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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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10 Astonishing Things You Should Know About the Milky Way
Anne Dirkse, Flickr // CC BY-SA 2.0
Anne Dirkse, Flickr // CC BY-SA 2.0

Our little star and the tiny planets that circle it are part of a galaxy called the Milky Way. Its name comes from the Greek galaxias kyklos ("milky circle") and Latin via lactea ("milky road"). Find a remote area in a national park, miles from the nearest street light, and you'll see exactly why the name makes sense and what all the fuss is about. Above is not a sky of black, but a luminous sea of whites, blues, greens, and tans. Here are a few things you might not know about our spiraling home in the universe.

1. THE MILKY WAY IS GIGANTIC.

The Milky Way galaxy is about 1,000,000,000,000,000,000 kilometers (about 621,371,000,000,000,000 miles) across. Even traveling at the speed of light, it would still take you well over 100,000 years to go from one end of the galaxy to the other. So it's big. Not quite as big as space itself, which is "vastly, hugely, mind-bogglingly big," as Douglas Adams wrote, but respectably large. And that's just one galaxy. Consider how many galaxies there are in the universe: One recent estimate says 2 trillion.

2. IT'S JAM-PACKED WITH CELESTIAL STUFF.

artist's illustration of the milky way galaxy and its center
An artist's concept of the Milky Way and the supermassive black hole Sagittarius A* at its core.
ESA–C. Carreau

The Milky Way is a barred spiral galaxy composed of an estimated 300 billion stars, along with dust, gas, and celestial phenomena such as nebulae, all of which orbits around a hub of sorts called the Galactic Center, with a supermassive black hole called Sagittarius A* (pronounced "A-star") at its core. The bar refers to the characteristic arrangement of stars at the interior of the galaxy, with interstellar gas essentially being channeled inward to feed an interstellar nursery. There are four spiral arms of the galaxy, with the Sun residing on the inner part of a minor arm called Orion. We're located in the boondocks of the Milky Way, but that is OK. There is definitely life here, but everywhere else is a question mark. For all we know, this might be the galactic Paris.

3. FOR A SPIRAL GALAXY, IT'S PRETTY TYPICAL …

If you looked at all the spiral galaxies in the local volume of the universe, the Milky Way wouldn't stand out as being much different than any other. "As galaxies go, the Milky Way is pretty ordinary for its type," Steve Majewski, a professor of astronomy at the University of Virginia and the principal investigator on the Apache Point Observatory Galactic Evolution Experiment (APOGEE), tells Mental Floss. "It's got a pretty regular form. It's got its usual complement of star clusters around it. It's got a supermassive black hole in the center, which most galaxies seem to indicate they have. From that point of view, the Milky Way is a pretty run-of-the-mill spiral galaxy."

4. …AND YET IT STANDS OUT AMONG ALL GALAXIES.

On the other hand, he tells Mental Floss, spiral galaxies in general tend to be larger than most other types of galaxies. "If you did a census of all the galaxies in the universe, the Milky Way would seem rather unusual because it is very big, our type being one of the biggest kinds of galaxies that there are in the universe." From a human perspective, the most important thing about the Milky Way is that it definitely managed to produce life. If they exist, the creatures in Andromeda, the galaxy next door (see #9), probably feel the same way about their own.

5. FIGURING OUT ITS STRUCTURE FROM THE INSIDE IS A CHALLENGE.


John McSporran, Flickr // CC BY 2.0

We have a very close-up view of the phenomena and forces at work in the Milky Way because we live inside of it, but that internal perspective places astronomers at a disadvantage when it comes to determining a galactic pattern. "We have a nice view of the Andromeda galaxy because we can see the whole thing laid out in front of us," Majewski says. "We don't have that opportunity in the Milky Way."

To figure out its structure, astronomers have to think like band members during a football halftime show. Though spectators in the stands can easily see the letters and shapes being made on the field by the marchers, the band can't see the shapes they are making. Rather, they can only work together in some coordinated way, moving to make these patterns and motions on the field. So it is with telescopes and stars.

6. INTERSTELLAR DUST BLOCKS OUR VIEW OF SOME PARTS OF THE GALAXY.

Interstellar dust further stymies astronomers. "That dust blocks our light, our view of the more distant parts of the Milky Way," Majewski says. "There are areas of the galaxy that are relatively obscured from view because they are behind huge columns of dust that we can't see through in the optical wavelengths that our eyes work in." To ameliorate this problem, astronomers sometimes work in longer wavelengths such as radio or infrared, which lessen the effects of the dust.

