Do you wonder how astronomers find all those exoplanets orbiting stars in distant solar systems?
Mostly they use the transit method. When a planet travels in between its star and an observer, the light from the star dims. That’s called a transit. If astronomers watch a planet transit its star a few times, they can confirm its orbital period. They can also start to understand other things about the planet, like its mass and density.
The planet Mercury just transited the Sun, giving us all an up close look at transits.
One-one thou… That’s how long it takes for the International Space Station, traveling at over 17,000 mph (27,300 kph), to cross the face of the Full Moon. Only about a half second! To see it with your own eyes, you need to know exactly when and where to look. Full Moon is best, since it’s the biggest the moon can appear, but anything from a half-moon up and up will do.
The photo above was made by superimposing 13 separate images of the ISS passing in front of the Moon into one. Once the team knew when the pass would happen, they used a digital camera to fire a burst of exposures, capturing multiple moments of the silhouetted spacecraft.
The ISS transits the Full Moon in May 2016
The ISS is the largest structure in orbit, spanning the size of a football field, but at 250 miles (400 km) altitude, it only appears as big as a modest lunar crater. While taking a photo sequence demands careful planning, seeing a pass is bit easier. As you’d suspect, the chances of the space station lining up exactly with a small target like the Moon from any particular location is small. But the ISS Transit Findermakes the job simple.
Click on the link and fill in your local latitude, longitude and altitude or select from the Google maps link shown. You can always find your precise latitude and longitude at NASA’s Latitude/Longitude Finderand altitude at Google Maps Find Altitude. Next, set the time span of your Moon transit search (up to one month from the current date) and then how far you’re willing to drive to see the ISS fly in front of the Moon.
When you click Calculate, you’ll get a list of events with little diagrams showing where the ISS will pass in relation to the Moon and sun (yes, the calculator also does solar disk crossings!) from your location. Notice that most of the passes will be near misses. However, if you click on the Show on Map link, you’ll get a ground track of exactly where you will need to travel to see it squarely cross Moon or Sun. Times shown are your local time, not Universal or UT.
The map also includes Recalculate for this location link. Clicking that will show you a sketch of the ISS’ predicted path across the Moon from the centerline location along with other details. I checked my city, and while there are no lunar transits for the next month, there’s a very nice solar one visible just a few miles from my home on Feb. 8. Remember to use a safe solar filter if you plan on viewing one of these!
While you might attempt to see a transit of the ISS in binoculars, your best bet is with a telescope. Nothing fancy required, just about any size will do so long as it magnifies at least 30x to 40x. Timing is crucial. Like an occultation, when the moon hides a background star in an instant, you want to be on time and 100% present.
Make sure you’re set up and focused on the moon or sun (with filter) at least 5 minutes beforehand. Keep your cellphone handy. I’ve found the time displayed at least on my phone to be accurate. One minute before the anticipated transit, glue your eye to the eyepiece, relax and wait for the flyby. Expect something like a bird in silhouette to make a swift dash across the moon’s face. The video above will help you anticipate what to expect.
Even if you never go to the trouble of identifying a “direct hit”, you can still use the transit finder to compile a list of cool lunar close approaches that would make for great photos with just a camera and tripod.
The Transit Finder isn’t the only way to predict ISS flybys. Some observers also use the excellent satellite site, CalSky. Once you tell it your location, select the Lunar/Solar Disk Crossings and Occultations link for lots of information including times, diagrams of crossings, ground tracks and more.
I use Stellarium (above) to make nifty simulated paths and show me where the Moon will be in the sky at the time of the transit. When you’ve downloaded the free program, get the latest satellite orbital elements this way:
* Move you cursor to the lower left of the window and select the Configuration box
* Click the Plugins tab and scroll down to Satellites and click Configure and then Update
* Hover the cursor at the bottom of the screen for a visual menu. Slide over to the satellite icon and click it once for Satellite hints. The ISS will now be active.
