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Say Hello to the Tiniest Planet Ever Discovered

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By Chris Gayomali

Say hello to Kepler-37b: the tiniest planet ever discovered. At about one-third the diameter of Earth, Kepler-37b is about as big as our moon. It was first discovered by Thomas Barclay at the Ames Research Center in northern California using NASA's planet-hunting Kepler telescope, which simultaneously keeps an eye on 150,000 stars in the night sky for hints of new, potentially inhabitable exoplanets.

Whenever a shadow—large or small—appears in front of one of Kepler's stars, astronomers take a closer took to determine if the visual obstruction might be a previously unseen world. 

Kepler-37b is one such planet. It's 210 light years away, in the constellation Lyra. It took three years for scientists to confirm that the tiny speck passing in front of its host star, Kepler-37, was indeed a floating planet all its own. Unfortunately, 37-b is also a little too close to its host star to be habitable, with surface temperatures soaring to 700 degrees Fahrenheit. 

Although its discovery probably won't help sad Pluto get its planet status back anytime soon (37-b is 3,965 km across versus Pluto's 2,400 km), it does significantly raise the possibility that there are other tiny planets bouncing around the galaxy. Maybe one of these baby Earths will even fall into the Goldilocks Zone of just-right surface conditions for humans to potentially live on.

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NASA / SwRI / MSSS / Gerald Eichstädt / Seán Doran © PUBLIC DOMAIN
Here's the Closest View of Jupiter's Great Red Spot That Humans Have Ever Seen
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NASA / SwRI / MSSS / Gerald Eichstädt / Seán Doran © PUBLIC DOMAIN

NASA's Juno spacecraft completed perijove 7 yesterday, flying nearest to Jupiter in its 53-day orbit and collecting intimate science a mere 5600 miles above the gas giant's cloud tops. This flyby took the spacecraft directly over Jupiter's Great Red Spot, a centuries-old, 10,000-mile-wide vermilion vortex that has long perplexed scientists. Among the storm's unknowns are its depth and perpetuating forces. The first raw images of the Earth-sized hurricane were released today.

"This is a storm that we've been tracking ever since the dawn of modern astronomy, and we're the first generation to get this exquisite level of detail," Leigh Fletcher, a planetary scientist at the University of Leicester, tells Mental Floss. He says that from the spacecraft's perspective, the Great Red Spot would have stretched from horizon to horizon.

Juno has thus far given us a startling new vision of Jupiter—one of teeming teals and swirling storms—and caused scientists to sharpen their pencils and rewrite much of what they knew about the solar system's largest planet. Today's initial image data promise no less a revolution in the scientific understanding of Jupiter.

What does the Great Red Spot look like from an expert's perspective? "I see a swirl of red cloud material as the vortex spirals anti-clockwise, a deep-red heart that coincides with the calm center of the powerful winds, and clusters of small-scale clouds that stand above the red depths," says Fletcher. "There's even evidence of waves in the spiral arms in these breathtaking images. It's an incredible level of detail in an image that's set to become instantly iconic."

sequential views of the great red spot of jupiter
Enhanced, filtered, and color-adjusted images of the Great Red Spot, in sequential order, showing the changing view from the spacecraft as it passed over the 10,000-mile-wide storm.

Today's image release is just a taste of what is to come, of course. The spacecraft had all nine of its science instruments active during the pass, and data are being blasted back to the Deep Space Network at the speed of light. "For me, the real science always starts with spectroscopy," says Fletcher, "assessing the fingerprints of the gaseous composition and aerosols that are present within the storm." Juno's science payload allows scientists to peer hundreds of miles beneath Jupiter's clouds. "For years we've tried to understand how deep [the Great Red Spot] penetrates into the atmosphere, and what might be sustaining it. By probing below the clouds with the microwave instrument, we might just find the answers we've been looking for."

The Juno spacecraft launched on August 5, 2011 and achieved orbit around Jupiter on July 4, 2016. The next flyby of Jupiter will take place on September 1. It will mark the spacecraft's eighth orbit and seventh science flyby.

Want to see more amazing images? Head over to NASA's JunoCam.


Mercury, the diminutive planet closest to the Sun, was notoriously mysterious due to its difficulty to explore. That changed on March 18, 2011, when the MESSENGER spacecraft from Johns Hopkins' Applied Physics Laboratory achieved orbit around Mercury. The mission spent the next four years transforming scientists' understanding of how Mercury works and what it is made of. Mental Floss spoke to Sean Solomon, the principal investigator of MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), to learn what's most interesting about the first rock from the Sun.


Mercury is the smallest terrestrial planet of the solar system. Comparatively, Mercury is about midway in size between Earth's moon and the planet Mars. (Mars is a lot smaller than you might think, and our moon a lot larger.) Mercury is 3032 miles in diameter, which is, as the crow flies, just a little less than the distance from Anchorage to Dallas. Its gravity is 38 percent of Earth's, which means if you weigh 150 pounds here, you'd weigh 57 pounds on Mercury (the same as you would on Mars).

One day on Mercury lasts 59 Earth days, and one year lasts 88, which would make figuring out your age a thorny algebra problem. As you might imagine, days on Mercury can get pretty hot—around 800°F. On Earth a brick of coal at that temperature would burst into flames. (This is not a problem on Mercury, as the planet lacks an atmosphere.) Its nights, meanwhile, are a brisk -280°F. This is the widest day-to-night temperature variation of any planet in the solar system, and would make packing for a trip there very difficult indeed.


