In search of life beyond our galaxy, many scientists have their eyes fixed on spheres like the Earth: rocky planets. After exploring a rocky planet slightly larger than Earth last fall, a research team launched a campaign to capture additional images with the Spitzer Space Telescope, the only telescope in space currently in existence can capture a planet directly infrared light. The telescope produced images smaller than 1 pixel (1/94 inch), like a speck of dust that makes predictions about the planet's habitability.
By observing multiple orbits of the planet, scientists were able to map the temperature of its surface and create models of its atmosphere – capabilities that scientists are just beginning to develop for rocky planets. Much of what explorers learn about exoplanets is based on what they know about the stars that orbit them.
"People say we only know one planet as well as we do the star, because basically, we're counting on things that are based on what we're measuring over the star," said Laura Schaefer , an Assistant Professor of Earth Sciences at the Stanford School of Earth's Energy & Environmental Sciences (Stanford Earth) and co-author of a study to characterize a planet published in Nature August 19
The The team's analysis shows that the 48.6-light-distant planet LHS 3844b is much hotter than Earth and possibly covered in dark volcanic rock, and in just 11 hours orbits a star smaller than the Sun. The star is an M Dwarf – the most abundant and long-lived star type, possibly hosting a high percentage of the galaxy's planets – and the rocky planet's atmosphere is the first to orbit an M dwarf that is characterized The researchers found that the planet has no or little atmosphere and therefore can not support life – an important insight into the understanding of atmospheres of similar rocky planets around M dwarfs.
Stanford News Service spoke with Schaefer to learn more about the results and their significance.
Why do scientists want to explore exoplanets?
In general, trying to understand planet formation better. We pretty much understand the planets in our own solar system, but that only gives us a snapshot of how planet formation works. By going out and finding planets around other stars, we have discovered many crazy new things that we did not know happened when planets formed. For example, we found a class of planets nobody expected to exist, called Jupiter. These are actually the first types of exoplanets that have been discovered.
The other major goal when looking at exoplanets is to find another planet, such as the Earth, that may have life on it. I focus on the smaller rocky planets, not on the big gas giants. The ultimate goal is to find a planet in the so-called "habitable zone," a region in orbital space where liquid water could be stable on the surface of a planet such as Earth.
To determine if a planet has life, we need to be able to measure its atmosphere and see if life has affected it, as we know it here on Earth, where our oxygen atmosphere is generated is from life. Before life was widespread on Earth, its atmosphere was very different. So, if we can look at the atmosphere of the planets in the habitable zone and find out what they are made of, we might be able to say if these planets have life. This is a first small step on the way there.
How did the team map the temperature of such a distant planet?
Watching the planet at various points along its orbit We see several fractions of the planet's daytime side. When we look at the star's light, we see a big slump as the planet passes in front of the star we call Transit. On the way behind the star, we see a smaller burglary, which we call a secondary eclipse. The height of this burglary gives us a limitation on the surface temperature of the planet. We can also look for variations in the starlight that give a temperature map with the day and night side.
We can pretty well limit orbit; We know how close it is to its star and how bright the star is, so we essentially know how much light the planet receives from the star. We use models of stellar evolution to understand how much light the planet has received throughout its life.
What did the data tell you about its atmosphere?
An atmosphere can absorb heat from the stern and move it. If the planet has no atmosphere then one would expect a big contrast between the day and the night side. Two signatures of the atmosphere are a shift of the highest temperature point and a lower amplitude of this signature, indicating that heat is being moved. With this particular planet – one of the first rocky planets from which this kind of measurement could be performed – we found a large temperature contrast between the day and night side and no offset of that temperature point. This indicated that the atmosphere had to be very thin.
My contribution was to determine if the atmosphere is stable by performing models to study how much atmosphere the planet could potentially lose over the life of the planet for a range of parameters. If the planet had started with about the same amount of gases as water and carbon dioxide, or even more like Earth, it would have lost all those gases in the course of its life because the star has heated the atmosphere and allowed it to flee – that's one Mechanism for escape from the atmosphere. We looked at another model that limited the lower end of the atmosphere the planet could have, and found that these thin atmospheres are not stable on this planet.
Why Do You Focus Your Research on Atmospheric Escape Models?  I started understanding early planet atmospheres a few years ago, before I even started school. For me it is one of the most interesting problems because it is the early state of the planet that really seems to determine how it develops in the course of its life. This is really important for Earth, because we do not know much about its early history in the first half billion years – but at that time, life began. So my perspective is that you have to start at the beginning. And that actually means starting with it before the planet forms and trying to understand all the processes that are leading to the creation of the planet, and what are the initial conditions from which it eventually evolves. When we look at these hot, rocky exoplanets, we can test our understanding of these processes.
A rocky exoplanet in Earth size lacks an atmosphere
Laura Kreidberg et al. Lack of a dense atmosphere on the terrestrial exoplanet LHS 3844b, Nature (2019). DOI: 10.1038 / s41586-019-1497-4
Questions and answers: Scientists model the atmosphere of an exoplanet (2019, 23 August)
retrieved on 23rd August 2019
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