The Hunt For New Planets

The Extra Terrestrial Planets

They have been among the hottest things in astronomy for more than a decade. But let's face it. Giant Jupiters, fried Neptunes, inflated fat Saturns, pairs of giants in resonant orbits - these are just lead-ups to the main act. What we really need to know about are Exo-Earths. By an "Earth," astronomers generally mean a planet that's small enough to have a solid, rocky surface but big enough to hold a considerable atmosphere, and maybe with the possibility of "liquid" water. In other words, a place where life as we know it might arise and remains. Perhaps even, with a lot of luck, a place where humans could someday walk around on it's surface with nothing more than an oxygen tube, or even without.

Even the most dispassionate giant-planet specialist will admit that these are the prospects that get the heart pounding. Astronomers discovered the first planet circling a distant Sun-like star in 1995. Since then they've confirmed more than 300 exoplanets. Most are gas giants at least as massive as Jupiter (with 318 Earth masses), which means they're mostly hydrogen and helium. But with improving technology the list now includes worlds having only about the mass of Neptune(17 Earths) and even some with as little as about 5 Earths. This last group, the "super-Earths," are the closest that astronomers have come to finding worlds like ours around other stars. As we went to press, eight of them were cataloged in the range of about 5 to 10 Earth masses.

Most of these are hot worlds orbiting their stars very closely, because fast-orbiting planets are the easiest to find. Worlds with mild temperatures would generally have longer orbital periods,and finding one of these takes a lot of time on the world's best planet-hunting equipment. But even as the crop of hot super-Earths grows, NASA is preparing to launch its Kepler mission, a $572-million satellite to find planets more like ours. Within three years we may know for sure how common, or rare, "Earths" actually are. Astronomers define super-Earths as planets with masses from 2 to 10 times ours. They base the 10-Earth cutoff on the idea that any larger collection of rock and iron would start gathering up large amounts of gas from the protoplanetary disk that it forms in - a runaway process that leads to a massive atmosphere like those of Jupiter and Saturn. Even so, the 10-Earth limit is a "contrived definition,”says exoplanet modeler Sara Seager of MIT. For one thing, the theory does not explain why Uranus and Neptune (with 14.5 and 17.1 Earth masses,respectively) stopped growing as soon as they did. Nor does the definition distinguish between planets of different compositions;a world with 5 Earth masses could consist of a 3-Earth-mass rock-and-iron core deep inside a very massive atmosphere, or it could be a 5-Earth-mass rock ball exposed to space.

The current best guess is that the larger super-Earths probably resemble Uranus and Neptune: with small, rocky cores surrounded by thick mantles of "ices" (water, methane, and ammonia in solid form under high pressure) under deep atmospheres of mostly hydrogen and helium. Until recently, scientists could only assume that super-Earths ought to exist because bigger planets do. Now the list contains eight between 4 and 10 Earth masses. Three of these,with as little as 4.2, 6.8, and 9.1 Earths, orbit HD 40307, an orange K dwarf 42 light-years away in the constellation Pictor. More are coming. Last summer Christophe Lovis and Michel Mayor (both of Geneva Observatory, Switzerland) and their colleagues announced 45 candidate objects that range from about 4 to 30 Earth masses. The astronomers used the High Accuracy Radial-velocity Planet Searcher (HARPS)spectrograph, a European Southern Observatory instrument on a 3.6-meter telescope at La Silla, Chile. Like similar planet-hunting equipment at other observatories,HARPS is a special-purpose spectrograph that measures minuscule wavelength changes in the absorption lines in a star's spectrum. A planet orbiting a star induces, by its gravitational pull, tiny changes in the star's velocity toward and away from Earth (its radial velocity). These changes show up as very tiny Doppler shifts in every line in the star's spectrum. The HARPS instrument can detect Doppler shifts of slightly less than 1 meter per second (2 mph) - far better than the 15 m/s accuracy of the Elodie spectrograph with which Mayor and Didier Queloz detected the half-Jupiter-mass planet around 51 Pegasi back in 1995.

