Astronomy & Science

Kepler Team Releases Final Exoplanet Catalog

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The most comprehensive and detailed catalog released yet marks the end of an era for the planet-hunting scope.

That’s a wrap for the Kepler Space Telescope — at least its primary mission. On June 19th, astronomers released the eighth and final mission catalog with data gathered from Kepler’s first 3.5 years of life. With 219 new planet candidates, 10 of which are near-Earth size and orbiting in their star’s habitable zone, the catalog will help astronomers answer a question central to the telescope’s mission: How many Earth-like planets orbit Sun-like stars?

Small planets come in two sizes

Researchers have discovered a gap in the distribution of planet sizes, indicating that most planets discovered by Kepler so far fall into two distinct size classes: the rocky Earth-size and super-Earth-size (similar to Kepler-452b), and the mini-Neptune-size (similar to Kepler-22b). This histogram shows the number of planets per 100 stars as a function of planet size relative to Earth. The orange line shows regions where counting data is complete, while the white line marks incomplete data. (Though there appears to be a gap between smaller, Earth-size planets and rocky super-Earths, there's not enough data there to tell if that second gap is real.)
NASA / Ames Research Center / CalTech / University of Hawai'i / B.J. Fulton

By the Numbers

In total, Kepler discovered 4,034 planet candidates, of which 2,335 are bona fide exoplanets. Although the community has watched those numbers creep up over the last four years as astronomers continued to analyze the data pouring in from the planet-hunting scope, those numbers have now reached their peak. This is the last catalog that will be released from Kepler’s primary mission — the one that scoured 160,000 stars in a patch of sky in the constellation Cygnus.

In addition, Kepler discovered 50 near-Earth size habitable zone candidates, more than 30 of which have been verified. “Kepler really and truly has opened up our eyes to the existence of these small terrestrial-sized worlds,” Susan Mullally (SETI Institute) said at Monday’s news conference at the NASA’s Ames Research Center in California.

This final catalog reprocessed the entire set of data from Kepler’s primary mission with a new trick: The team introduced their own simulated and false signals into the dataset to determine which types of planets were overcounted and which were undercounted. This will allow teams going forward to better characterize planets across the galaxy and answer one of astronomy’s most compelling questions: How many habitable worlds are there?

“The reason why I'm so excited about [Mullally’s] results, is that this catalog — because it was done in such a sophisticated, methodical way — really enables studies of habitable-zone-planet occurrence for sun-like stars in a way previous catalogs did not,” Courtney Dressing (Caltech) says. “It's laying fundamental ground-work.”

The Family Tree Divides Exoplanet Family Tree

This sketch illustrates the family tree of exoplanets. Planets are born out of swirling disks of gas and dust called protoplanetary disks, which give rise to giant planets such as Jupiter, as well as smaller planets between the size of Earth and Neptune. Researchers have discovered that latter group can be cleanly divided into two smaller groups: rocky Earth- and super-Earth-like planets and gaseous mini-Neptunes.
NASA / Ames Research Center / JPL-Caltech / Tim Pyle

Already, Benjamin Fulton (University of Hawai‘i at Manoa) and his colleagues have taken advantage of Kepler’s latest dataset, posting their result on the arXiv preprint server. It helps astronomers better understand one of Kepler’s greatest surprises: that planets form at masses between those of Earth and Neptune. Previously thought forbidden, astronomers classify these planets as super-Earths — rocky worlds a little larger than Earth with deep, crushing atmospheres — or mini-Neptunes — gaseous planets smaller than Neptune that don’t have a surface.

Although most of the exoplanets across the galaxy fall into this forbidden regime between Earth and Neptune, astronomers have struggled to find a dividing line between the two classes. Where does a super-Earth stop and a mini-Neptune begin? Fulton’s work might provide the answer.

He has found that between 1.75 and 2 times the size of the Earth, very few planets form, leaving a gap — and a dividing line — between the two classes. Any planet that is smaller than 1.75 times the size of Earth is likely a super-Earth and any planet that is greater than twice the size of Earth is likely a mini-Neptune.

“This is a major new division in the family tree of exoplanets, somewhat analogous to the discovery that mammals and lizards are separate branches on the tree of life,” said Fulton.

Although Fulton is unsure why nature prefers to keep these two planets clearly separated, he suspects that it all comes down to the lightest two elements: hydrogen and helium. “A very small amount of light hydrogen and helium gases goes a long way to inflate the size of planets,” he says. “Adding a tiny amount of hydrogen to one of these rocky planets, say about 2% by mass, would cause the planet to jump the gap and move into the group of larger planets.”

