Planet

It’s time to redefine what qualifies as a planet

Pluto
PLANET OR NOT? A group of planetary scientists label Pluto and many other orbs in the solar system as planets, despite the definition set down by the International Astronomical Union in 2006.

Pluto is a planet. It always has been, and it always will be, says Will Grundy of Lowell Observatory in Flagstaff, Arizona. Now he just has to convince the world of that.

For centuries, the word planet meant “wanderer” and included the sun, the moon, Mercury, Venus, Mars, Jupiter and Saturn. Eventually the moon and sun were dropped from the definition, but Pluto was included, after its discovery in 1930. That idea of a planet as a rocky or gaseous body that orbited the sun stuck, all the way up until 2006.

Then, the International Astronomical Union narrowed the definition, describing a planet as any round object that orbits the sun and has moved any pesky neighbors out of its way, either by consuming them or flinging them off into space. Pluto failed to meet the last criterion (SN: 9/2/06, p. 149), so it was demoted to a dwarf planet.

Almost overnight, the solar system was down to eight planets. “The public took notice,” Grundy says. It latched onto the IAU’s definition — perhaps a bit prematurely. The definition has flaws, he and other planetary scientists argue. First, it discounts the thousands of…

New solar system found to have 7 Earth-size planets

seven planets
seven planets

Astronomers have just identified a nearby solar system hosting seven Earth-sized planets. Most intriguing: Three planets that orbit its central star — known as TRAPPIST-1 — may even be within a habitable zone. That means they fall within a region that could support life as we know it. As such, these newfound worlds are good sites to focus a search for alien life.

TRAPPIST-1’s big planetary family also hints that many more cousins of Earth may exist than astronomers had thought.

“It’s rather stunning that the system has so many Earth-sized planets,” says Drake Deming. He’s an astronomer at the University of Maryland in College Park. It seems like every stable spot where a planet could be, there is an Earth-sized one. And that, he adds, “bodes well for finding habitable planets.”

Astrophysicist Michaël Gillon works at the University of Liège in Belgium. He was part of a team that last year announced they had found three Earth-sized planets around TRAPPIST-1. This dwarf star is only about the size of Jupiter. It’s also much cooler than the sun. And it’s a relative neighbor to Earth, a mere 39 light-years away in the constellation Aquarius.

Follow-up observations with the Spitzer Space Telescope and additional telescopes on the ground now show that what first had appeared to be a third planet is actually a quartet of Earth-sized ones. Three of these may be habitable.

If those planets have Earthlike atmospheres, their surfaces may even host oceans of liquid water. Or at least that’s what Gillon and his colleagues reported online February 22 in Nature. Their data also offer signs of a seventh, outermost planet.

How they spotted the new worlds

All seven planets were detected by watching how their star dims as each passes — or transits — in front of it. Scientists measured how much of the star’s light each transit blocked from Earth’s view. Knowing how big a planet would have to be to do that, the astronomer calculated that all seven must have roughly the same radius as Earth.

Those dips in starlight also showed how fast the planets orbit their star: The innermost one makes a round trip in 1.5 Earth days. The outermost one takes roughly 20 days.

The planets’ masses range from about half to 1.5 times that of Earth. To figure that out, the researchers looked at the way the six inner planets tug on each other. The mass and size data then allowed the team to calculate the planets’ densities. All of this suggested that the inner six are rocky, as Earth is.

The length of each planet’s day — how quickly it spins on its axis — may sync with its sun’s orbit. That would make the innermost planet’s day 1.5 Earth days long and the outermost one’s 20 Earth days long. That would be like Earth rotating once in 365 days…

Astronomers Find Seven ‘Earth-Like’ Planets Orbiting a Cool Star

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 1200F.) The researchers write that three of the planets—E, F, and G—“could harbor water oceans on their surfaces,…

5 Cratered Facts About Mercury

Mercury is the closest planet to the sun, and you would be forgiven for thinking it’s just a boring lump of rock. After all, there’s nothing interesting on it like aliens or places to drink. Still, Mercury is probably a more interesting place than you think…

1. IT HAS WATER ICE.

Although you would be forgiven for thinking Mercury, being the closest planet to the Sun, is one of the hottest places in the solar system, it in fact is subject to enormous temperature fluctuations. Some regions reach 800°F, but without an atmosphere to retain heat, Mercury’s poles and nightside of the planet plunge well below even the coldest temperatures recorded on Earth. With some regions never getting above -279°F, conditions are perfect for ice to form. Mercury’s regolith is home to possibly a trillion tons of ice, which would make it one of the wettest places in the solar system.