* Set the clock and location (lower left again) for the precise time and location, then do a search for the Moon, and you’ll see the ISS path.
There you have it — lots of options. Or you can simply use the Transit Finder and call it a day! I hope you’ll soon be in the right place at the right time to see the space station pass in front of the Moon. Checking my usual haunts, I see that the space station will be returning next weekend (Jan. 27) to begin an approximately 3-week run of easily viewable evening passes.
There’s a remote chance that inexplicable light variations in a star in the Northern Cross may be caused by the works of an alien civilization.
1,480 light years from Earth twinkles one of the greatest mysteries of recent times. There in the constellation Cygnus the Swan, you’ll find a dim, ordinary-looking point of light with an innocent sounding name — Tabby’s Star. Named for Louisiana State University astronomer Tabetha Boyajian, who was the lead author on a paper about its behavior, this star has so confounded astronomers with its unpredictable ups and downs in its brightness, they’ve gone to war on the object, drilling down on it with everything from the Hubble to the monster 393.7-inch (10-meter) Keck Telescope in Hawaii. Continue reading “The Search Is On For Alien Signals Around Tabby’s Star”
On May 9, 2016, Mercury passed directly between the Sun and Earth. No one had a better view of the event than the space-based Solar Dynamics Observatory, as it had a completely unobstructed view of the entire seven-and-a-half-hour event! This composite image, above, of Mercury’s journey across the Sun was created with visible-light images from the Helioseismic and Magnetic Imager on SDO, and below is a wonderful video of the transit, as it includes views in several different wavelenths (and also some great soaring music sure to stir your soul).
Mercury transits of the Sun happen about 13 times each century, however the next one will occur in only about three and a half years, on November 11, 2019. But then it’s a long dry spell, as the following one won’t occur until November 13, 2032.
Simple choices can sometimes lead to dramatic turns of events in our lives. Before turning in for the night last night, I opened the front door for one last look at the night sky. A brighter-than-normal auroral arc arched over the northern horizon. Although no geomagnetic activity had been forecast, there was something about that arc that hinted of possibility.
It was 11:30 at the time, and it would have been easy to go to bed, but I figured one quick drive north for a better look couldn’t hurt. Ten minutes later the sky exploded. The arc subdivided into individual pillars of light that stretched by degrees until they reached the zenith and beyond. Rhythmic ripples of light – much like the regular beat of waves on a beach — pulsed upward through the display. You can’t see a chill going up your spine, but if you could, this is what it would look like.
Auroras can be caused by huge eruptions of subatomic particles from the Sun’s corona called CMEs or coronal mass ejections, but they can also be sparked by holes in the solar magnetic canopy. Coronal holes show up as blank regions in photos of the Sun taken in far ultraviolet and X-ray light. Bright magnetic loops restrain the constant leakage of electrons and protons from the Sun called the solar wind. But holes allow these particles to fly away into space at high speed. Last night’s aurora traces its origin back to one of these holes.
The subatomic particles in the gusty wind come bundled with their own magnetic field with a plus or positive pole and a minus or negative pole. Recall that an ordinary bar magnet also has a “+” and “-” pole, and that like poles repel and opposite poles attract. Earth likewise has magnetic poles which anchor a large bubble of magnetism around the planet called the magnetosphere.
Field lines in the magnetosphere — those invisible lines of magnetic force around every magnet — point toward the north pole. When the field lines in the solar wind also point north, there’s little interaction between the two, almost like two magnets repelling one another. But if the cloud’s lines of magnetic force point south, they can link directly into Earth’s magnetic field like two magnets snapping together. Particles, primarily electrons, stream willy-nilly at high speed down Earth’s magnetic field lines like a zillion firefighters zipping down fire poles. They crash directly into molecules and atoms of oxygen and nitrogen around 60-100 miles overhead, which absorb the energy and then release it moments later in bursts of green and red light.
