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Astronomers Find Seven "Earth-Like" Planets Orbiting a Cool Star

An artist's conception of what it might be like to stand on the surface of the exoplanet TRAPPIST-1f. Image credit: NASA/JPL-Caltech.

 
Astronomers say they’ve discovered seven Earth-sized planets in tight orbit around a cool, dim star about 39 light-years from us—and all seven are located in the habitable zone that could potentially host life. This is the first time a planetary system oriented to this kind of star has been detected—and its discovery holds the potential to lead us to a lot more exoplanets. An international team of researchers reported their findings in a letter published today in the journal Nature.

“It’s the first time we have seven planets in this temperate zone … that can be called terrestrial,” lead author Michaël Gillon, of Belgium’s Université de Liège, said in a press briefing. “So many is really, really surprising.”

TRAPPIST-1 is an ultracool dwarf star that’s 1/80th the brightness of the Sun and similar in size to Jupiter. All seven planets in its system are within 20 percent of the size and mass of Earth, and their density measurements indicate they’re likely of rocky composition. They’re clutched by TRAPPIST-1 in tight orbits—all would fit well within the orbit of Mercury. But unlike in our solar system, where such closeness to a hot star renders life impossible, the TRAPPIST-1 planetary system, with its cool celestial heart, could potentially host liquid water and organic molecules.

The first three planets were spotted in early 2016 by some of the same researchers involved in the current findings, including Gillon. As the planets cross in front of the star during their orbits, they cause the star, which emits light in the infrared, to briefly dim. Such transits, or eclipses, provide a common way for astronomers to detect exoplanets.

Using telescopes in Chile, South Africa, Spain, the UK, and Morocco, the researchers followed up on these transit signals multiple times in 2016, most notably in late September with a 20-day, nearly continuous monitoring of the star using the Spitzer Space Telescope, currently located about 145 million miles from us in an Earth-trailing orbit around the Sun. By moving our view off the Earth, researchers were able to detect 34 separate transits. This turned out to be the result of seven planets—six in near-resonant orbit—crossing in front of their home star. (The transit of the seventh was detected only once, so the orbit of this planet, known as TRAPPIST-h, hasn’t been determined yet.)

The planets have relatively narrow surface temperature fluctuations—about 100 degrees—despite their proximity to their home star. (Compare that to Mercury, which has temperature variations of nearly 1200°F.) The researchers write that three of the planets—E, F, and G—“could harbor water oceans on their surfaces, assuming Earth-like atmospheres.”

They’re probably tidally locked, meaning the same hemisphere of each planet always faces the star. Because they’re so close to each other, they can influence each other’s movements, causing eccentric orbits. The result is a planetary system that looks more like Jupiter and its Galilean moons than our own solar system. The planets likely formed outside the system and were pulled into it, and it’s entirely possible the seven so far identified are not alone.

Top row: Artist conceptions of the seven planets of TRAPPIST-1 with their orbital periods, distances from their star, radii, and masses as compared to those of Earth. Bottom row: Data about Mercury, Venus, Earth, and Mars. Image Credit: NASA/JPL-Caltech.

“It’s an exciting discovery," University of Montreal astrophysicist Lauren Weiss tells mental_floss. "The TRAPPIST-1 system demonstrates that even the smallest stars in our galaxy can form a multitude of planets.”

Weiss, who was not involved in the current study, researches exoplanetary systems—their masses, density, composition, and orbital dynamics. “These planets are all of sizes that are consistent with rocky compositions," she says of the TRAPPIST-1 system. "In addition, the mass measurements the authors have conducted are consistent with rocky compositions for the planets.”

Most planet-hunting efforts have been focused on brighter stars and bigger planets—and these efforts have been fruitful. Consider NASA’s Kepler mission: As of today, astronomers using the space telescope have detected 2330 exoplanets.

But the TRAPPIST-1 discovery suggests that we shouldn’t overlook the potential that even cool, dim stars have to lead us to new planets. About 15 percent of stars in our neighborhood are ultracool dwarfs like TRAPPIST-1. Moreover, M dwarf stars like this one are by far the most abundant in the galaxy, says astronomer Jackie Faherty, senior scientist at the American Museum of Natural History, who studies them.

“When I heard that the number of planets around TRAPPIST-1 had increased from three to seven, I was taken aback,” Faherty tells mental_floss. “The thought that the galaxy must be bursting at the seams with planets immediately sprung into my head.”