How to make a super-Earth / mini-Neptune

This diagram illustrates how planets are assembled and sorted into two distinct size classes. First, the rocky cores of planets are formed from smaller pieces. Then, the gravity of the planets attracts hydrogen and helium gas. Finally, the planets are "baked" by the starlight and lose some gas. At a certain mass threshold, planets retain the gas and become gaseous mini-Neptunes; below this threshold, the planets lose all their gas, becoming rocky super-Earths.
NASA / Ames Research Center / JPL-Caltech / R. Hurt

To boot, the results sharpen the dividing line between planets that are potentially habitable and those that would not make comfortable abodes. Anything above twice the size of the Earth would certainly not hold life as we know it and anything less than 1.75 times the size of the Earth just might, but it’s not a guarantee.

Although Monday’s news conference was hailed as the end of an era — it has, after all, been 23 years since William Borucki and David Koch first dreamed of a telescope that could catch the stellar dimmings required to spot planets — the aroma of the newsroom contained more excitement than sadness.

“It feels a bit like the end of an era — but actually I see it as a new beginning,” Mullally said. “I'm really excited to see what people are going to do with this catalog because it's the first time we have a population that is really well characterized and we can now do these statistical studies and really start to understand the Earth analogues out there.”

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China Launches Its First X-ray Observatory

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China's Hard X-ray Modulation Telescope (HXMT), the country's first X-ray observatory — not to mention its first astronomical satellite — headed to orbit last week.


An artist's conception of the HXMT in space.
Xinhua/China Academy of Sciences/SASTIND

China entered the orbital X-ray astronomy game last week as a Long March 4B rocket roared to life, hoisting the nation's first astronomical observatory into space.

The launch occurred from China's Jiuquan space center on Thursday, June 15th at 3:00 UT / 11:00 p.m. EDT on June 14th. HXMT was renamed Huiyan (Chinese for “insight”) shortly after launch. Three other smaller Earth-observing satellites, including Argentina's NuSat-3 and two of the Chinese company Orbita's Zhuhai satellites also hitched a ride to space along with the observatory.

To study the high-energy universe, you have to get up above the Earth's atmosphere, which absorbs cosmic X-rays on their way to our planet's surface. In fact, it was brief sounding-rocket flights that carried out our very first X-ray observations of the Sun, using captured Nazi V-2 rockets that were launched from the White Sands missile range in 1949. Now, several X-ray observatories circle Earth, including NASA's Chandra, NuSTAR, and Swift telescopes, as well as the European Space Agency's XMM-Newton.

China's HXMT joins this cohort to study X-ray sources such as supermassive black holes in distant galaxies, as well as stellar-mass black holes and pulsars in our own galaxy. HMXT will also survey the sky across the galactic plane on the hunt for new, short-lived X-ray sources, mapping out the cosmic X-ray background in the process. HXMT sees in gamma rays as well: It can catch gamma-ray bursts in the range of 3 million electron-volts (3 MeV). The mission could work in tandem with China's new Five-hundred-meter Aperture Spherical radio Telescope (FAST) to study the properties of millisecond pulsars, in hopes of using these sources as navigational beacons for deep-space missions.

HXMT followed a long road to the launch pad. The mission was first proposed 1993 and moved ahead into the early development stage in 2000. Originally, China hoped to launch the observatory in 2010, but funding constraints pushed the mission over three of China's Five Year Plans for development to 2017.

HXMT in the lab

HXMT undergoes testing in the lab.
Institute of High Energy Physics (Chinese Academy of Sciences)

The Tsinghua University, the Ministry of Science and Technology of China, and the Chinese Academy of Sciences' Institute for High-Energy Physics (IHEP) collaborate in operating the telescope. The HXMT team has released a statement on their website on June 19th, which reports that the spacecraft is healthy and its low and medium energy detectors have successfully switched on.

HXMT joins China's Dark Matter Particle Explorer (DAMPE) as the second premier astronomical observatory in low-Earth orbit.

Want to catch a glimpse yourself? HXMT is in a 43°-inclined, 536- by 546-kilometer orbit, going around the Earth once every 95 minutes. This orbit means that — like Hubble and Hitomi — the observatory will be visible to naked-eye satellite observers from about latitude 45°N to 45°N. Heavens-Above is a great place to check for local passes: HXMT's NORAD ID is (42758) 2017-034A.