But Earth, by comparison, induces a mere 0.1 m/s Doppler shift in the spectrum of the Sun. Moreover, the shift would take a whole Earth year to track well enough to confirm.

Predicted sizes of different kinds of planets compared to a sun-like star.

How Many Planets?

Astronomers expect terrestrial planets to be fairly common- more common in fact than gas giants, which require a lot more material to form. Exoplanet hunter Geoff Marcy (University of California, Berkeley) says that his team's radial-velocity survey and others indicate that roughly 10% of nearby stars have a planet at least as massive as Jupiter within 5 astronomical units, Jupiter's distance from the Sun. And 15% of stars have at least a Jupiter or Saturn within 10 a.u., Saturn's distance. Some very different work by Joshua Eisner (also at Berkeley) and his colleagues supports these estimates. In July his team announced that less than 10% of the new born stars that they observed in the Orion Nebula star-forming region have enough material in their surrounding disks to create a Jupiter-mass planet. On the other hand, the average disk in the team's survey (having a thousandth the Sun's mass) could form a planet twice as massive as Neptune. That's enough stuff for 34 Earths. Eisner and his colleagues used the Combined Array for Research in Millimeter Astronomy (CARMA), a millimeter-wave (extremely far-infrared) telescope in California. It currently lacks the sensitivity to detect sparser disks from which only lower-mass planets could form. (Of course, small planets can also coalesce in heavier disks along with giants.) But other observations of young stars show that roughly 80% have disks massive enough to make Earth-like terrestrial planets, says Marcy. It's therefore likely, he concludes, that 80% of all stars have rocky planets. Increasingly, astronomers are looking for super-Earths around the lowest-mass stars: dim, red M dwarfs. These typically emit less than 1% the light and heat of the Sun,so their habitable zones - where temperatures would allow liquid water on a planet's surface - are very narrow and lie within about 0.1 a.u of the star. Low-mass stars respond more to planets' tugs, but their faintness demands long observations to extract a conclusive signal from the background noise. Nevertheless, that's where the super-Earth action is happening now.

What Lies Beneath

Radial-velocity measurements have detected four times as many exoplanets as all other techniques combined. But they provide only a minimum mass value, and they say nothing about a planet's diameter or makeup. The key to knowing what super-Earths really are like will lie not with radial-velocity studies, but with the transit method. Transit searchers look for slight, periodic dips in a star's brightness caused by a planet crossing in front of it in silhouette. Astronomers had confirmed 52 transiting exoplanets as of late October, all of them gas giants. The amount of dimming reveals the planet's diameter (assuming the star's own diameter can be accurately estimated). And because we must see the orbit nearly edge-on to observe a transit, the star's radial-velocity wobble yields the planet's actual mass, not just its minimum mass. The diameter and mass together tell the planet's average density. This in turns says a lot about its makeup in particular, whether it's made of mostly gas or rock. In a few cases, astronomers have even identified gases in the atmospheres of transiting giants spectroscopically. Until recently, scientists thought super-Earth interiors would be too complex to model successfully, says Dimitar Sasselov (Harvard-Smithsonian Center for Astrophysics).

BACKYARD TRANSIT

Millions of people watched Venus transiting our own Sun on June 4, 2004. Venus is 95% as wide as Earth,

but because it was nearly4 times closer to us than the Sun is, it looks here the way a 3.4-Earth-diameter planet would from afar. Right: During the transit, Dennis di Cicco caught this image showing a trace of sunlight filtering through Venus's atmosphere.

This light was extremely weak compared to the total light of the Sun.

The challenge for exoplanet researchers is to compare the spectrum of a star with and without this tiny atmospheric filter in front of it. They've already done it for giant planets, and it should some day be possible even for an Earth-size world.

