Space Friday: Sunspot, Starshot, and Planet Hunting

A Giant Sunspot Gazes Earthward

About a week ago, a new active region on the Sun’s surface rotated into view from our star’s farside. Labelled AR 2529, it is a sunspot about the size of Earth that quickly grew larger, becoming visible without telescopic magnification and easily seen at sunset, when the Sun’s brightness is greatly-dimmed by our atmosphere. AR2529 recently morphed from a circular, Earth-sized spot to a much larger heart shape in which about half a dozen Earths could fit, and the Sun’s slow rotation has positioned the spot about halfway across the solar disk. For observers who still want to see AR2529 for themselves, that leaves about a week before it rotates out of sight. However, it will likely continue to evolve during that time, as sunspots are transient phenomena that may last from a few days to a few weeks.

Sunspots are dark areas on the Sun’s roiling surface where our star’s tangled magnetic field lines loop in and out through its visible layer, or photosphere. Suppressing the convection that churns the Sun’s hot, incandescent gases up from below, this cools the site by several thousand degrees—from 5,800 Kelvins (10,000° Fahrenheit) to 3,800K (6,000°F). As a result, sunspots look darker than the rest of the Sun, but only by comparison. They are associated with solar flares and coronal mass ejections, which can spit out large quantities of charged particles, interacting with the magnetic fields and atmospheres of any planets in their path. In the case of our own world, solar activity causes the auroras and can affect not only radio transmissions but also delicate electronics, with consequences for satellite operations, weather forecasting, and airline travel, to name only a few effects. They are all aspects of space weather interactions taking place in the Sun-Earth environment.

Observers wishing to see sunspots should NEVER look directly at the Sun without proper eye protection. You can purchase safe filters in the form of polymerized films and goggles intended for solar viewing. Another safe alternative is to project the image of the Sun backward through a small telescope or pair of binoculars onto a white surface. Or you can view a video stream or daily image feed on the Internet. Learning about safe methods of solar viewing will come in handy not just for observing giant sunspots, but also for the transit of Mercury occurring on the morning of May 9 and the total solar eclipse that cuts across the U.S. next year on August 21. Of course, Science Today will bring you more information as those events draw near. –Bing Quock

Riding the Light

On April 12, 1961, Yuri Gagarin became the first human to go to space, orbiting Earth once and returning home. It was a monumental achievement that changed the way we perceived what was possible. If we can go to space and come home safely, we can do anything!

Fifty-five years later, Yuri Milner, a Russian billionaire named for Gargarin, announced an even more ambitious initiative: Milner wants to send spacecraft to another star. For context, if we sent the fastest spacecraft we currently have toward the closest star system (Alpha Centauri), it would take over 70,000 years to reach that destination. Milner thinks he can do it in only 20 years.

The plan is called Starshot, and the idea is to build a swarm of tiny spacecraft on the order of a few grams—about the size of the electronics in a smartphone if you subtract the case and the display systems. Then attach each of these micro-electronics to a solar sail.

A solar sail is exactly what it sounds like: a large sail that moves by pressure from the Sun. Light hits the sail and imparts a tiny bit of momentum, and with constant pressure from the light, the craft experiences constant acceleration, so it can reach very high speeds. Milner wants to adjust this idea in a significant way. Rather than relying on the force of the Sun, the Starshot program would fire 100 gigawatts of laser light on the sails for two straight minutes. By the time the lasers shut off, the craft would be nearly one million kilometers (600,000 miles) away and travelling at one-fifth the speed of light: 60,000 kilometers (37,000 miles per second) per second.

In a word: awesome. But can it really be done? Stephen Hawking thinks so. So does Mark Zuckerberg, who’s signed on to help with the financing. But it’s going to take a long time—up to 20 years—to develop the technology. To start with, nobody’s ever built a laser array that could produce 100 gigawatts of energy: with current technology, it would require 2.5 square kilometers (that’s a square mile) of lasers. Then, you have to coordinate and sync all those lasers to fire at exactly the right time and into exactly the same one square meter (nine square foot) sail. All of this done using adaptive optics to account for atmospheric interference, which we’ve never attempted on such a huge scale. And if the sail isn’t perfectly reflective and absorbs even one percent of the light, it will simply fry to a crisp rather than blast to the stars.

