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 instead of in 24 hours.
Such a spin would keep the same side of a planet facing its star all the time (much as one side of our moon always faces Earth). This would give each of TRAPPIST-1’s planets permanent day sides and night sides. Astronomers feared that would make the planets too hot on the day side and too cold on the night side to be habitable.
But if they have Earthlike atmospheres, three of the planets would still be warm enough all over to have liquid water. And that’s one requirement for a so-called livable “Goldilocks” zone — an environment that’s not too hot or cold to support life. This solar system’s seventh planet probably is icy, Gillon says, perhaps like Jupiter’s moon Europa.
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“We are on the right angle to see this system and its Earth-sized planets,” notes Deming, who was not involved in the study. “For every system we see, there are dozens more that we don’t.” Stars like TRAPPIST-1 with Earth-sized planets are probably not rare, he now suspects. If they were, it could have taken many more observations to find some. In fact, the pilot project by Gillon’s group to study ultracool dwarf stars spotted one quickly. That this star had all of these Earth-size exoplanets suggests that it may be quite normal for such stars to harbor planets similar to Earth.
Studying the atmospheres of such planets could reveal if they have life. One thing to look for: the gases methane and oxygen. Astronomers can look for those atmospheres (if they exist) with the Hubble Space Telescope or its successor, the James Webb Space Telescope (which is due to launch in 2018).
Deming is cautious, however, about how easy it will be to probe for details of planetary atmospheres. The light from ultracool dwarf stars can vary, he notes. And it can be hard to understand how the planets’ atmospheres might behave.
Didier Queloz is a bit more optimistic. He’s an astronomer at the University of Cambridge, in England, and one of the new study’s authors. “We have no idea what these planets look like now. They could be wet or dry. We just don’t know,” he says. “But for the first time since the first exoplanet was discovered 25 years ago, we may be able to answer the question about life beyond our solar system.”
alien A non-native organism. (in astronomy) Life on or from a distant world.
angle The space (usually measured in degrees) between two intersecting lines or surfaces at or close to the point where they meet.
astronomical unit A unit of distance in space that is roughly equal to the average distance of Earth from the sun.
atmosphere The envelope of gases surrounding Earth or another planet.
axis The line about which something rotates. On a wheel, the axis would go straight through the center and stick out on either side.
colleague Someone who works with another; a co-worker or team member.
constellation Patterns formed by prominent stars that lie close to each other in the night sky. Modern astronomers divide the sky into 88 constellations, 12 of which (known as the zodiac) lie along the sun’s path through the sky over the course of a year. Cancri, the original Greek name for the constellation Cancer, is one of those 12 zodiac constellations.
environment The sum of all of the things that exist around some organism or the process and the condition those things create for that organism or process. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature, humidity and placement of components in some electronics system or product.
Europa One of the moons of Jupiter and the sixth-closest satellite to the planet. Europa, 1,951 miles across, has a network of dark lines on a bright, icy surface.
exoplanet A planet that orbits a star outside the solar system. Also called an extrasolar planet.
Goldilocks zone A term that astronomers use for a region out from a star where conditions there might allow a planet to support life as we know it. This distance would be not too close to its sun (otherwise the extreme heat would evaporate liquids). It also can’t be too far (or the extreme cold would freeze any water). But if it’s just right — in that so-called Goldilocks zone — water could pool as a liquid and support life.
habitable A place suitable for humans or other living things to comfortably dwell.
Jupiter (in astronomy) The solar system’s largest planet, it has the shortest day length (10 hours). A gas giant, its low density indicates that this planet is composed of light elements, such as hydrogen and helium. This planet also releases more heat than it receives from the sun as gravity compresses its mass (and slowly shrinks the planet).
light-year The distance light travels in one year, about 9.48 trillion kilometers (almost 6 trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.
mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
methane A hydrocarbon with the chemical formula CH4 (meaning there are four hydrogen atoms bound to one carbon atom). It’s a natural constituent of what’s known as natural gas. It’s also emitted by decomposing plant material in wetlands and is belched out by cows and other ruminant livestock. From a climate perspective, methane is 20 times more potent than carbon dioxide is in trapping heat in Earth’s atmosphere, making it a very important greenhouse gas.
moon The natural satellite of any planet.
online A term that refers to things that can be found or done on the internet.
orbit The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.
oxygen A gas that makes up about 21 percent of the atmosphere. All animals and many microorganisms need oxygen to fuel their metabolism.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now includes eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
radius A straight line from the center to the circumference of a circle or sphere.
range The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. (in math or for measurements) The extent to which variation in values is possible. Also, the distance within which something can be reached or perceived.
solar system The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets. Or a similar system of celestial objects orbiting one or more related stars.
star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.
sun The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Or a sunlike star.
telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.
transit (in astronomy) The passing of a planet across the face of a star, or of a moon or its shadow across the face of a planet.