Fitting, then, that the ancient Chinese called it the Water Star.

2. IT ONCE HAD AN IMAGINARY FRIEND CALLED VULCAN.

In the 19th century, scientists were smug in the knowledge that they knew everything there was to know. There were of course a couple of curiosities that they could not understand, one being the precession in the orbit of Mercury. That is to say, Mercury goes around the Sun in an ellipse rather than in a circle, and the ellipse changed the direction it was pointing from time to time. It was thought that there must be a new planet between Mercury and the sun that was changing its orbit—the planet Vulcan. But despite trying, the planet could not be observed.

Albert Einstein eventually disproved the theory of Vulcan via the general theory of…

5 Strange Facts About the Planet Earth

You know what it’s like: You live somewhere all your life but never realize just how great it is until someone comes to visit. While it’s just a shame we don’t get any visitors to marvel at all the peculiarities of our home planet, here are five facts you might still appreciate.

1. EARTH’S A GIANT DYNAMO.

The core of the Earth is a solid lump of nickel and iron, rotating in a sea of molten iron and nickel. This rotation functions the same way winding up a hand-held generator does, giving Earth an enormous magnetic field that extends up to 50,000 kilometers out into space. This magnetic field is crucial for life on Earth, as without it we would be exposed to the full force of the Sun’s radiation. As well as causing cancers and other radiation-aggravated conditions, the radiation’s sheer force would blow our atmosphere into space, as happened with Mercury, and to a lesser extent, Mars. Instead, charged particles are (mostly) harmlessly deflected away, giving rise to the auroras.

It’s not all good though: Any particles that hit the Earth head-on tend to get trapped in the field and can’t get out. These so-called Van Allen Radiation Belts can pose a hazard for astronauts who leave low Earth orbit.

2. IT’S THE DENSEST PLANET IN THE SOLAR SYSTEM.

While Earth may not be the biggest planet in the system, it is the biggest rocky planet in the solar system, and also the densest. Therefore, Earth has by far the highest surface gravity of any terrestrial object in the solar system. This is both a blessing and a curse.

The reason for the high density is the large deposits of heavy elements in the Earth’s makeup. Elements such as lead and uranium are much rarer on other worlds, which gives us a huge advantage in the amount and variety of construction materials available here on Earth. The high gravity has also demanded that humans develop the reflexes and endurance necessary to cope with such gravity, meaning we are far more durable than the potential delicately boned, sloth-like creature we could be had we evolved in low gravity.

Unfortunately, that high gravity makes Earth the worst place…

Venus Will Be Bright and a Little Strange Tonight

Look at the sky just after sunset tonight, January 12, and you’re guaranteed to know immediately which point of light is Venus. Hint: It will be the astonishingly bright one. Now get your telescope. As the planet arrives at its greatest angular distance to the east of the Sun, you won’t be able to see surface features, as you sometimes can when looking at Mars, or identify stunning cloud swirls as you might see when viewing Jupiter, because Venus’s thick and unforgiving clouds conceal the planet’s mountains below. But with the help of a telescope, you’ll be able to see that Venus doesn’t appear to be a full circle.

Here’s why: it’s at greatest eastern elongation tonight. What’s that? Elongation is the angle between the planet and the Sun from Earth. To understand elongation, point at the Sun as it sets. With your other hand, point at Venus. Simply put, the angle that your arms make is the elongation. Because planets are ever in motion and orbiting at speeds different from one another, that angle is ever in flux. Repeat this process in March and you’ll notice a big difference in the directions of your arms.

The largest this angle will ever get in an orbit is its greatest elongation. When greatest elongation occurs at sunset, it’s said to be at greatest eastern elongation. That’s what we have tonight. When it occurs at sunrise, it is at greatest western…