Let's imagine you're vacationing on WASP-76b, a gas-giant exoplanet in the constellation Pisces, some 640 light-years away. The good news is that you're on the super-hot planet's "cool" side. The bad news? Molten iron is raining down and drifting your way from the hot side. "One could say that this planet gets rainy in the evening, except it rains iron," notes astronomer David Ehrenreich of the University of Geneva in Switzerland.
A hellish place to visit
Image source: Dotted Yeti/ShutterstockWASP-76b is one of the most extreme exoplanets astronomers have laid eyes on so far, and both its day and night sides are way to hot for us. It was first identified using the Very Large Telescope (VLT) at the European Southern Observatory (ESO) Cerro Paranal, 2,600 meters above sea level in the super-dry Atacama Desert in Chile. Recent observations of the gas giant's chemistry by the ESO VLT's ESPRESSO instrument revealed its bizarre extremes and iron rain. ESPRESSO is an acronym for the "Echelle SPectrograph for Rocky Exoplanet and Stable Spectroscopic Observations."
WASP-76b is "tidally locked" to its star, meaning that the same side is always facing its sun. The day side gets up to 2400° C (4352°F) while the night side maxes out at a balmy 1,500° C (2732° F).
That day side is so hot that molecules separate into atoms, and metals like iron evaporate into the blistering atmosphere. ESPRESSO's data posed an intriguing question. "The observations show," says astrophysicist María Rosa Zapatero Osorio, "that iron vapor is abundant in the atmosphere of the hot day side of WASP-76b." However, that strong iron signature at the evening boundary between the two sides of WASP-76b was nowhere to be seen near the morning edge. Where could it go? Astronomers believe that the atmospheric and rotational winds carry a fair amount of the iron vapor to the night side where it falls as molten iron rain.
Because exoplanets are always so normal?
Image source: Jurik Peter/Shutterstock
Well, not really. While searching for Earth-like exoplanets, astronomers keep finding a planets that definitely don't qualify.
WASP-76b is way too hot, for example, but it's hardly the only orbiting inferno. Consider HD 149026b, whose surface temperature is just slightly cooler than WASP-76b, coming in at just over 2,000° C (3632°F).
Then there's OGLE-2005-BLG-390Lb, a big — 5.5 times the size of Earth — rocky sphere that's so so cold: -220° C (-364°F). Brrr.
SWEEPS-10 is really close to its star, just 1.2 million kilometers, and races around its sun every 10 hours, as opposed to our roughly 365 days. That proximity means that it's particularly vulnerable to eventually being pulled apart, unless it slips away. And CoKu Tau/4 is just a mere babe, only a million years old.
An immediate win for ESPRESSO
Image source: ESO/M. Zamani/Wikimedia
Credit for the new, deeper understanding of WASP-76b must go to ESPRESSO, which was designed to help search for Earth-like planets around Sun-like stars. In fact, the WASP-76b insights were derived from the instrument's very first observations made in September 2018 by the scientific consortium of experts from Portugal, Italy, Switzerland, Spain and the ESO responsible for the instrument.
Insights such as these have changed the way scientists see ESPRESSO. It's not just for finding exoplanets, but also helping explain them. "We soon realized that the remarkable collecting power of the VLT and the extreme stability of ESPRESSO made it a prime machine to study exoplanet atmospheres," says Pedro Figueira, ESPRESSO instrument scientist at ESO in Chile.
Meanwhile the search for additional exoplanets continues. At the time of writing, 4,135 exoplanets have been found with 55 of them considered potentially habitable by life as we know it.
While exoplanets appear to be plentiful outside our solar system, the moons that we might expect to be orbiting them are another story. Indeed, last spring it looked like astronomers had finally found one — it was dubbed Neptmoon because of its great size — but that finding now appears less certain.
In light of this quandary, a new paper, published on June 27, looks at what might be happening to "exomoons" that orbit large gas-giant planets migrating inward toward their stars, such as our own Jupiter seems to have done.
The researchers — astrophysicist Mario Sucerquia and colleagues — hypothesize that these satellites break free of their tidal connection to their "parent" planets as they move nearer to their sun. The paper suggests that, at this point, they're not quite moons anymore — or planets — but "ploonets."
What's more, our own moon, the researchers say, may meet a similar fate one day, even though Earth isn't a gas giant. Warns Sucerquia:
"Earth's tidal strength is gradually pushing the moon away from us at a rate of about 3 centimeters a year. Therefore, the moon is indeed a potential ploonet once it reaches an unstable orbit."
Image source: JPL/BigThink
The research in the new paper is grounded on the manner in which large gas giants have been observed to slowly move inward through their solar systems toward their respective suns. It suggests that, as such a body draws close to the star, its moon's orbit — affected at that juncture by both the gravitational pull of the planet and the host star — experiences an increase in energy, which becomes unstable. This, eventually, releases the moon from the gravitational bonds of its parent parent.
Further, the paper's conclusions are based on a series of computer simulations that researchers conducted regarding what would happen to a moon orbiting a migrating gas giant. What was discovered?