How the Hard X-ray Modulating Telescope "Sees" X-rays

HXMT couples an incredible range of energy coverage, from 1 to 250 keV, with wide sky coverage. To achieve this, the observatory uses three detectors with overlapping energy regimes: low energies (1–15 keV), medium energies (5–30 keV), and high energies (20–250 keV). The high-energy detector is the main science instrument and has the greatest sensitivity of the three detectors. Another instrument, designed to help calibrate the high-energy detector, can also detect gamma-ray bursts between 300 keV and 3,000 keV in energy.

Rather than using complex and expensive nested mirrors to focus the X-rays, HXMT employs an innovative focusing technique, filtering out all X-rays except those that are coming in parallel to a specified direction. Thanks to this method, the telescope doesn't have to narrow its field of view, making it an ideal instrument for large sky surveys. The trade-off for its broad energy and sky coverage is its sensitivity (that is, the faintest source it can pick up), which is on par with the now-retired Rossi Timing X-ray Explorer (RXTE).

HXMT Range

The sensitivities of the three detectors aboard the HXMT versus the sensitivities of NuSTAR, INTEGRAL / IBIS and RXTE / HEXTE.
BCAS / Koglin et al.

The Future of X-ray Astronomy

The European Space Agency (ESA) plans to collaborate with the Chinese observatory, enabling comparisons between observations from HXMT and ESA's XMM-Newton. In a 2016 interview, HXMT Principle Investigator Zhang Shuangnan also mentions possible collaboration with NASA's NuSTAR team.

Perhaps the mission will usher in a new era of international scientific collaboration along with a new era for X-ray astronomy.

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QHYCCD’s Latest All-in-One CCD Camera

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QHY16200AQHYCCD introduces the latest model in its "all-in-one" product series, the QHY16200A ($3,999) CCD camera. Its 4,540 x 3,630 (16-megapixel) array APS-H format detector measures 27 x 21.6 mm with 6-micron-square pixels, providing high resolution in a compact area. The camera's dual-stage thermoelectric cooling is capable of stable temperatures of 40° below ambient. The QHY16200A includes a removable off-axis guider and an internal 5-position filter wheel that accepts 2-inch filters, as well as a USB slave port with a locking clip to power an autoguiding camera. Each purchase comes with a 1-meter 12V threaded power cord, a 1.8-m USB 2.0 cable, and a 2-inch nosepiece. See the manufacturer's website for additional details.'s New Product Showcase is a reader service featuring innovative equipment and software of interest to amateur astronomers. The descriptions are based largely on information supplied by the manufacturers or distributors. Sky & Telescope assumes no responsibility for the accuracy of vendors statements. For further information contact the manufacturer or distributor. Announcements should be sent to Not all announcements will be listed.

The post QHYCCD’s Latest All-in-One CCD Camera appeared first on Sky & Telescope.

This Week’s Sky at a Glance, June 16 – 24

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Friday, June 16

• By the time it's fully dark this week, Altair is shining well up in the east. A finger-width above it or to its upper left is its little sidekick Tarazed (Gamma Aquilae), actually an orange giant that's far in the background. Altair is 17 light-years from us; Tarazed is about 460.

Saturday, June 17

• The last-quarter Moon rises late tonight, around 1 a.m. daylight-saving time. Watch for it to breach the horizon to the lower right of the Great Square of Pegasus.

Sunday, June 18

• Here it is almost summer. But as twilight fades, look very low in the north-northwest for wintry Capella very out of season. The farther north you are, the higher it will appear. You may need binoculars. If you're as far north as Portland or Montreal, Capella is actually circumpolar.

Moon and Venus at dawn, June 20, 21, 22, 2017

The waning crescent Moon leapfrogs past Venus between the mornings of the 20th and 21st for North America.

Monday, June 19

• A double shadow transit occurs on Jupiter tonight from 10:04 to 10:38 p.m. EDT, when both Io and Europa are casting their tiny black shadows onto opposite sides of the planet's face.

• Dawn on Tuesday the 20th find Venus shining to right of the waning crescent Moon, as shown here.

Tuesday, June 20

• This is "Midsummer's Night," the shortest night of the year in the Northern Hemisphere. Astronomical summer begins at the solstice, 12:24 a.m. EDT (4:24 UT) on the 21st; that's 9:24 p.m. on the 20th PDT.