So do great forces act on the tiniest of things to produce a vibrant display of northern lights. Last night’s show began at nightfall and lasted into dawn. Good news! The latest forecast calls for another round of aurora tonight from about 7 p.m. to 1 a.m. CDT (0-6 hours UT). Only minor G1 storming (K index =5) is expected, but that was last night’s expectation, too. Like the weather, the aurora can be tricky to pin down. Instead of a G1, we got a G3 or strong storm. No one’s complaining.
So if you’re looking for that perfect last minute Mother’s Day gift, take your mom to a place with a good view of the northern sky and start looking at the end of dusk for activity. Displays often begin with a low, “quiet” arc and amp up from there.
Aurora or not, tomorrow features a big event many of us have anticipated for years — the transit of Mercury. You’ll find everything you’ll need to know in this earlier story, but to recap, Mercury will cross directly in front of the Sun during the late morning-early evening for European observers and from around sunrise (or before) through late morning-early afternoon for skywatchers in the Americas. Because the planet is tiny and the Sun deadly bright, you’ll need a small telescope capped with a safe solar filter to watch the event. Remember, never look directly at the Sun at any time.
If you’re greeted with cloudy skies or live where the transit can’t be seen, be sure to check out astronomer Gianluca Masi’s live stream of the event. He’ll hook you up starting at 11:00 UT (6 a.m. CDT) tomorrow.
The table below includes the times across the major time zones in the continental U.S. for Monday May 9:
Be sure to mark your calendar for May 9. On that day, the Solar System’s most elusive planet will pass directly in front of the Sun. The special event, called a transit, happens infrequently. The last Mercury transit occurred more than 10 years ago, so many of us can’t wait for this next. Remember how cool it was to see Venus transit the Sun in 2008 and again in 2012? The views will be similar with one big difference: Mercury’s a lot smaller and farther away than Venus, so you’ll need a telescope. Not a big scope, but something that magnifies at least 30x. Mercury will span just 10 arc seconds, making it only a sixth as big as Venus.
If I might make a suggestion, consider buying a sheet of Baader AstroSolar aluminized polyester film and cutting it to size to make your own filter. Although the film’s crinkly texture might make you think it’s flimsy or of poor optical quality, don’t be deceived by appearances.
The material yields both excellent contrast and a pleasing neutral-colored solar image. You can purchase any of several different-sized films to suit your needs either from Astro-Physicsor on Amazon.com. Prices range from $40-90.
Nov. 8, 2006 Transit of Mercury by Dave Kodama
With filter material in hand, just follow these instructions to make your own, snug-fitting telescopic solar filter. Even I can do it, and I kid you not that I’m a total klutz when it comes to building things. If for whatever reason you can’t get a filter, go to Plan B. Put a low power eyepiece in your scope and project an image of the Sun onto a sheet of white paper a foot or two behind the eyepiece.
Since May 9th is a Monday, I’ve a hunch a few of you will be taking the day off. If you can’t, pack a telescope and set it up during lunch hour to share the view with your colleagues. Mercury will spend a leisurely 7 1/2 hours slowly crawling across the Sun’s face, traveling from east to west. The entire transit will be visible across the eastern half of the U.S., most of South America, eastern and central Canada, western Africa and much of western Europe. For the western U.S., Alaska and Hawaii the Sun will rise with the transit already in progress.
At first glance, the planet might look like a small sunspot, but if you look closely, you’ll see it’s a small, perfectly circular black dot compared to the out-of-round sunspots which also possess the classic two-part umbra-penumbra structure. Oh yes, it also moves. Slowly to be sure, but much faster than a typical sunspot which takes nearly two weeks to cross the Sun’s face. With a little luck, a few sunspots will be in view during transit time; compared to midnight Mercury their “black” umbral cores will look deep brown.
