Space Friday: Phobos and Distant Objects

Phobos in Trouble

The larger of Mars’s two moons, Phobos, is in the process of being torn apart by its parent planet, according to findings presented at the annual Meeting of the Division of Planetary Sciences of the American Astronomical Society this week.

Phobos, a small, irregularly shaped moon has has long, shallow grooves across its surface. For years, scientists believed these were the result of an massive collision with an asteroid that created the moon’s six-mile (9.5-kilometer) wide Stickney Crater and nearly shattered the 11–17 mile (18–28 kilometer) diameter moon. However, new analysis of these grooves show that they do not originate from the crater, but rather from another nearby point. Instead of shatterlines, these grooves may be “stretch marks” that occur as Phobos is deformed by tidal forces from Mars—which makes them early signs of structural failure that will eventually destroy the moon.

Just like Earth and its moon, Mars and its moon gravitationally tug on each other, which causes the tidal forces. The closer the two objects, the stronger the pull, and Phobos is closer to Mars than any other moon to its planet—a mere 3,700 miles (6,000 kilometers) compared to the Earth–Moon distance of about 240,000 miles (390,000 kilometers). At that distance, the tidal force inflicted in Phobos generates enough stress to create the grooves.

Phobos’s structure does not help. The moon has a low density, meaning it is not solid rock. Instead, the exterior is thought to be fairly cohesive and elastic while internally it is more of a delicate rubble pile. This kind of interior has little strength and distorts easily, forcing the outer layer to adjust. The outer layer is mostly flexible enough to make this adjustment, but it builds up stress that eventually cause the exterior to fail and fracture the surface.

And Mars’s influence will only get stronger, as it drags the moon about 6.6 feet (two meters) closer every century. This probably means Phobos is rapidly approaching its demise—sometime in the next 30–50 million years. –Elise Ricard

New potential dwarf planet? Far Out!

Japan’s Subaru telescope—a 27-foot (8.2-meter) powerhouse of an observatory—has just gathered the first ever evidence of a new solar system object dubbed V774104. The new object is thought to be the farthest object observed within the confines of our solar system—a whopping nine billion miles (15 billion kilometers) from our sun. Observing the object from such a great distance, we don’t know much about it, but estimates on its size lie between 500 meters and a kilometer (0.3 to 0.6 miles) in diameter. In order for scientists to refine their estimate of the object’s size, more information is needed. The next step will be to determine the trajectory of the object to find out whether it is inbound (heading toward the Sun) or outbound (heading away from the Sun).

Far-flung objects such as this are of great interest to astronomers since they give clues to how our solar system formed. Other exciting far outer solar system objects include Pluto and its companion Charon (visited recently by the New Horizons mission), Eris and its moon Dysnomia (the previously “farthest known” objects), and a potential ninth planet in the Solar System (a theoretical “superearth” that may have influenced the orbits of nearby dwarf planets).

Where do these worlds come from? Where are they going? There are lots of unknowns in this part of the Solar System, the best we can do to learn more is keep looking! (Find out more about objects in our solar system in the upcoming Morrison Planetarium show, Incoming! Opening in March 2016.) –Josh Roberts

“The Most Important Planet Ever Found Outside the Solar System”?

Astronomers are excited by a recently-announced exoplanet circling a star in the southern constellation Vela the Sail. The star, designated GJ 1132 (after Wilhelm Gliese and Hartmut Jahreiss, who included it in a catalog of nearby stars that they compiled) is 230 trillion miles away—a distance that takes its light 39 years to cross—so astronomers say it’s 39 “light years” away.

GJ 1132b has an estimated diameter about 16 percent larger than Earth’s, with a similar density to our home planet. Unlike the giant, gaseous planets that make up most of the known exoplanets, GJ 1132b may be a rocky, or ”terrestrial,” body. However, unlike Earth, which orbits the Sun once every 365.25 days at a comfortable distance of 93 million miles, the orbital period of GJ 1132b is less than two days, indicating that it’s extremely close to its star—only about 1.4 million miles (2.2 million kilometers) out, or 1/25 the distance from Mercury to the Sun. Normally, planets orbiting so close to sunlike stars are heated to thousands of degrees, but GJ 1132 is a red dwarf (also called an “M-dwarf”), a type of star much smaller and cooler than our sun, with a diameter 21 percent that of the Sun and only 0.5 percent the brightness. As a result, the planet’s surface temperature is estimated to be in the neighborhood of 500°F (260°C): “significantly cooler than any other exoplanet confirmed to be rocky,” according to a statement from the Harvard-Smithsonian Center for Astrophysics. Although that’s still too hot for liquid water to exist, and even though the planet receives nearly 20 times more radiation from its star than Earth gets from the Sun, astronomers think its atmosphere may not have been blasted away by the heat of the nearby star.

At its 39 light year distance, GJ 1132b is one of the closest exoplanets to Earth, and the relative sizes of it and its star make the effect of a transit more noticeable as the planet passes between Earth and the distant star, blocking off a miniscule portion of its light. This makes measurements easier, leading to more accurate calculations of its mass, density, and orbital distance. With next-generation telescopes lining up on the horizon, including the James Webb Space Telescope, the larger, more powerful successor to the venerable Hubble Space Telescope, planetary scientists hope to be able to examine the atmosphere of GJ 1132b, possibly determining its chemical composition, color, wind patterns.

According to Zachory Berta-Thompson, lead author of the research paper, this gives scientists the opportunity to actually test theories that have been developed regarding planet formation by backing them up with direct observation. For this reason, GJ 1132b may be a “planetary Rosetta Stone,” leading the way to a better understanding of planet formation processes, a prospect that has prompted astronomer Derek Deming (who was not involved in the GJ 1132b research) to call it “arguably the most important planet ever found outside the solar system.” –Bing Quock

​Image: NASA/JPL-Caltech/University of Arizona