What makes them especially appealing is that, because they are dim and small, a relatively substantial amount of light is blocked when a near object—like a planet in a close orbit—crosses in front of one. That makes planetary transits easier to spot.

What does this discovery suggest about the number of Earth-like planets in the galaxy? “There are 200 billion stars in our galaxy, so do the count. You multiply by 10, and you have the number of Earth-size planets in the galaxy—which is a lot,” study co-author Emmanuël Jehin, of the Université de Liège, said in the press briefing.

And as for finding life on one of the TRAPPIST-1 planets? Gillon said that, short of traveling to one and collecting a sample, we can’t say for certain whether life exists on any of them, but the presence of certain molecules in combination with one another will be a likely indicator. “If you have methane, oxygen or ozone, and CO2, you have a strong indication of life and biological activity,” he said in the press briefing. The combination is key—the presence of any one of these on its own isn’t enough to indicate biological life, Gillon noted.

According to Gillon, the James Webb Space Telescope—an infrared telescope slated for launch in October 2018—will greatly help in this effort. “Methane and, for instance, water could be detected with the James Webb telescope, and give us a very good insight on the atmospheric properties of the planet,” he said.

Of course, other scientists are continuing their own search for exoplanets. One high-profile initiative coming soon is NASA's Transiting Exoplanet Survey Satellite (TESS), which will study more than 200,000 of the brightest stars for two years in hopes of discovering thousands of exoplanets. It is slated to launch in early to mid 2018.

TESS project scientist Stephen Rinehart tells mental_floss that the TRAPPIST-1 planetary system "actually dovetails very nicely with what TESS is expected to discover. At present, there are only a handful of known exoplanets that are suitable for more detailed study. By focusing on finding planets around bright, nearby stars, we hope that TESS will find some 'siblings' for Trappist-1—other systems nearby with multiple planets in the habitable zone of their host star."

But it's not just identifying more exoplanets that's important—it's closer study of individual planets that we need. Rinehart points out that while planets located in the "habitable zone" sound promising, we don't yet know if even one of them can host life. "We know that there are a lot of small, rocky planets in the habitable zones of their host stars, but look at our own solar system," he says. "Venus, Earth, and Mars are all in (or very nearly in) the habitable zone, all three are small, rocky planets, but the three are completely different! So, if we find an exoplanet that is about the same size and mass as Earth, and that planet is in the habitable zone of its host star, we know that it has the potential to be habitable, but we can’t know that it is habitable without more careful study."

The TRAPPIST-1 researchers are going to continue their own search with the project SPECULOOS (Search for Planets EClipsing ULtra-cOOl Stars).

“We’ve taken a crucial step of finding life out there,” said co-author Amaury Triaud, of the University of Cambridge. “Here if life managed to thrive and release gases similar to those we have on Earth, we will know. We have the right target.”

Editor's note: This post has been updated with additional commentary from TESS project scientist Stephen Rinehart.

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NASA, JPL-Caltech
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It's Official: Uranus Smells Like Farts
NASA, JPL-Caltech
NASA, JPL-Caltech

Poor Uranus: After years of being the butt of many schoolyard jokes, the planet's odor lives up to the unfortunate name. According to a new study by researchers at the University of Oxford and other institutions, published in the journal Nature Astronomy, the upper layer of Uranus's atmosphere consists largely of hydrogen sulfide—the same compound that gives farts their putrid stench.

Scientists have long suspected that the clouds floating over Uranus contained hydrogen sulfide, but the compound's presence wasn't confirmed until recently. Certain gases absorb infrared light from the Sun. By analyzing the infrared light patterns in the images they captured using the Gemini North telescope in Hawaii, astronomers were able to get a clearer picture of Uranus's atmospheric composition.

On top of making farts smelly, hydrogen sulfide is also responsible for giving sewers and rotten eggs their signature stink. But the gas's presence on Uranus has value beyond making scientists giggle: It could unlock secrets about the formation of the solar system. Unlike Uranus (and most likely its fellow ice giant Neptune), the gas giants Saturn and Jupiter show no evidence of hydrogen sulfide in their upper atmospheres. Instead they contain ammonia, the same toxic compound used in some heavy-duty cleaners.