I want to alert you to four key times to have your eye glued to the telescope; all occur during the 3 minutes and 12 seconds when Mercury enters and exits the Sun. They’re listed below in Universal Time or UT. To convert UT to EDT, subtract 4 hours; CDT 5 hours; MDT 6 hours, PDT 7 hours, AKDT 8 hours and HST 10 hours.
First contact (11:12 UT): Watch for the first hint of Mercury’s globe biting into the Sun just south of the due east point on along the edge of disk’s edge. It’s always a thrill to see an astronomical event forecast years ago happen at precisely the predicted time.
Second contact (11:15 UT): Three minutes and 12 seconds later, the planet’s trailing edge touches the inner limb of the Sun at second contact. Does the planet separate cleanly from the solar limb or briefly remain “connected” by a narrow, black “line”, giving the silhouette a drop-shaped appearance?
This “black drop effect”is caused primarily by diffraction, the bending and interfering of light waves when they pass through the narrow gap between Mercury and the Sun’s edge. You can replicate the effect by bringing your thumb and index finger closer and closer together against a bright backdrop. Immediately before they touch, a black arc will fill the gap between them.
Third contact (18:39 UT): A minute or less before Mercury’s leading edge touches the opposite limb of the Sun at third contact, watch for the black drop effect to return.
Fourth contact (18:42 UT): The moment the last silhouetted speck of Mercury exits the Sun. Don’t forget to mark your calendar for November 11, 2019, date of the next transit, which also favors observers in the Americas and Europe. After that one, the next won’t happen till 2032.
Other interesting visuals to keep an eye out for is a bright ring or aureole that sometimes appears around the planet caused when our brain exaggerates the contrast of an object against a backdrop of a different brightness. Another spurious optical-brain effect keen-eyed observers can watch for is a central bright spot inside Mercury’s black disk. Use high power to get the best views of these obscure but fascinating phenomena seen by many observers during Mercury transits.
While I’ve been talking all “white light” observation, the proliferation of relatively inexpensive and portable hydrogen-alpha telescopes in recent years makes them another viewing option with intriguing possibilities. These instruments show solar phenomena beyond the Sun’s limb, including the flaming prominences normally seen only during a total eclipse. That makes it possible to glimpse Mercury minutes in advance of the transit (or minutes after transit end) silhouetted against a prominence or nudging into the rim furry ring of spicules surrounding the outer limb. Wow!
One final note. Be careful never to look directly at the Sun even for a moment during the transit. Keep your eyes safe! When aiming a telescope, the safest and easiest way to center the Sun in the field of view is to shift the scope up and down and back and forth until the shadow the tube casts on the ground is shortest. Try it.
I hope the weather gods smile on you on May 9, but it they don’t or if you live where the transit won’t be visible, Italian astrophysicist Gianluca Masi will stream it live on his Virtual Telescope websitestarting at 11:00 UT (6 a.m CDT).
“Bizarre.” “Interesting.” “Giant transit”. That were the reactions of Planet Hunters project volunteers when they got their first look at the light curve of the otherwise normal sun-like star KIC 8462852 nearly.
Of the more than 150,000 stars under constant observation during the four years of NASA’s primary Kepler Mission (2009-2013), this one stands alone for the inexplicable dips in its light. While almost certainly naturally-caused, some have suggested we consider other possibilities.
You’ll recall that the orbiting Kepler observatory continuously monitored stars in a fixed field of view focused on the constellations Lyra and Cygnus hoping to catch periodic dips in their light caused by transiting planets. If a drop was seen, more transits were observed to confirm the detection of a new exoplanet.
And catch it did. Kepler found 1,013 confirmed exoplanets in 440 star systems as of January 2015 with 3,199 unconfirmed candidates. Measuring the amount of light the planet temporarily “robbed” from its host star allowed astronomers to determine its diameter, while the length of time between transits yielded its orbital period.