The term "Midsummer's Night" is left over from when the seasons were commonly defined as beginning and ending around the cross-quarter days. Be like your ancestors — build a bonfire tonight and organize some all-night revelry while magic is afoot. Dawn will come soon enough — when you'll see Venus over the crescent Moon, as shown above.

Wednesday, June 21

• Do you know about the dark Propeller in the M13 star cluster in Hercules? With no Moon in the sky, take advantage of the dark to visit Sue French's six favorite summer deep-sky objects, which she features in the July Sky & Telescope, page 54.

Thursday, June 22

• Leo the Lion is a constellation of late winter and spring. But he's not gone yet. As twilight ends, look due west, rather low, for Regulus, his brightest and now lowest star: the forefoot of the Lion stick figure. The Sickle of Leo extends upper right from Regulus. The rest of the Lion's constellation figure extends for almost three fist-widths to the upper left, to end with his tail star, Denebola, the highest. He's walking down to the western horizon.

Friday, June 23

• This is the time of year when, after dark, the dim Little Dipper floats straight upward from Polaris (the end of its handle) — like a helium balloon on a string escaped from some summer evening party. Through light pollution, however, all you may see of the Little Dipper are Polaris at its bottom and Kochab, the lip of the Little Dipper's bowl, at the top.

• New Moon (exact at 10:31 p.m. EDT).

Saturday, June 24

• This is the time of year when the two brightest stars of summer, Arcturus and Vega, are equally high overhead soon after dark: Arcturus in the southwest, Vega toward the east.

Arcturus and Vega are 37 and 25 light-years away, respectively. They represent the two commonest types of naked-eye stars: a yellow-orange K giant and a white A main-sequence star. They're 150 and 50 times brighter than the Sun, respectively — which, combined with their nearness, is why they dominate the evening sky.


Want to become a better astronomer? Learn your way around the constellations! They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

This is an outdoor nature hobby. For an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential guide to astronomy.

Pocket Sky Atlas, jumbo edition

The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6 — which may sound like a lot, but it's less than one per square degree on the sky. Also plotted are many hundreds of telescopic galaxies, star clusters, and nebulae. Shown above is the Jumbo Edition for easier reading in the night. Larger view. Sample chart.

Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or Jumbo Edition), which shows stars to magnitude 7.6.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many. The next up, once you know your way around, is the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.

You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, or the bigger Night Sky Observer's Guide by Kepple and Sanner.

Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically (meaning heavy and expensive). And as Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."

This Week's Planet Roundup Jupiter on June 1, 2017

The non-Red-Spot side of Jupiter on June 1st, imaged by Christopher Go in the Philippines. South is up.

Mercury is hidden in the glare of the Sun.

Venus (magnitude –4.3) shines bright in the east during dawn. A telescope shows it just on the gibbous side of dichotomy (half-lit phase).

Mars is lost in the sunset.

Jupiter (magnitude –2.1, in Virgo) shines high and bright in the southwest during evening. Spica, magnitude +1.0 and noticeably bluer, glitters 10° left of it. In a telescope, Jupiter has shrunk to 39 arcseconds wide.

Saturn (magnitude 0.0, in southern Ophiuchus) is just past opposition. It glows yellowish low in the southeast in twilight, 15° left of fiery Antares. By about midnight Saturn is at its highest in the south, in the steadiest air for telescopic observing.

Uranus (magnitude 5.9, in Pisces) is low in the glow of dawn.

Neptune (magnitude 7.9, in Aquarius) is well up in the southeast before the first light of dawn.


All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America.

Eastern Daylight Time (EDT) is Universal Time (UT, UTC, or GMT) minus 4 hours.


"This adventure is made possible by generations of searchers strictly adhering to a simple set of rules. Test ideas by experiments and observations. Build on those ideas that pass the test. Reject the ones that fail. Follow the evidence wherever it leads, and question everything. Accept these terms, and the cosmos is yours."
— Neil deGrasse Tyson, 2014

"Objective reality exists. Facts are often determinable. Vaccines do stop diseases. Carbon dioxide does warm the globe. Science and reason are no political conspiracy; they are how we discover reality. Civilization's survival depends on our ability, and willingness, to do so."
— Alan MacRobert, your Sky at a Glance editor

"Facts are stubborn things; and whatever may be our wishes, our inclinations, or the dictates of our passions, they cannot alter the state of facts and evidence."
— John Adams, 1770


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