When the James Webb Space Telescope launches in December, astronomers around the world are expecting to find the unexpected, said Sara Seager, astrophysicist and planetary scientist at the Massachusetts Institute of Technology.
The space observatory will be able to look into the distant universe as well as observe objects in our own solar system. But the telescope has almost become synonymous with exoplanets, or worlds outside of our solar system, that it will be able to observe in unique ways.
Telescope time has been granted to a number of proposals by astronomers who want to observe an intriguing range of exoplanets. Some of these could share similar characteristics with planets we recognize in our own cosmological backyard, while others couldn't be more opposite. Webb was not designed to find signs of life on other planets, but it can shed light on the mysteries of planetary evolution, as well as their atmospheres and what chemistry exists within them.
Webb, which observes in infrared light invisible to the human eye, will build on the observations made by other space and ground-based telescopes to help researchers better understand fully formed planets as well as those that are still forming.
Within our solar system, Webb will study our planetary neighbors to see how they have evolved over time in comparison to Earth.
"We are going to look at all the objects in the solar system, beginning with Mars and going all the way out," said John Mather, Webb senior project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. Some key targets of Webb include ocean worlds in our solar system, like Jupiter's moon Europa and Saturn's moon Titan, he noted. Future missions will investigate if life or the chemistry that leads to life is possible on these worlds.
Peering inside planetary atmospheres
Outside of our solar system, bigger questions remain. Webb could shed more light on the types of planets that exist out there beyond our tiny corner of the universe. But it won't be looking into the atmospheres of Earth-like planets around sun-like stars.
Instead, the planets Webb will observe are located around much smaller, cooler stars, which are very common in our galaxy. It's possible that those planets can still be habitable, said Klaus Pontoppidan, Webb project scientist at the Space Telescope Science Institute in Baltimore.
In addition to observing exoplanets and their surfaces, Webb will actually be able to peer inside their atmospheres, if these planets have them. There, a veritable rainbow of information awaits. The gases inside the atmosphere of a planet absorb light in specific colors, which will allow scientists to identify them and see the atmospheric composition of an exoplanet.
"Webb means different things to different exoplanet scientists, but for a large number of them it's about studying the atmospheres of exoplanets," Seager said. "And when a planet passes in front of its host star, some of the starlight shines through the atmosphere. And by seeing what light makes it through and what is blocked, we can identify the gases in an atmosphere."
While the Hubble Space Telescope has allowed scientists to do this already, this new telescope "will take us to the next level," Seager said.
Imagine viewing Earth from afar. From that perspective, our atmosphere would appear like a tiny layer of haze above the planet. This is why it's so difficult to sense atmospheres.
"Our dream is to study rocky planets and to see water vapor, which indicates liquid water oceans," Seager said. "If we can establish that rocky planets with water vapor are common, that indicates that rocky planets with water oceans are common. And water is necessary for all life as we know it. So that would be a huge milestone."
And detecting gases that aren't expected or understood could be an even more intriguing find, leading to more questions than answers, she said.
Webb will also contribute more data that helps scientists see planets in 3D, said Nikole Lewis, astrophysicist and an assistant professor of astronomy at Cornell University. This includes temperature, cloud formation and even being able to understand the weather occurring on other planets to create a better picture of extraterrestrial climates.
"This decade will really be the decade of understanding small planets around small stars and then the decades beyond, we'll be turning our eyes towards small planets around sun-like stars."
Zooming in on TRAPPIST-1
Astronomers announced their discovery of seven Earth-size planets orbiting a star 40 light-years from Earth in February 2017. With the help of the space-based Spitzer telescope, the seven exoplanets were all found in tight formation around an ultracool dwarf star called TRAPPIST-1.
Three of the planets are within the star's habitable zone, where liquid water could collect on the surface of the planet and potentially support life.
Determining whether the planets have atmospheres is the first step. The planets currently orbit an ultracool dwarf star, but they weren't always in such suitable conditions. The star is half the temperature and a tenth the mass of the sun. But it was much hotter earlier in its life span, which would have caused the nearby planets to lose atmosphere, oceans or ice.