Volunteers with the Planet Hunters project, one of many citizen science programs under the umbrella of Zooniverse, harness the power of the human eye to examine Kepler light curves (a graph of a star’s changing light intensity over time), looking for repeating patterns that might indicate orbiting planets. They were the first to meet up with the perplexing KIC 8462852.
This magnitude +11.7 star in Cygnus, hotter and half again as big as the Sun, showed dips all over the place. Around Day 800 during Kepler’s run, it faded by 15% then resumed a steady brightness until Days 1510-1570, when it underwent a whole series of dips including one that dimmed the star by 22%. That’s huge! Consider that an exo-Earth blocks only a fraction of a percent of a star’s light; even a Jupiter-sized world, the norm among extrasolar planets, soaks up about a percent.
Exoplanets also show regular, repeatable light curves as they enter, cross and then exit the faces of their host stars. KIC 8462852’s dips are wildly a-periodic.
Whatever’s causing the flickering can’t be a planet. With great care, the researchers ruled out many possibilities: instrumental errors, starspots (like sunspots but on other stars), dust rings seen around young, evolving stars (this is an older star) and pulsations that cover a star with light-sucking dust clouds.
What about a collision between two planets? That would generate lots of material along with huge clouds of dust that could easily choke off a star’s light in rapid and irregular fashion.
A great idea except that dust absorbs light from its host star, warms up and glows in infrared light. We should be able to see this “infrared excess” if it were there, but instead KIC 8462852 beams the expected amount of infrared for a star of its class and not a jot more. There’s also no evidence in data taken by NASA’s Wide-field Infrared Survey Explorer (WISE) several years previously that a dust-releasing collision happened around the star.
After examining the options, the researchers concluded the best fit might be a shattered comet that continued to fragment into a cascade of smaller comets. Pretty amazing scenario. There’s still dust to account for, but not as much as other scenarios would require.
Being fragile types, comets can crumble all by themselves especially when passing exceptionally near the Sun as sungrazing comets are wont to do in our own Solar System. Or a passing star could disturb the host star’s Oort comet cloud and unleash a barrage of comets into the inner stellar system. It so happens that a red dwarf star lies within about 1000 a.u. (1000 times Earth’s distance from the Sun) of KIC 8462852. No one knows yet whether the star orbits the Kepler star or happens to be passing by. Either way, it’s close enough to get involved in comet flinging.
So much for “natural” explanations. Tabetha Boyajian, a postdoc at Yale, who oversees the Planet Hunters and the lead author of the paper on KIC 8462852, asked Jason Wright, an assistant professor of astronomy at Penn State, what he thought of the light curves. “Crazy” came to mind as soon he set eyes on them, but the squiggles stirred a thought. Turns out Wright had been working on a paper about detecting transiting megastructures with Kepler.
In a recent blog, he writes: “The idea is that if advanced alien civilizations build planet-sized megastructures — solar panels, ring worlds, telescopes, beacons, whatever — Kepler might be able to distinguish them from planets.” Let’s assume our friendly aliens want to harness the energy of their home star. They might construct enormous solar panels by the millions and send them into orbit to beam starlight down to their planet’s surface. Physicist Freeman Dyson popularized the idea back in the 1960s. Remember the Dyson Sphere, a giant hypothetical structure built to encompass a star?
From our perspective, we might see the star flicker in irregular ways as the giant panels circled about it. To illustrate this point, Wright came up with a wonderful analogy:
“The analogy I have is watching the shadows on the blinds of people outside a window passing by. If one person is going around the block on a bicycle, their shadow will appear regularly in time and shape (like a regular transiting planet). But crowds of people ambling by — both directions, fast and slow, big and large — would not have any regularity about it at all. The total light coming through the blinds might vary like — Tabby’s star.”
Even Wright admits that the “alien hypothesis” should be seen as a last resort. But to make sure no stone goes unturned, Wright, Boyajian and several of the Planet Hunters put together a proposal to do a radio-SETI search with the Green Bank 100-meter telescope. In my opinion, this is science at its best. We have a difficult question to answer, so let’s use all the tools at our disposal to seek an answer.