The models found that 44 percent of the moons would meet their demise by being pulled into their planets (this could explain some of the planetary rings that have been observed). The system's star would seize and destroy another 6 percent. Significant amount of exomoons, however, — about 48 percent of them — would split off from their planets and begin orbiting their star as "ploonets." Around 2 percent would be blown out of their solar system altogether.
This would certainly explain why we haven't definitively found any evidence of exomoons yet.
We know now today that our planet isn’t special in certain other respects. In terms of inhabiting a “goldilocks zone” which could harbor life, Earth is not the only planet that’s neither too hot nor too cold. Liquid water and an atmosphere are no longer considered luxuries, either. In short, given this and the mathematical possibilities, many scientists believe it’s only a matter of time before we find life somewhere else. After all, hundreds of billions of stars inhabit our galaxy alone, which assumes hundreds of planets the right size within the habitable zone. And that’s a very conservative assessment.
Kepler is a space telescope designed to scour a section of the Milky Way in order to find exoplanets. An exoplanet is one found beyond our solar system. Before Kepler, astronomers were wondering if planets themselves were common or rare. Launched in 2009, Kepler has discovered over 2,500 confirmed exoplanets, and 30 in the habitable zone—each less than twice the size of Earth. Today, there are currently 3,500 confirmed exoplanets total.
The Kepler spacecraft. Credit: NASA.
The problem with Kepler is that it collected reams of reams of data, so much that no one could go through it all. Scientists chose to select those candidates with strong signals. Weak signals could be a treasure trove for A.I., however. So Christopher Shallue, a senior software engineer at Google Brain, and Andrew Vanderburg, a NASA Sagan Postdoctoral Fellow at the University of Texas at Austin, decided to have a crack at it. They employed machine learning, a fascinating new A.I. field that’s making some incredible headway. Their A.I. utilizes artificial “neural networks” modeled after our brain, albeit a far simpler version.
In a recent press conference, Shallue and Vanderburg explained how they trained their A.I. program to identify exoplanets from the Kepler database. According to the scientists, you train neural networks not by programming them but by exposing them to what you want them to recognize. For instance, if you want it to identify puppies and kittens, you show it plenty of pictures of them. After a while, it’ll get good at recognizing them.
Except, here they didn’t show it pets. Instead, they taught it to read minuscule light changes in the brightness of a star which occur when a planet transits or passes in front of it. After enough practice, they let it loose on light recordings captured by Kepler. What the A.I. discovered is that our solar system isn’t as unique as we thought. Instead of being the only eight-planet one, we’re now one of two (that we know of).
NASA introduced the Kepler 90 system in an historic announcement on December 14, 2017. "The Kepler-90 star system is like a mini version of our solar system. You have small planets inside and big planets outside, but everything is scrunched in much closer," said Andrew Vanderburg, astronomer and NASA Sagan Postdoctoral Fellow at The University of Texas, Austin.
Experts speculate that an eight-planet solar system may in fact be common. It also seems that having a solar system where the smaller planets are in the front and the larger ones in the back, may not be so rare. Hopefully, future explorations can help to understand exactly how planetary systems form, as the discovery of new exoplanets has disrupted many of the theories astronomers developed from studying our solar system.
Credit: NASA/Ames Research Center/Wendy Stenzel.
The discovery began with just one planet. The A.I. found a weak transit signal from a planet known as Kepler-90i that had been previously missed. It’s in a planetary system called Kepler-90, located in the constellation Draco, some 2,545 light years from Earth. The new planet is extremely hot, with an average surface temperature of 800 degrees Fahrenheit—about as hot as mercury. Its year is incredibly short. It orbits its star once every 14.4 days. This system probably isn’t the best candidate for life. What’s game changing besides our solar system slipping from its unique place, is the ability to use machine learning to detect previously unrecognized exoplanets. The program also located a 6th planet in the newly discovered TRAPPIST system.
A.I. had been used previously to scour the Kepler database. But these findings show that artificial neural networks are particularly adept at the task. The idea was first posited by Google’s Shallue, who while studying astronomy in his free time, heard about how the discipline was drowning in data. "Machine learning really shines in situations where there is so much data that humans can't search it for themselves,” he said.
Paul Hertz is the director of NASA’s Astrophysics Division in Washington. He said, “Just as we expected, there are exciting discoveries lurking in our archived Kepler data, waiting for the right tool or technology to unearth them.” He added, “This finding shows that our data will be a treasure trove available to innovative researchers for years to come.” Shallue and Vanderburg consider this a successful proof of concept study. In it, explained in a paper to be published in The Astronomical Journal, the A.I. scanned 670 stars. In the future, they plan to have it study all 150,000 stars Kepler has identified.
Today, we still consider our planet special, as it’s the only known place to harbor life. One wonders for how long it’ll hold this lofty location. So far, the A.I. used can’t determine whether an exoplanet is a good candidate for life. But with technology and computing power moving so fast, that ability shouldn’t be too far away.
To see NASA’s video of this discovery, click here:
