One Star, Seven Planets

Many worlds around HD 10180
An artist's impression of the many worlds orbiting the Sunlike star HD 10180.

Nestled in the Chilean Andes at altitude of 7,900 feet (2,400 m), the European Southern Observatory's 3.6-m reflector at La Silla is neither the highest nor the largest telescope in the region. Far from it. However, coupled with the incredible HARPS spectrograph, this telescope is cranking out discoveries of extrasolar planets faster than any other observatory on Earth.

Today, at a gathering of planet-hunting astronomers in France, Christophe Lovis (Geneva Observatory) announced that his team has identified a solar system packed with planets — five for sure, and probably seven — using the La Silla facility. It's the most populous exoplanet system known.

The central star is HD 10180, a type G dwarf situated 137 light-years away in the southern constellation Hydrus. It's one of 400 nearby Sunlike stars that astronomers have been monitoring from La Silla for years. HARPS can't see these worlds directly; instead it detects minuscule Doppler shifts in the stars' light, caused by back-and-forth wobbles in their motion that, over time, reveal the presence of planets circling around them. In the case of HD 10180, the telescope amassed 190 nights of observations over six years.

You could make a case that spotting alien planets has gotten a little ho-hum — after all, the total count (including those orbiting the Sun) now tops 500, and 15 systems involve at least three planets.

But HD 10180 definitely raises the bar. Its five sure-things, dubbed C, D, E, F, and G, have Neptune-class masses 13 to 25 times that of Earth. But all five are quite close to the star, in orbits that range from 0.06 to 1.4 astronomical units (5½ to 130 million miles out). So much mass packed so close together is bound to incite gravitational tussles among them, and future observations will follow the long-term evolution of the system.




Solar systems compared
A comparison of our solar system with that of HD 10180 and other stars encircled by at least three planets. Black lines indicate the range of distance due to eccentric orbits.

But wait — there's more! Lovis and his team are fairly certain there's a sixth planet, H, with at least 65 Earth masses (making it Saturn-ish) and orbiting 3.4 a.u. from the star. There's also strong evidence for a seventh sibling, B, zipping just 0.02 a.u (2 million miles) from the star. Although not yet confirmed, this innermost planet might be very close to Earth in mass. It causes a wobble in HD 10180 only about 2 miles (3 km) per hour — "slower than walking speed," notes team member Damien Ségransan — and is thus very hard to measure.

For all the details, read the team's article in the current issue of Astronomy & Astrophysics. At upper right is Figure 13, a concise comparison of solar systems with three or more planets (one is missing, however: the four-member set circling pulsar PSR 1257+12).

Now that we know of a system as crowded with planets as our own, isn't it about time we starting finding some that resemble Earth? That bar-raising exoplanet discovery may not be long in coming — NASA's Kepler spacecraft is looking for them right now.

The Incredible Shrinking Moon

At one point during the Apollo 17 mission, moonwalking astronauts Gene Cernan and Harrison "Jack" Schmitt tried to drive their lunar rover up the face of a 200-foot-high rise known as the Lincoln-Lee Scarp. It didn't seem that imposing a task, but the rover's wheels slipped so much that the astronauts were forced to climb it at an angle, much as a sailboat tacks into a stiff wind.


Thrust fault diagram
Thrust faults occur when the lunar crust is compressed laterally, breaking the rocky materials below and forming a long scarp on the surface.

Lincoln-Lee is the kind of feature created when one slab of rock overrides another due to horizontal compression (what geologists term a thrust fault). On Earth, good (if enormous) examples of thrust faults occur where crustal plates collide — think how the towering Andes rim the west coast of South America, and you get the idea.

What Cernan and Schmitt couldn't have known back in 1972 is that Lincoln-Lee is not an isolated feature but one of likely hundreds of small thrust faults all over the Moon. That revelation came to light only recently, thanks to the incredibly detailed images of the lunar surface being beamed to Earth by two Narrow Angle Cameras aboard NASA's Lunar Reconnaissance Orbiter. (These same cameras have taken snapshots of the historic Apollo 11 landing site and others.)


Map of lunar scarps
The distribution of lobate scarps known as of mid-2010. Black dots indicate previously known features, while white dots mark those found in images by the Lunar Reconnaissance Orbiter. Click here for a larger version.

It wasn't until LRO arrived on the scene that geologists realized the subtle fractures are pervasive. After poring over images from LRO, a team led by Thomas Watters (Smithsonian Institution) has identified 14 new scarps in addition to the three dozen already known. As the map here shows, half of the new finds are poleward of 60° in latitude. (Most of the previously recognized scarps had been spotted by cameras mounted into the orbiting command modules of Apollos 15, 16, and 17 — but these craft never strayed far from the lunar equator.)

In the August 20th issue of Science, the researchers reach a startling and unexpected conclusion: "We have now found that these lobate scarps occur everywhere on the Moon," Watters explains, "which means the Moon has been contracting or shrinking globally."


Scarp in Gregory crater
A thrust fault crossing the floor of Gregory crater on the lunar farside.

All told, the lunar diameter hasn't changed much, probably only about 700 feet (200 m). But the scarps look so fresh that they must have formed in the recent past, geologically speaking. "These scarps can't be any older than 800 million to 1 billion years," Watters noted during a press briefing yesterday, and they could be much younger. "We're finding the Moon is a truly dynamic planet," adds Michael Wargo, chief lunar scientist at NASA headquarters. "Who'd have thought that tectonic processes would still be occurring today?"

The scarps escaped notice until now because they're only a mile or two long and just tens of feet high — completely invisible to backyard telescopes and even to previous lunar-orbiting craft. Most likely, they result from the gradual contraction of the lunar interior as it cools, a process that apparently didn't end when the last maria filled with lava some 3 billion years ago.

Thrust faults appear on the surfaces of Mars and especially Mercury, but they're huge by comparison. Some of the Mercurian scars are hundreds of miles long and more than a mile high, implying that the planet shrank by at least a couple of miles as its molten interior cooled and contracted.

Yet at one time the Moon must have been really, really hot as well. After all, it likely accreted by picking up the white-hot pieces after something enormous collided early on with Earth. So shouldn't there likewise be giant thrust faults jutting skyward across lunar landscape? Some researchers think the Moon did undergo substantial contraction, but the surface scars from that have been erased over time. Watters thinks otherwise. "Our results are really more consistent with a cooler initial starting temperature," he explains, "one that didn't allow the entire Moon to melt."


Astronaut collecting lunar soil
Astronaut Harrison "Jack" Schmitt uses a special rake to collect lunar soil during the Apollo 17 mission in December 1972.

As I listened to yesterday's press briefing, I wondered what Cernan and Schmitt were thinking as they struggled up the slope of Lincoln-Lee Scarp all those years ago. So I asked one of them.

"We were well aware of the Lee-Lincoln scarp and that it is a potential thrust fault or wrinkle ridge," Schmitt recalls. "We drove up it to Station 2 at the base of the South Massif, and along and down it going to Station 4 [Shorty crater]. It was entirely covered by a light mantle or avalanche deposit. The avalanche probably flowed off the South Massif about 100 million years ago, as it appears to have been triggered by [nearby impacts from] Tycho ejecta. Lee-Lincoln would be at least older than the avalanche and I suspect much older than that."

Watters and other lunar geologists should eventually be able to say more about how and when all these lobate scarps formed. As of now, LRO has mapped only about 10% of the lunar surface at high resolution. But give it another three years (assuming the funding holds out), and there'll be enough coverage to inspect the entire lunar glove down to a resolution of just a few feet.

A Solar Tsunami

Solar flare on August 1, 2010
A false-color composite shows the
solar flare (bright area at lower left) and large magnetic disturbance
that rippled across the Sun's disk on August 1, 2010. Recorded by
NASA's Solar Dynamics Observatory at extreme-ultraviolet wavelengths,
the hues correspond to coronal gases at temperatures of 1 to 2 million
kelvins. Click here for a larger view.

After oversleeping for more than a year, our star is finally stirring from hibernation. Experts don't expect solar activity to peak until until mid-2013 (and a weak one at that), but the signs of awakening are clearly evident.

A few days ago the Sun let loose with a massive belch. On August 1st at 8:55 Universal Time, orbiting satellites witnessed a sizable flare erupting from the large sunspot region designated 1092. The strength of this outburst was pegged at C3, modest as flares go, but it still triggered an impressive coronal mass ejection, or CME, that shot out from the solar disk at more than 600 miles (1,000 km) per second. Watch the amazing video from NASA's STEREO spacecraft here.

When the flare erupted, NASA's recently-launched Solar Dynamics Observatory also looked on as the magnetic disturbance caused an enormous filament of superheated gas to pulse across the Sun's disk.

All this tumult occurred on the Earth-facing side of the Sun, and skywatchers at far northern and southern locations briefly enjoyed colorful auroral displays on the night of August 3-4.

The Sun has since quieted down, but the big spot in region 1092 has been joined by a second, smaller group (1093) that rotated into view yesterday. Go have a look — but, as always, never try to look at the Sun by eye or using optical aid without using a safe solar filter.

Meanwhile, when the Sun's activity finally does peak, solar physicists will have more spacecraft at their disposal than ever before — despite the news that NASA managers pulled the plug on the Transition Region and Coronal Explorer (TRACE) on June 21st after 12 years of operation.

The workhorse Solar and Heliospheric Observatory (SOHO), a collaboration between NASA and the Rueopean Space Agency, just keeps on chugging (it's nearly 15 years old). Even with the loss of TRACE, NASA still has SDO, STEREO, and ACE (Advanced Composition Explorer) in its arsenal of orbiting sentinels. (By the way, teh STEREO team has a great iPhone app showing the Sun in 3D.)

In addition, the Japan Aerospace Exploration Agency continues to receive great high-resolution images from its Hinode spacecraft.

An Evening Dance of Planets

Saturn and Mars are five magnitudes fainter than Venus and thus only about 1% as bright. They're side by side in the sky, separately by three or four fingers held together at arm’s length. They'll spend the coming week sliding to the right with respect to Venus, creating a planetary triangle that changes shape from day to day. The crescent Moon joins the twilight planet scene on Thursday, August 12th (when it’s below Venus), and on Friday, the 13th (when it’s left of Venus).

An Evening Dance of Planets

Step outside as evening twilight fades, and from now through the middle of August you’ll find three planets shining low in the west — one much brighter than the other two. Venus will leap out at you, but you may need to wait for the sky to darken a bit more before fainter Saturn and Mars glimmer into view.

Three evening planets, Aug. 2010
In the western sky at dusk, bright Venus lights the way to fainter Mars and Saturn. The crescent Moon joins them on August 12th and 13th. Can you spot Mercury far to their lower right? Binoculars help.

Although the three planets look close together, they’re not. Venus is currently 6 light-minutes (73 million miles) from us, Mars is 17 light-minutes (190 million miles) distant, and Saturn is far in the background 85 light-minutes (950 million miles) away.

Venus's proximity is one of three reasons why it's so much brighter than the other two planets. It’s also much closer to the Sun, so it’s illuminated more intensely, and it’s covered with white, brilliantly reflective clouds. Mars and Saturn look similarly bright for reasons that cancel out: Saturn is 35 times larger than Mars, but it’s much farther both from us and from the Sun.

Don’t miss this chance to do some easy astronomy from your backyard, balcony, or rooftop

Dark Nights for the Perseids

The last time the annual Perseid meteor shower happened during a run of good moonless nights was in 2007. It turns out that every three years, the same phase of the Moon returns to roughly the same date each month (2.2 days earlier, on average). So in 2010 we're on for moonless Perseids again!

The shower lasts for many days, but according to the International Meteor Organization this year's peak should occur during a half-day-long window centered on 1:00 Universal Time on August 13th, which is ideal timing for skywatchers in Eurasia. For North Americans, the best viewing will probably be late Thursday night and early Friday morning, August 12-13, or possibly the night before.

In any case, prime viewing for the Perseids is from about 11 p.m. or midnight (local time) until the first light of dawn. This is when the shower's radiant (its perspective point of origin) is well up in your sky. The higher the radiant, the more meteors you'll see.

Many longtime skywatchers remember the fine displays the Perseids put on in the early 1990s, around when the shower's parent comet, 109P/Swift-Tuttle, last passed through the inner solar system. Those days are gone; the comet won't be back until 2126. But even now some, thin, dense filaments of meteoroids that the comet shed in recent centuries continue to liven up the shower's behavior. Strands left behind by the comet in 441 and 1479 might be in play this year, though only a little enhancement is expected from them.

At a very dark, rural site, you can probably expect to see 100 or more meteors per hour when the radiant (in northern Perseus) is highest in your sky before the first light of dawn. Any light pollution will cut down on the numbers, as will the radiant's lower altitude earlier in the night. But the brightest few meteors shine right through light pollution, and the few that happen when the radiant is low are especially long, skimming the upper atmosphere and flying far across the sky.

To get the most enjoyment while watching for Perseids, find a dark spot with an open sky view, bundle up thoroughly in blankets or a sleeping bag (for mosquito shielding as well as warmth, and don't forget the repellent), and lie back in a reclining chair. Gaze into the stars, and be patient. The best direction to watch is wherever your sky is darkest, usually straight up, perhaps with a little inclination toward the radiant. That's all there is to it!



If you're a little more ambitious, you can make a careful meteor count and report it to the International Meteor Organization. Such counts are analyzed to yield the shower's zenithal hourly rate, which is the number of meteors that a single observer would see per hour under ideal conditions: with the radiant directly overhead (at the zenith) and the sky dark enough to reveal 6.5-magnitude stars.

Not all the meteors you'll see are Perseids. In addition to occasional random, sporadic meteors, the weaker Delta Aquarid shower is also active during Perseid season. The Delta Aquarids are slower, often yellower, and track away from a radiant point in eastern Aquarius. Weaker still are the Kappa Cygnids, identifiable by their flight direction away from Cygnus in an altogether different part of the sky.

Don't forget that the Perseid shower lasts for more than one night! Rates are about a quarter to half the peak for one or two nights before and after. A few forerunners of the shower may show up as early as July 20th, and stragglers have been recorded as late as August 24th.