"During our solar system's formation, the balance between nitrogen and sulfur (and hence ammonia and Uranus’s newly detected hydrogen sulfide) was determined by the temperature and location of planet’s formation," research team member Leigh Fletcher, of the University of Leicester, said in a press statement. In other words, the gases in Uranus's atmosphere may be able to tell us where in the solar system the planet formed before it migrated to its current spot.

From far away, Uranus's hydrogen sulfide content marks an exciting discovery, but up close it's a silent but deadly killer. In large enough concentrations, the compound is lethal to humans. But if someone were to walk on Uranus without a spacesuit, that would be the least of their problems: The -300°F temperatures and hydrogen, helium, and methane gases at ground level would be instantly fatal.

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Scott Butner, Flickr // CC BY-NC-ND 2.0
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Look Up! The Lyrid Meteor Shower Arrives Saturday Night
Scott Butner, Flickr // CC BY-NC-ND 2.0
Scott Butner, Flickr // CC BY-NC-ND 2.0

There is a thin line between Saturday night and Sunday morning, but this weekend, look up and you might see several of them. Between 11:59 p.m. on April 21 and dawn on Sunday, April 22, the Lyrid meteor shower will peak over the Northern Hemisphere. Make some time for the celestial show and you'll see a shooting star streaking across the night sky every few minutes. Here is everything you need to know.

WHAT IS THE LYRID METEOR SHOWER?

Every 415.5 years, the comet Thatcher circles the Sun in a highly eccentric orbit shaped almost like a cat's eye. At its farthest from the Sun, it's billions of miles from Pluto; at its nearest, it swings between the Earth and Mars. (The last time it was near the Earth was in 1861, and it won't be that close again until 2280.) That's quite a journey, and more pressingly, quite a variation in temperature. The closer it gets to the Sun, the more debris it sheds. That debris is what you're seeing when you see a meteor shower: dust-sized particles slamming into the Earth's atmosphere at tens of thousands of miles per hour. In a competition between the two, the Earth is going to win, and "shooting stars" are the result of energy released as the particles are vaporized.

The comet was spotted on April 4, 1861 by A.E. Thatcher, an amateur skywatcher in New York City, earning him kudos from the noted astronomer Sir John Herschel. Clues to the comet's discovery are in its astronomical designation, C/1861 G1. The "C" means it's a long-period comet with an orbit of more than 200 years; "G" stands for the first half of April, and the "1" indicates it was the first comet discovered in that timeframe.

Sightings of the Lyrid meteor shower—named after Lyra, the constellation it appears to originate from—are much older; the first record dates to 7th-century BCE China.

HOW CAN I SEE IT?

Saturday night marks a first quarter Moon (visually half the Moon), which by midnight will have set below the horizon, so it won't wash out the night sky. That's great news—you can expect to see 20 meteors per hour. You're going to need to get away from local light pollution and find truly dark skies, and to completely avoid smartphones, flashlights, car headlights, or dome lights. The goal is to let your eyes adjust totally to the darkness: Find your viewing area, lay out your blanket, lay down, look up, and wait. In an hour, you'll be able to see the night sky with great—and if you've never done this before, surprising—clarity. Don't touch the smartphone or you'll undo all your hard ocular work.

Where is the nearest dark sky to where you live? You can find out on the Dark Site Finder map. And because the shower peaks on a Saturday night, your local astronomy club is very likely going to have an event to celebrate the Lyrids. Looking for a local club? Sky & Telescope has you covered.

WHAT ELSE IS GOING ON UP THERE?

You don't need a telescope to see a meteor shower, but if you bring one, aim it south to find Jupiter. It's the bright, unblinking spot in the sky. With a telescope, you should be able to make out its stripes. Those five stars surrounding it are the constellation Libra. You'll notice also four tiny points of light nearby. Those are the Galilean moons: Io, Europa, Ganymede, and Callisto. When Galileo discovered those moons in 1610, he was able to prove the Copernican model of heliocentricity: that the Earth goes around the Sun.

THERE'S BAD WEATHER HERE! WHAT DO I DO?

First: Don't panic. The shower peaks on the early morning of the 22nd. But it doesn't end that day. You can try again on the 23rd and 24th, though the numbers of meteors will likely diminish. The Lyrids will be back next year, and the year after, and so on. But if you are eager for another show, on May 6, the Eta Aquariids will be at their strongest. The night sky always delivers.

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