few other metal‑like elements.
But what's really interesting in their paper is that they are not looking at a day‑side atmosphere like has previously been done for some hot Jupiters; instead they are directly detecting light leaving the surface. That sounds pretty amazing and it is. But let's put it into some context. What they really do is take a series of infrared spectra before, during, and after secondary transit. They then use the data obtained during secondary transit – which would be just the star – to correct off the starlight and view the light just from the exoplanet before and after secondary transit.
One of the first things they noticed was a lack of atmospheric gases such as carbon dioxide or sulfur dioxide, again meaning they were getting light right from the surface. They claim their spectrum to be a featureless thermal spectrum of a dark surface, although I see a slight absorption around 11 microns. Dr. Ziba and collaborators rule out a silica‑rich surface like Earth's and instead prefer a basaltic surface like the Moon's or in Earth's mantle, but could also be iron‑ and magnesium‑rich lava. However, if the origin were a volcano‑dominated world, there would be an atmosphere as volcanoes release a lot of gas. The lack of carbon dioxide or sulfur dioxide, common volcanic gases, likely rules out a lava‑volcano world. Instead, Kua'kua is more likely an old space‑weathered rocky planet like our Moon or Mercury just in a larger version.