"These small red dwarf stars, you have to think of them like they had a very long, very badly behaved teenage phase," Seager said. "And during that time, they were very hot and they gave off a lot of energy. So a planet that's in the habitable zone today was bombarded by heat and high energy and people think it might have lost its atmosphere during that time. So whether or not the atmosphere could be replenished, we're not sure."
The researchers who discovered the TRAPPIST planets found them using the transit method. The astronomers saw shadows, like little eclipses, periodically interrupting the steady pattern of starlight as they observed the star through a telescope. This is called transiting. The shadows indicated planets, and further observation confirmed it.
The same starlight that is used to detect planets passes through the planet's atmosphere. When that happens, astronomers could detect the composition of the atmosphere, if it exists. This method also reveals colors representing different wavelengths of light.
One of the planned targets for Webb is TRAPPIST-1e, which is potentially habitable. The researchers will be looking for signs of an atmosphere, and then specific components, like signatures of carbon dioxide or water, Lewis said.
"Nature has given us a sample of seven roughly Earth-size planets in one system," Lewis said. "In solar system science, we compare planets against each other and that helps us understand how they formed and why Earth is the only planet that currently is known to host life in the solar system. And so the TRAPPIST system is going to give us that same opportunity in an exoplanetary system to look at each of the planets."
The Webb Telescope could sneak a peek at the potential atmospheres of all seven TRAPPIST planets. Observing these worlds can help scientists better understand the evolution of planets, Seager said.
Investigating mysterious worlds
The range of planets Webb will observe is wonderfully diverse, including "Hot Jupiters," "warm Neptunes," planets that get blasted with heat as they closely orbit their star and even worlds that orbit dead white dwarf stars.
Scientists are eager to get a closer look at Beta Pictoris, a young system 63 light-years away that includes at least two planets and small, rocky bodies in a dusty disk. Another target for early in the mission is WASP-18b, a blazing "hot Jupiter" with an atmosphere, according to NASA.
Webb could reveal the physical processes that drive exoplanet diversity, said Natalie Batalha, professor of astronomy and astrophysics at the University of California, Santa Cruz. Batalha also served as co-investigator and Kepler mission scientist for the Kepler mission, which helped find thousands of exoplanets.
"One of the takeaways from Kepler is that the diversity of planets in the galaxy far exceeds diversity of planets in our solar system," Batalha said. "So I think that has very significant impacts on the study of planetary habitability and where the most likely abodes of life are going to be. I'm hoping the Webb sorts that out and that it gives us this new lens on diversity."
One mystery are so-called stripped-core planets. Imagine a planet that formed like the ice giant Neptune, yet close to its star. Over time, the planet's hydrogen envelope was stripped away, leaving a rocky core that looks like Earth, but must be very different inside, Batlaha said.
"So is that stripped core conducive to life? Is that additional real estate for the possibility of life? I don't know," Batalha said.
These are different from how Earth's atmosphere formed. Determining atmospheric tracers using Webb could reveal more about these stripped-core worlds.
Another mystery uncovered by Kepler are planets between the size of Earth and Neptune. Depending on their exact size, they are referred to as Super-Earths or sub-Neptunes, said Johanna Teske, Earth and Planets Laboratory staff scientist at the Carnegie Institution for Science in Washington, DC.
Teske is co-lead on a project led by Batalha's daughter, Natasha Batalha, a scientist at NASA's Ames Research Center, to observe 11 of these planets around eight stars. It's the largest exoplanet program selected for Webb's first observing cycle.
Kepler helped scientists realize "the most common type of planet (out there) is a planet we don't even have in our solar system and it kind of bridges the tiny terrestrial planets that orbit close to their stars and the big gas giants that are a bit far away," Natalie Batalha said.
These planets are common around sun-like stars, often appear in multi-planet systems and appear to be a very common outcome for planet formation, Teske said.
Scientists want to understand if these worlds are more similar to Earth or Neptune -- or if they are something completely different, Teske said. Their project is designed to address how these planets formed as well as their composition and potential atmospheres.
It's possible that the planets are gaseous, terrestrial or even water worlds, so the team will be looking to see if water is in their atmospheres.
The James Webb Space Telescope "is going to break open the atmospheric characterization landscape for exoplanets," Teske said.
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