Big, rogue planets — ones without parent stars — are rare.

A new census of free-floating Jupiter-mass planets determined that these worlds are a tenth as common as previous estimates suggested. The results appear online July 24 in Nature.

Planets can go rogue in two ways: They can get kicked out of their parent planetary systems or form when a ball of gas and dust collapses (SN: 4/4/15, p. 22).

In the new study, Przemek Mróz of the Astronomical Observatory of the University of Warsaw and colleagues estimated the number of large, rogue planets in our galaxy using a technique called microlensing. When an object with a mass of a planet passes in front of a distant, background star, the gravity of the planet acts as a gravitational magnifying glass. It distorts and focuses the light, giving up the planet’s existence.
Mróz and colleagues looked at 2,617 microlensing events recorded between 2010 and 2015 and determined which were caused by a rogue planet. For every typical star, called main sequence stars, there are 0.25 free-floating Jupiter-mass planets, the analysis suggests.

The new result sharply contrasts an estimate published in 2011, which suggested that rogue Jupiters are almost twice as common as main sequence stars. About 90 percent of stars in the universe are main sequence stars, so if that estimate were accurate, there should be a lot of free-floating Jupiters.

“That result changed our conceptual framework of the universe just a little bit,” says astronomer Michael Liu of the University of Hawaii in Honolulu. It challenged long-held ideas about how planets go rogue because the known methods wouldn’t generate enough planets to account for all the wanderers.

The 2011 result was based on a relatively small sample of microlensing events, only 474. Since then, infrared telescope images haven’t detected as many free-floating planets as expected. “Over the years, serious doubts were cast over the claims of a large population of Jupiter-mass free-floaters,” Mróz says.

David Bennett, coauthor of the 2011 study, agrees that the new census failed to find evidence for a large population of Jupiter-mass rogue planets. He notes, however, that the new data do reveal four times as many Jupiter-mass failed stars called brown dwarfs than predicted in the original census. So some of the rogues that were originally classified as planets may, in fact, be failed stars. Bennett, of NASA’s Goddard Space Flight Center in Greenbelt, Md., and colleagues are working on a new analysis of potential rogues with nearly 3,000 microlensing events and plan to compare their results with those from the new census.
Liu says the latest census is much more in line with theories of how planets form. Most rogues should be Earth-mass or a little heavier. Those lighter planets get tossed out of their planetary systems much easier than behemoths like Jupiter. Still, the smaller planets are harder to detect.

The new microlensing analysis did identify several events in which stars brightened and dimmed in less than half a day. Such short events hint at the existence of Earth-mass free-floaters because smaller planets with less gravity should brighten a distant star more briefly than more massive stars. Determining whether those small planets are really rogue and counting how many there are will take better telescopes, the team notes.

A species gets only one chance to explore its solar system for the first time.

For humans, that chance began 40 years ago this month, when the twin Voyager spacecraft embarked on their “grand tour” of the solar system. A new PBS documentary airing on August 23, The Farthest: Voyager in Space, chronicles their journey to send home the first close-ups of the giant planets and to bring a message about life on Earth to the stars.
Voyagers’ launch dates took advantage of a rare planetary alignment. In 1977, the giant planets — Jupiter, Saturn, Uranus and Neptune — lined up in such a way that a spacecraft could swing past all four in less than 15 years, stealing some gravitational oomph from each world as it went.

That lucky alignment happens only once every 176 years. When NASA’s administrator went to President Richard Nixon to ask for funding for Voyager, he allegedly said: “The last time the planets were lined up like that, President Jefferson was sitting at your desk. And he blew it.”

The Voyagers almost blew it, too. The first craft (Voyager 2, confusingly) launched on August 20, 1977. It experienced so much shaking that its onboard computer — which had as much computing power as a modern car key fob — thought it was failing and put itself in safe mode.
Engineers got it back on track and fixed the problem for Voyager 1’s launch. Then that spacecraft’s rocket had a fuel leak during launch. The craft was within 3½ seconds of running out of gas before it accelerated enough to reach Jupiter.

These nail-biters are mostly told through personal, entertaining anecdotes from Voyager team members. Historical footage from press conferences and newscasts grounds the story in its era. Everyone has big ’70s computers and big ’70s hair. Cuts from shots of the scientists today to their younger selves emphasize how much time has passed. It’s strange that such a high-tech and ambitious mission seems so vintage.