Material Studying

Unlike gas giants, which must be mostly hydrogen and helium, a small planet of a given size and density could form from a variety of recipes mixing hundreds of substances, including metals, rocks, ices, and/or thick atmospheres. However, planetary studies and theory narrow down the possibilities. It turns out that at the extremely high pressures within these planets(100,000 to several million times Earth's atmospheric pressure), materials behave in a limited number of predictable ways. Scientists refer to the "equation of state" that describes a material's condition at a given pressure and temperature. Yet even if the likely materials have very different equations of state, different mixes of them could still produce a planet of the same density and size. "And that's a bummer," says Sasselov. Luckily, some constraints are more clear cut. Sasselov says that a mass/radius range exists for which a planet must be "water-rich," made of 2% to 10% (or more) water. Earth isn't one of these: our thin oceans make up only0.05% of Earth's mass. Seager, Marc Kuchner (NASA/Goddard Space Flight Center), and their colleagues say that, for planets with robust mass and radius estimates and no substantial atmosphere, the likely proportions of iron, silicates, and water (including water-ice at high temperature and pressure) could be determined with just 5% uncertainty. But it will take a lot of these observations for astronomers to constrain general models for what super-Earths are actually like. That, Sasselov says, is where the Kepler mission's mother lode of data will come in.

COROT and Kepler

Ground-based transit observations have limited precision because of problems with Earth's atmosphere, and they also suffer the added annoyance of daytime. So the best planet-transit work has to be done from space. The European Space Agency's COROT (Convection, Rotation, and planetary Transits) mission, launched in December 2006, should be able to detect transits of much smaller exoplanets than are possible to see from the ground almost down to Earth's diameter, depending on the diameters of the stars they orbit. COROT scientists are conducting uninterrupted, 5-month observing runs measuring the brightnesses of up to 12,000 stars at a time, with an unprecedented precision of up to one part per million (0.000001 magnitude)for bright stars. COROT has detected one signal that may prove to be an exoplanet with about 1.7 times Earth's diameter (suggesting maybe 6 Earth masses), but its other candidates so far are giants. The chance of a transit happening at all depends on the ratio of the star's diameter to the orbit's diameter. So the smaller the orbit, the better. For a planet 1 a.u. from a Sun-like star, the chance of ever seeing transits is only1 in 210. And even if an Earth-size planet does transit, it would dim the star by only 1 part in 10,000 for just a few hours once a year. To confirm a transit, you need to see several identical dips at regular intervals. So COROT's 5-month stints limit it to worlds that orbit their stars closer than Mercury circles the Sun. The really interesting exoplanets - those in their stars' habitable zones - generally lie farther out. Identifying them will require monitoring the star steadily for several years. NASA has built the Kepler mission to do just that. Scheduled for launch this April, Kepler will spend 3½ years staring at the same 100,000 stars in a large patch of sky in Cygnus and Lyra, as shown in the sky map below.

KEPLER'S CHOSEN FIELD

NASA's Kepler spacecraft will stare continuously at a squarish field in Cygnus and Lyra for at least 3½ years, measuring the brightnesses of 100,000 stars every 30 minutes to high precision in search of transiting planets.

Its 1.4-meter primary mirror will collect about 25 times as much light as COROT's, and it should achieve an accuracy of at least 1 part in 50,000 (0.00002 magnitude) for 12th-magnitude stars. Its photometer will cover 105 square degrees of sky, about as much sky as your fist covers at arm's length. Within this area Kepler will watch a preselected catalog of stars, from puny M dwarfs up to type-A and B stars bigger, hotter, and more massive than our Sun. Astronomers have already spent several years observing and classifying 3 million stars in Kepler's field, says mission principal investigator William Borucki (NASA/Ames Research Center), in order to choose the best to watch. With 3.5 years of Kepler data in hand, project scientists expect to find between 50 and 640 candidates with periods of roughly a year. Twelve percent of stars should have at least two. Kepler will also likely find several hundred exoplanets transiting with very short periods, as well as a few dozen giants orbiting beyond 1 a.u. These predictions assume that all the target stars have one Earth-sized planet in their habitable zones, Borucki explains. They might not. But even if Kepler detects nothing(unlikely but possible), the result would have major significance, scientifically and philosophically.

From the events that Kepler counts, astronomers will be able to extrapolate to say reliably how many planets of what types circle stars throughout the Milky Way and the universe. It's important to remember that almost every transit that Kepler or other studies detect is only a candidate for at planet, until its star's radial-velocity wobble can be measured. For the lowest-mass worlds, that may never be possible. For actual Earth analogs, Kepler's data will produce"a lot of 'maybes' and a few 'yeses,'" says Seager. Still, the overall statistical picture should be reliable. NASA planners also see Kepler as a forerunner that will provide an atlas of data to guide future missions such as the Terrestrial Planet Finder (TPF), which is just a star in NASA's eye for now. TPF's goal will be to image Earth-like exoplanets directly and study their atmospheres spectroscopically. In the nearer term, the James Webb Space Telescope (scheduled to launch in 2013) will study planet formation in the infrared and may be able to image young Jupiter-mass exoplanets directly by their heat glow. Kepler will also provide plenty of fodder for followups by ground-based radial-velocity instruments. Traditional transit studies from the ground still have vital roles as well. Many are currently at work. The M-Earth project plans to survey roughly 2,000 nearby M-dwarfs for habitable super-Earths. Transit search, a group formed by Tim Castellano (NASA/Ames Research Center) and Greg Laughlin (University of California, Santa Cruz), works with amateur astronomers to discover and verify transits. Transit search amateurs currently hold the record for detecting the longest-period transiter: HD 17156b in Cassiopeia, with a period of 21 days in a highly eccentric orbit. But only Kepler will be able to provide the immense amount of information necessary to understand super-Earths well, says Sasselov. "We may have found another 15 to 20 super-Earths" by the time Kepler's data start pouring in, he explains. "But Kepler will introduce a completely different scale of knowledge."

CoRot measures the light variations coming from stars with a very high accuracy. Every perturbation of this intensity, caused by the instrument itself or by the spacial environment, must therefore be minimized, and in particular, the parasitic light generated by the Earth must be minimized. This is what has determined the optical design of the instrument. CoRoT is a wide-field telescope with a diameter of 27 cm, working in the visible domain, which collects and concentrates photons, and form an image of the sky on the detectors installed in the focal block.

An equipment compartment contains all the electronic equipement required for the function of the instrument, and the on board calculator responsible for data processing.

The First Second Earth

Worlds in habitable zones with liquid water don't necessarily imply life. For all we know, life may be such a fluke that even the simplest bacteria are rare in the universe. But with exceptionally good transit data, astronomers may be able to detect the composition of atmospheres surrounding terrestrial exoplanets. If they find chemicals that only exist as byproducts of non-equilibrium processes(free oxygen, for instance, which persists in Earth's atmosphere only because photosynthesis keeps creating it), this could be the first evidence of alien life. Such worlds might look nothing like ours. Several astronomers think the best bet for finding the first exo-Earth in a habitable zone will be around an M or K dwarf. Such a star would be a large, orange orb in an other worldly sky and, if it's reasonably young, it might be magnetically active enough to produce spectacular auroras and other space-weather phenomena. The star might hold the planet in a tidal lock, with one side facing the star in eternal day and the other facing away in eternal night."I have no idea what star it will be, nor even what type of star it will be," Geoff Marcy says. "But I do know this. Humanity will discover the first plausibly habitable planet within the next three years. A question posed by ancient Greek philosophers about the uniqueness of our Earth will finally be answered." Meanwhile, we look up and wait.

Newly found planets, such as Fomalhaut b seen above orbiting its parent star, have supplied new frontiers to search.

Comparison of the Solar system and the system around Fomalhaut.

First map of extraterrestial planet HD189733b, Spitzer Space Telescope.

Updates on August 30, 2011, National Geographic News

New Planet May Be Among Most Earthlike - Weather Permitting Alien world could host liquid water if it has 50 percent cloud cover, study says.

The unpoetically named HD85512b was discovered orbiting an orange dwarf star in the constellation Vela. Astronomers found the planet using the European Southern Observatory's High Accuracy Radial velocity Planet Searcher, or HARPS, instrument in Chile. Radial velocity is a planet-hunting technique that looks for wobbles in a star's light, which can indicate the gravitational tugs of orbiting worlds.