Logic would suggest that Mercury is the hottest planet, considering its proximity to the giant fusion reactor at the center of our solar system that is 1,400,000,000,000,000,000,000,000,000,000 cubic meters in volume. The hottest planet honor, however, belongs to its neighbor Venus, one planet away, where the average surface temperature is 864°F. On Venus, lead would melt the way an ice cube melts on Earth.


Pretty much everything about Mercury should astound the casual observer, but what most surprises the principal investigator of MESSENGER, the first orbiter mission there? "The chemistry—that was the biggest surprise," says Solomon, who is also director of the Lamont-Doherty Earth Observatory at Columbia University. "We still don't have a good physical and chemical model for planet formation, and so the result that Mercury is this iron-rich planet, in which the silicate fraction is not only not depleted in elements easily removed by high temperatures, but is more abundant in some of those elements than Earth." The big takeaway from Mercury's chemical profile, Solomon says, is that "we don't really understand how the planets were assembled."


"How did we end up with four bodies of rock and metal that are quite different?" asks Solomon. "Venus and Earth are different because of their different atmospheres. The different evolution of the climate, and the feedback between climate and interior, led to very different tectonic evolution."

Mars and Earth are different because Mars is so much smaller than Earth, only 10 percent of Earth's mass, he explains. As for Mars and Venus: "A lot of Mars's atmosphere was stripped away by the solar wind, so it turned into this cold, barren desert world, whereas Venus has this dense CO2 atmosphere. Runaway greenhouse [effect] turned it into a hothouse world." Earth is in between.

Mercury suggests that the process of planet forming depends on more than simply planet size, solar distance, and differences in atmosphere. The original building blocks of planets also varied across the inner solar system in important ways. "The chemistry varied, volatile abundances varied, and some conditions must have helped during planet formation that can't be ascribed to late-stage processes like a collision," Solomon says.

Now that we've performed one comprehensive study of Mercury, scientists can endeavor to explain the diversity of the terrestrial planets. "We now have filled in the last missing piece in describing the four siblings of that process [of planetary formation]. They're all different, and yet the parental processes, if you will, must have been in common, so it's a kind of planetary genome expression," Solomon says. "How the heck can gene expression be so different among these four siblings, given that they all started out at the same time by the same processes, in just slightly different places in the inner solar system?"


"There are faults all over the surface, and most of those faults involve horizontal shortening," or shrinking. The idea goes all the way back to Mariner 10, a robotic space probe launched by NASA in 1973, says Solomon. "The faults that accommodate horizontal shortening are seen on top of every kind of terrain, and they have a wide range of orientations. The Mariner 10 proposed—and the MESSENGER team confirmed—that contraction has dominated the history of the planet, and is consistent with the planet shrinking over time as the result of interior cooling and contraction of the interior." This tectonic activity has been active over most of the history of the planet, as the planet continues to cool.

But were you to stand on Mercury's surface, you couldn't expect Seti Alpha VI-like cataclysms as the planet suddenly contracts. "Were we to send a seismic experiment to Mercury, we would probably see mercury-quakes not anywhere near the frequency or size of earthquakes, but something more akin to moonquakes," Solomon says.


The orientation of craters found on the poles of Mercury allows for permanently shadowed regions—that is, areas that never receive sunlight, no matter the planet's rotational position or place in its revolution. The conditions in those craters are amenable to stable water ice, on or mere centimeters below the planet's surface. MESSENGER's nuclear spectrometer yielded measurements consistent with water ice on the north pole, and its camera later captured optical-light images of that ice.


Only two missions have thus far explored Mercury: the Mariner 10 space probe in 1974, and the MESSENGER orbiter in 2011. This is in part because of the tremendous challenges associated with visiting the planet. "Mercury is in a challenging environment," says Solomon. "The Sun is 11 times brighter than it is at Earth. The surface temperature of the day-side is very hot. The night-side temperature, however, is quite cold, so the swings in temperature are large. The radiation environment that close to the Sun is challenging, as we anticipated going into the mission. We were hit directly by streams of energized particles from the Sun."

Mariner 10 performed three fast flybys of Mercury, and scientists spent the next three decades working largely from the close-up science it performed. Mariner's findings and the questions they raised would further contribute to the scientific rationale of an orbiter—what would be the eventual MESSENGER spacecraft.

A Mercury orbiter, of course, is no small order, and placing a spacecraft in orbit around that planet is one of the great achievements of the American space program. You can't just fly to Mercury and enter orbit. A spacecraft would be moving at a velocity far too great for that, as Mercury lacks the atmosphere to allow aerobreaking. Instead, a trajectory had to be calculated in which MESSENGER bounced around the solar system, from Earth, around the Sun and back to Earth; around the Sun and to Venus; around the Sun and back to Venus; and around the Sun four more times, flying closer and closer to Mercury each time, until at last it could enter Mercury's orbit. In essence, MESSENGER borrowed the gravity of other planets to compensate for what Mercury could not provide on a direct flight.

Due to this circuitous route, MESSENGER had to travel 5 billion miles over six-and-a-half years to reach a planet 100 million miles away. Once there, the challenge continued. The spacecraft had to maintain an orientation that kept between its scientific payload and the Sun a giant sunshade, lest the Sun fry the instruments. But extreme heat wasn't the only problem. So was extreme cold. When the spacecraft crossed into Mercury's shadow, an onboard heater had to warm the spacecraft lest the instruments freeze.

Despite the challenges, we're going back. The next mission bound for Mercury will launch in 2018. BepiColombo, a joint mission between the European and Japanese Space Agencies, will place two satellites in orbit around Mercury, where they will study its composition, tenuous atmosphere, and magnetosphere. Like MESSENGER, the spacecraft will require a complex trajectory—and a very long time to reach its target. It will achieve orbit around Mercury in December 2025.


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