7. THE MILKY WAY SPINS, BUT ITS SPEED DOESN'T ADD UP …

Astronomers can make pretty reasonable estimates of the mass of the galaxy by the amount of light they can see. They can count the galaxy's stars and calculate how much those stars should weigh. They can account for all the dust in the galaxy and all of the gas. And when they tally the mass of everything they can see, they find that it is far short of what is needed to account for the gravity that causes the Milky Way to spin.

In short, our Sun is about two-thirds of the way from the center of the galaxy, and astronomers know that it goes around the galaxy at about 144 miles per second. "If you calculate it based on the amount of matter interior to the orbit of the Sun, how fast we should be going around, the number you should get is around 150 or 160 kilometers [93–99 miles] per second," Majewski says. "Further out, the stars are rotating even faster than they should if you just account for what we call luminous matter. Clearly there is some other substance in the Milky Way exerting a gravitational effect. We call it dark matter."

8. … AND WE BLAME DARK MATTER FOR THAT.

Dark matter is a big problem in galactic studies. "In the Milky Way, we study it by looking at the orbits of stars and star clusters and satellite galaxies, and then trying to figure out how much mass do we need interior to the orbit of that thing to get it moving at the speed that we can measure," Majewski says. "And so by doing this kind of analysis for objects at different radii across the galaxy, we actually have a fairly good idea of the distribution of the dark matter in the Milky Way—and yet we still have no idea what the dark matter is."

9. THE MILKY WAY IS ON A COLLISION COURSE WITH ANDROMEDA. BUT DON'T PANIC.

andromeda galaxy
The Andromeda galaxy
ESA/Hubble & NASA

Sometime in the next 4 or 5 billion years, the Milky Way and Andromeda galaxies will smash into each other. The two galaxies are about the same size and have about the same number of stars, but there is no cause for alarm. "Even though there are 300 billion stars in our galaxy and a comparable number, or maybe more, in Andromeda, when they collide together, not a single star is expected to hit another star. The space between stars is that vast," Majewski says.

10. WE'RE THROWING EVERYTHING WE HAVE AT STUDYING IT.

There are countless spacecraft and telescopes studying the Milky Way. Most famous is the Hubble Space Telescope, while other space telescopes such as Chandra, Spitzer, and Kepler are also returning data to help astronomers unlock the mysteries of our swirling patch of stars. The next landmark telescope in development is NASA's James Webb Space Telescope. It should finally launch in 2019. Meanwhile, such ambitious projects as APOGEE are working out the structure and evolution of our spiral home by doing "galactic archaeology." APOGEE is a survey of the Milky Way using spectroscopy, measuring the chemical compositions of hundreds of thousands of stars across the galaxy in great detail. The properties of stars around us are fossil evidence of their formation, which, when combined with their ages, helps astronomers understand the timeline and evolution of the galaxy we call home. 

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Mysterious 'Hypatia Stone' Is Like Nothing Else in Our Solar System
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In 1996, Egyptian geologist Aly Barakat discovered a tiny, one-ounce stone in the eastern Sahara. Ever since, scientists have been trying to figure out where exactly the mysterious pebble originated. As Popular Mechanics reports, it probably wasn't anywhere near Earth. A new study in Geochimica et Cosmochimica Acta finds that the micro-compounds in the rock don't match anything we've ever found in our solar system.

Scientists have known for several years that the fragment, known as the Hypatia stone, was extraterrestrial in origin. But this new study finds that it's even weirder than we thought. Led by University of Johannesburg geologists, the research team performed mineral analyses on the microdiamond-studded rock that showed that it is made of matter that predates the existence of our Sun or any of the planets in the solar system. And, its chemical composition doesn't resemble anything we've found on Earth or in comets or meteorites we have studied.

Lead researcher Jan Kramers told Popular Mechanics that the rock was likely created in the early solar nebula, a giant cloud of homogenous interstellar dust from which the Sun and its planets formed. While some of the basic materials in the pebble are found on Earth—carbon, aluminum, iron, silicon—they exist in wildly different ratios than materials we've seen before. Researchers believe the rock's microscopic diamonds were created by the shock of the impact with Earth's atmosphere or crust.

"When Hypatia was first found to be extraterrestrial, it was a sensation, but these latest results are opening up even bigger questions about its origins," as study co-author Marco Andreoli said in a press release.

The study suggests the early solar nebula may not have been as homogenous as we thought. "If Hypatia itself is not presolar, [some of its chemical] features indicate that the solar nebula wasn't the same kind of dust everywhere—which starts tugging at the generally accepted view of the formation of our solar system," Kramer said.

The researchers plan to further probe the rock's origins, hopefully solving some of the puzzles this study has presented.

[h/t Popular Mechanics]

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