The HARPS data show that the planet is 3.6 times the mass of Earth, and the new world orbits its parent star at just the right distance for water to be liquid on the planet's surface, a trait scientists believe is crucial for life as we know it.

"The distance is exactly the limit where you want to be to have liquid water," said study leader Lisa Kaltenegger of the Harvard-Smithsonian Center for Astrophysics and the Max Planck Institute for Astronomy. "If you scale it to our system, it's a bit further out than Venus is to our sun." At that distance, the planet likely receives a bit more solar energy from its star than Earth does from the sun.

Planet Sizes - presence in the galaxy related to Earth size

But Kaltenegger and colleagues calculate that a cloud cover of at least 50 percent would reflect enough of the energy back into space to prevent overheating. On average, Earth has 60 percent cloud cover, so partly cloudy skies on HD85512b are "not out of the question," she said. Of course, clouds of water vapor depend on the presence of an atmosphere similar to Earth's, something that can't be detected on such distant planets with current instruments.

Models of planet formation predict that planets with more than ten times Earth's mass should have atmospheres dominated by hydrogen and helium, Kaltenegger said. Less massive worlds, including HD85512b - are more likely to have Earthlike atmospheres, made mostly of nitrogen and oxygen.

New World "A Strong Candidate" for Habitability

So far, the newly detected planet is only the second rocky world outside our solar system to be confirmed in its star's habitable zone, the region around a star that's not too hot and not too cold for liquid water. The other contender, planet Gliese 581d, was previously discovered using the HARPS instrument. This world lies just on the cool edge of its star's habitable zone.

Another promising planet, Gliese 581g, was discovered in 2010 and dubbed the most Earthlike planet yet. But controversy surrounds the claim, with some experts declaring that the entire planet is actually a data glitch. Manfred Cuntz, director of the astronomy program at the University of Texas, Arlington, noted that more information is needed before anyone can speculate whether aliens are wandering around the new found planet.

"It's not their fault no extra information [about the planet's atmosphere] is available right now," Cuntz said of the research team. "It looks like this is a strong candidate, in principle."In addition to size and location, HD85512b has two other points in its favor for potentially harboring life, Cuntz said. The planet's orbit is nearly circular, which would provide a stable climate, and its parent star, HD85512, is older, and therefore less active, than our sun, which would lower the likelihood of electromagnetic storms damaging the planet's atmosphere.

Not only that, but in principle, the age of the system - 5.6 billion years - "gives life a chance to originate and develop," he said. By contrast, our own solar system is thought to be about 4.6 billion years old.

Given current limits on space travel, it's unlikely for now that humans will get to visit HD85512b. But if we could get there, the new found planet might seem like a fairly alien world: muggy, hot, and with a gravity 1.4 times that of Earth's, study leader Kaltenegger said. On the bright side, "hot yoga might be one of the things you don't have to pay for there," she quipped.

Known types of alien worlds. Credit: Space.com

Astronomers studying data from the Kepler space telescope estimate that 17 percent of stars in the Milky Way galaxy have planets about the size of Earth. This means that about one in six stars has an Earth-size companion exoplanet. As there are about 100 billion stars in the galaxy, there are at least 17 billion Earth-size worlds in this galaxy alone. Credit: Space.com

Earth vs Planet KO1 172.02
KOI-172.02, (K00172.02), is an unconfirmed (Kepler Object of Interest) candidate exoplanet discovered by Kepler Mission space observatory and first announced on January 7, 2013.
The candidate object, a Super-Earth, has a radius 1.54 times that of Earth. KOI-172.02 orbits a sun-like star, named KOI 172, within the "habitable zone" a zone where liquid water could exist on the surface of the planet. Scientists claim the exoplanet, if confirmed, could be a "prime candidate to host alien life". Credit: Wikipedia, Space.com

Infograph: NASA planetary missions, notice the increased number of Mars related missions.

Sources: S&T Magazine, Wikipedia, Spitzer Space Telescope, Astrophysique sur mesure, National Geographic News, and various..

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