So a lot of work still needs to happen. And even if they launch in 20 years, it’s going to take 20 more years to make the trip to Alpha Centauri. Along the way, any number of things could hit a spaceship and destroy it, which is why Milner wants to send several small and cheap crafts, hoping at least one will survive the trip. When a probe arrives at the distant star (and its potential exoplanets), it would fly past and take pictures and data as fast as it could. Then it would send those data back to Earth encoded on a much smaller laser. Which means we’ll get the information 4.4 years later. Twenty years to develop, twenty more to travel, and over four to transmit. Mark your calendars for 2061, if it all goes perfectly.

NASA, on the other hand, is currently testing a different solar sail technology called E-Sail. Conventional solar sails get their momentum from light, but as we noted in the sunspot article above, the Sun is also pouring solar wind into space all the time. Solar wind is a collection of high-energy and high-velocity particles, including protons, which have a positive electric charge. By putting a negative electric charge on the sail, the craft can continue accelerating up to three times farther away than conventional sails, and up to three times faster than our current fastest spacecraft! To do this, the E-Sail will be made of ten to twenty rods, 20 kilometers (12.5 miles) in length—and the thickness of a paperclip! Charging these rods will produce an electric field stretching between them, and protons repelled from that field will provide the thrust for the spacecraft, pushing it away from the Sun faster and faster. This project is also in its early stages of testing, but it is being physically tested, which is several steps closer to reality than the Starshot program.

This year, we celebrate 55 years of humans in space. In another 45 years, on the centennial of Yuri Gagarin’s first flight, we could be receiving data from our first close-up exploration of another star system. Talk about changing our idea of what’s possible. –Colin Elliott

Exoplanet Hunting—Old and New Techniques

“It’s amazing to think that two decades ago we’d only just confirmed exoplanets actually existed and now we’re able to refine and improve those methods for further discoveries.” That’s Peter Gao of Caltech. He and his colleagues published a paper this week in The Astrophysical Journal describing one of these refinements.

The team has a new method for finding low-mass exoplanets by examining the radial velocity in the near-infrared. A planet is obviously influenced by the gravity of the star it orbits; that’s what keeps it in orbit. The radial-velocity technique takes advantage of the fact that the planet’s gravity also affects the star in return. As a result, astronomers are able to detect the tiny wobbles the planet induces as its gravity tugs on the star.

Certain kinds of low-mass stars, however, cause problems for the standard radial velocity method, resulting in false positives—in other words, we can find something that looks like a planet, but isn’t.

The astronomers’ new method breaks through these limitations. “Switching from the visible spectrum to the near-infrared, the wobble effect caused by an orbiting planet will remain the same regardless of wavelength,” explains Jonathan Gagné of the Carnegie Institute of Science. “But looking in the near-infrared will allow us to reject false positives caused by sunspots and other phenomena that will not look the same in near-infrared as they do in visible light.”

Radial velocity work in the near-infrared wavelengths has been conducted before, but it has experienced a few technical challenges. The research team was able to develop a better calibration tool to improve the overall technology for near-infrared radial velocity work, which should make it a better option going forward.

They examined 32 low-mass stars using this technological upgrade at the NASA Infrared Telescope Facility atop Mauna Kea, Hawaii. Their findings confirmed several known planets and binary systems, and also identified a few new planetary candidates.

“Our results indicate that this planet-hunting tool is precise and should be a part of the mix of approaches used by astronomers going forward,” says Gao.

Another tool for exoplanet hunters going forward? Century-old astronomical glass plates, according to another recent study.

Jay Farihi of University College London discovered an exoplanetary system by accident, observing a stellar spectrum on one of these plates, dating back to 1917. Stellar spectra are recordings of the light emitted by distant stars. Spectra spread out all of the component colors of light, like a rainbow from a prism, and typically contain what are called absorption lines. These dark regions in the spectrum reveal a star’s chemical composition—and also indicate how the light emitted by a star is affected by the chemistry of things it passes through on its way to our telescopes here on Earth.

The clue for Farihi was in absorption lines that indicated the presence of heavy elements interfering with the light of a white dwarf. Within the last 12 years, it has become clear to astronomers that white dwarfs with heavy elements in their spectra represent a type of planetary system featuring vast rings of rocky planetary remnants that deposit debris into the stellar atmosphere. These recently discovered systems are called “polluted white dwarfs.” The star on the 1917 astronomical plate is a polluted white dwarf.

“The mechanism that creates the rings of planetary debris, and the deposition onto the stellar atmosphere, requires the gravitational influence of full-fledged planets,” Farihi explains. “The process couldn’t occur unless there were planets there.” –Molly Michelson

Image: Langkawi National Observatory, MALAYSIA