In the end, it’s probably not an alien megastructure, just like the first pulsar signals weren’t sent by LGM-1 (Little Green Men). But whatever’s causing the dips, Boyajian wants astronomers to keep a close watch on KIC 8462852 to find out if and when its erratic light variations repeat. I love a mystery, but answers are even better.
This has been on my bucket list for a while, but I’ve never had the opportunity to witness it myself: seeing the International Space Station transit the Moon. And now thanks to my friend Gadi Eidelheit, I want to see it for myself even more! He captured video and imagery of the ISS scooting in front of the Moon, from Ganot, Israel.
“This was just about 10km from my house!” Gadi told me via email. “I used a Canon 700d with Sigma 18-250 for the first clip and Canon sx50HS for the second clip (maximum zoom which equivalent to 1200mm). To find the best place I used Heavens-Above and a little trial and error! I just choose different locations near my home until I found the correct spot.”
For the first part of the video you need to look closely, as the transit happens quite quickly. But then Gadi slows it down, and later zooms in on the action. It still happens quite quickly but it’s very fun to see.
Gadi is an amateur astronomer and blogger who always has interesting things to post on social media or on his blog. You’ve really got me motivated now, Gadi!!
Early on the morning of Dec. 19, 2013, the pre-dawn sky above the coastal town of Kourou in French Guiana was briefly sliced by the brilliant exhaust of a Soyuz VS06 rocket as it ferried ESA’s “billion-star surveyor” Gaia into space, on its way to begin a five-year mission to map the precise locations of our galaxy’s stars. From its position in orbit around L2 Gaia will ultimately catalog the positions of over a billion stars… and in the meantime it will also locate a surprising amount of Jupiter-sized exoplanets – an estimated 21,000 by the end of its primary mission in 2019.
And, should Gaia continue observations in extended missions beyond 2019 improvements in detection methods will likely turn up even more exoplanets, anywhere from 50,000 to 90,000 over the course of a ten-year mission. Gaia could very well far surpass NASA’s Kepler spacecraft for exoplanet big game hunting!
“It is not just the number of expected exoplanet discoveries that is impressive”, said former mission project scientist Michael Perryman, lead author on a report titled Astrometric Exoplanet Detection with Gaia. “This particular measurement method will give us planet masses, a complete exoplanet survey around all types of stars in our Galaxy, and will advance our knowledge of the existence of massive planets orbiting far out from their host stars”.
The planets Gaia will be able to spot are expected to be anywhere from 1 to fifteen times the mass of Jupiter in orbit around Sun-like stars out to a distance of about 500 parsecs (1,630 light-years) from our own Solar System. Exoplanets orbiting smaller red dwarf stars will also be detectable, but only within about a fifth of that distance.
While other space observatories like NASA’s Kepler and CNES/ESA’s CoRoT were designed to detect exoplanets through the transit method, whereby a star’s brightness is dimmed ever-so-slightly by the silhouette of a passing planet, Gaia will detect particularly high-mass exoplanets by the gravitational wobble they impart to their host stars as they travel around them in orbit. This is known as the astrometric method.
A select few of those exoplanets will also be transiting their host stars as seen from Earth – anywhere from 25 to 50 of them – and so will be observable by Gaia as well as from many ground-based transit-detection observatories.
After some issues with stray light sneaking into its optics, Gaia was finally given the green light to begin science observations at the end of July and has since been diligently scanning the stars from L2, 1.5 million km from Earth.
With the incredible ability to measure the positions of a billion stars each to an accuracy of 24 microarcseconds – that’s like measuring the width of a human hair from 1,000 km – Gaia won’t be “just” an unprecedented galactic mapmaker but also a world-class exoplanet detector! Get more facts about the Gaia mission here.
The team’s findings have been accepted for publication in The Astrophysical Journal.