Even the Voyager footage of Jupiter and Saturn coming into view for the first time has a home video quality, especially compared with the sharp, colorful images that spacecraft send back from these planets today. Watching the footage felt like watching video of my parents’ wedding: I recognize everyone, but they look so different.

But the sense of awe that the Voyager images sparked is palpable. At the time, every picture was the best planetary picture ever taken. Much of what is known about the outer solar system now — Jupiter’s moon Io has volcanoes, Europa has an ocean, Neptune has a great churning hurricane that never stops — was glimpsed for the first time with Voyager.

The Voyager spacecraft are still out there, and one may have already left the solar system (SN: 8/23/14, p. 6). Good thing because both craft carry a message in a bottle: the Golden Record.

The Golden Record was a literal record to be played on a phonograph by any aliens that might encounter the spacecraft. The package included a needle, a speaker and graphical instructions on how to play the record. A listener would hear a two-hour sampling of sounds from Earth, including babies crying, whales singing, chimps screeching, trains, thunderstorms, Beethoven, Chuck Berry, greetings in 55 languages and astronomer Carl Sagan’s son saying, “Hello from the children of planet Earth.”

The Farthest weaves the story of exploration with the story of the making of the record. The record’s producers and champions recount how they pulled the whole thing together in just six weeks. What to leave in — a map to Earth, in case the aliens want to visit — and what to leave out — full frontal nudity — was fiercely debated.

At times, refrains of “Wow!” and “It was a first” feel repetitive. Some of the stock footage and spacecraft animations are a little cheesy. But The Farthest is a tender tribute, tinged with nostalgia and existential awe. For those like me, who weren’t alive or aware when the first pictures of Jupiter came back, The Farthest offers a sense of what we missed.

As luck (or exceptionally precise astronomical modeling) would have it, my new, small Oregon town happens to lie in the upcoming eclipse’s path of totality. For nearly two glorious minutes on August 21, we will look up and see the unworldly sight of the moon completely blocking the sun.

To put it mildly, Oregon is going bonkers. Local radio is warning of gas shortages and apocalyptic traffic. Schools and businesses are closing. Emergency services are ramping up for the expected onslaught. Every local store has a pile of eclipse glasses near the register, yours for a very reasonable $2. (Oregonians don’t price gouge.)

I bought glasses (the good kind) for my family and put them in a high drawer. But as a parent to a 2-year-old, I realize that my eclipse prep can’t stop there. I’ve seen what the girl does to regular sunglasses, so I’ve got a few ideas to preschooler-proof these eclipse glasses for her.

Except for during the brief window of totality (when the sun’s surface is completely blacked-out), you shouldn’t look directly at the sun during an eclipse without wearing proper, eclipse-specific eyewear. The powerful light can cause extensive, sometimes permanent eye damage, a condition called solar retinopathy.

As you may imagine, it might be hard to impress this risk on children. Take the cases of these three Australian kids. After watching the 2012 partial eclipse of the sun through binoculars, a 10-year-old boy hurt his eyes. Examinations three months after the injury revealed persistent damage. Another boy, this one 8 years old, stared at the same partial eclipse directly. His eyes showed signs of harm five months later. And an 11-year-old girl who peeked at the 2012 transit of Venus with only her right eye also suffered persistent eye damage.

Those cautionary examples, described in 2015 in the Journal of the American Association for Pediatric Ophthalmology and Strabismus, made me want to duct tape my children’s eclipse glasses to their heads, mummy-style.

In lieu of that, I’m opting for super thick and stretchy fabric bands that I’ll staple and tape to the arms of the glasses. I’m also experimenting with a headband to limit movement on the top of the head, and perhaps even a paper plate taped around the front of the glasses to block incidental light. You could even take a note from 1963 schoolchildren, who put big boxes over their heads to see a projection of an eclipse.
I was happy to see that my DIY ideas aren’t totally off: Amid its wealth of eclipse information, the American Astronomical Society recommends modifying eclipse glasses with elastic or tape around the back so they sit firmly on small faces.

Of course, if you have a little Houdini who regularly squirms out of constricting clothes, you may consider any tweaking to be too risky. A simple pinhole projector, which doesn’t require looking anywhere near the sun, might be better.

Clearly, eye protection is something to take seriously. But don’t let that worry keep you and your children from seeing this once-in-a-lifetime celestial event. It’s the type of natural phenomenon that people — especially really young ones — can grab onto and understand. After all, kids love shadows, and this is going to be one heck of a shadow.