NASA's Inspiring, Enlightening, and Successful Search for New Earths

Journey that began four decades before Columbus sailed for the New World finally ended when the Kepler space telescope snared a few errant photons as they shot past Earth’s orbit en route to infinity. The light had sped through space for 560 years, traveling more than three quadrillion miles from a star much like our sun. Captured by Kepler’s digital sensors, transformed into bytes of data, and downloaded to computers at NASA’s Ames Research Center near San Francisco, the processed starlight slowly revealed a remarkable story: A planet not much bigger than Earth was whipping around its native star at a blistering pace, completing an orbit—its version of a “year”—in just over 20 hours.

Aside from its size, the planet bears little resemblance to Earth. It circles so close to its star that its surface temperature probably exceeds 2,500 degrees Fahrenheit, hot enough to melt iron. Nevertheless, the planet’s detection was a technical and intellectual coup, a rite of passage for Kepler. The planet, dubbed Kepler-10 b when NASA announced its existence this past January, was the smallest world yet found beyond our solar system. Its discovery proved that the Kepler spacecraft, which was launched in March 2009, could indeed do what its designers had boldly promised: find small, Earth-size planets around distant stars, a task that once seemed so difficult as to border on the absurd.

Kepler-10 b was merely a preview. A month after the January announcement, NASA released its first full data set from the Kepler mission, and the results left astronomers straining for superlatives. “Frankly, we’re overwhelmed,” says Geoff Marcy, an astronomer at the University of California, Berkeley, and a member of the Kepler team. “What NASA is doing is akin to the transoceanic voyages of the 15th century—the voyages that opened up the whole world. With the Kepler telescope, we’re learning about the properties of planets across the cosmic ocean. This is history. It’s Armstrong stepping off the bottom rung.”

Kepler monitors 156,000 stars, less than 0.0001 percent of the galaxy’s population. Like some cosmic wildcat drilling operation, Kepler has struck a gusher, discovering 1,235 possible new worlds in its first four months of operation. That number doubles the previous total of just over 500, painstakingly gathered over the last 16 years. Prior to 1995, keeping track of all the known planets around other stars like the sun was easy—the tally stood at an even zero.

The bulk of Kepler’s data have not yet been studied, and the mission will keep going for at least two and a half more years. But it is already shredding the textbooks, showing that our galaxy (at least the fraction of it seen by the spacecraft) contains a far more exotic assortment of planets than astronomers expected to find. “We’re learning about a diversity of worlds in our universe that we had no clue about beforehand,” Marcy says. “Rocky planets, yeah, we thought there might be some of those. By the way, we’re finding some rocky planets that are even denser than Earth. But we’re also finding these mini-Neptunes, a class of planet for which we have no examples in our solar system. They’re like small Neptunes but with huge amounts of liquid water around a rocky core.”

Also on the list are 67 planets roughly the size of Earth, give or take a thousand miles or so in radius; 288 “super-Earths” up to twice Earth’s diameter; 662 Neptune-size planets; and 184 giants rivaling or exceeding Jupiter in size. The simple statistics from Kepler say that Earth-size planets are widespread.

Just two years into its mission, Kepler is well on its way toward determining whether planets like Earth are rare or common. But that is just the first domino as scientists try to topple the much bigger questions, the kinds that make the hairs stand up on the back of your neck. Does life exist on other planets? Are planets with life common? Are any other intelligent beings out there?

For the first time, we have a handle on the odds, and the numbers beaming in from Kepler are not only encouraging but staggering. “Our galaxy contains 200 billion stars,” Marcy says. “I would guess that at least 30 percent of them have an Earth-size planet. So 30 percent of 200 billion, that’s at least 60 billion Earth-size planets just in our galaxy alone”...



Distant light hints at size of space-time grains

THE light from a spectacular gamma-ray burst has been used to set the most stringent constraints yet on the size of the "grains" of space-time. The conclusion has been praised for pushing boundaries in the search for a theory of everything but has also drawn criticism for exaggeration.

A big challenge in physics is how to combine Einstein's general relativity, which describes gravity, with quantum mechanics to create a theory of quantum gravity. The best efforts on this front include string theory, which argues that everything in nature arises out of the vibration of tiny strings in 10-dimensional space-time, and loop quantum gravity (LQG), which shows mathematically that the fabric of space-time is woven out of gravitational field lines. Both approaches lack experimental support, however.

So cosmologists have been looking for signs of quantum gravity in the distant universe. One prediction of both string theory and LQG is that space-time is not smooth but "grainy" at extremely small scales. In theory, this graininess can be observed by studying particles from the same source but with different energies, to see if they are affected differently by the structure of space-time.

Over the past few years, terrestrial telescopes such as the Major Atmospheric Gamma-ray Imaging Cherenkov Telescope (MAGIC) in the Canary Islands and the High Energy Stereoscopic System (HESS) in Namibia have seen low-energy photons from a gamma-ray burst (GRB) arriving before their high-energy counterparts. While this could be due to delays in emission at the source, it could also be caused by the interaction of photons with the structure of space-time (New Scientist, 15 August 2009, p 26).

Now a team has used data from the Integral satellite, run by the European Space Agency (ESA), to study an entirely different effect: the polarisation of light of different energies from a GRB. In December 2004, Integral observed one of the brightest GRBs ever recorded. "We were lucky because the GRB was in the field of view of the instrument," says Philippe Laurent of CEA, the French government's atomic energy research institute in Saclay.

Earlier this year, astronomers used the Canada-France-Hawaii Telescope on Mauna Kea, Hawaii, to estimate the distance to the galaxy that produced this GRB, putting it some 275 million light years away.

Now Laurent and his colleagues have used the combined data to look for effects of quantum gravity. One idea is that if space-time is composed of indivisible grains, then this would polarise photons in a way that depends on their energy. This effect would accumulate over cosmological distances and so be observable from Earth. Yet the team found no such effect (Physical Review D, DOI: 10.1103/physrevd.83.121301).

Considering the extremely high quality of Integral's data, this puts an important limit on the size of the grains of space-time. According to an ESA press release, it means they must be smaller than 10-48 metres, many orders of magnitude smaller than the Planck length of 10-35 metres, the universe's smallest length scale.

However, this conclusion is drawing criticism. Carlo Rovelli, who studies LQG at the Centre for Theoretical Physics in Marseille, France, points out that grains of space-time cannot be smaller than the Planck length and says that the limit is "wrong".

The strange conclusion is a consequence of the way in which the data was analysed with respect to a very specific model of quantum gravity, which has fallen out of favour with some theorists. The main versions of both string theory and LQG don't support such a model. "This result does not contradict the main expectations [of these theories]," says Rovelli. Laurent agrees that the work has limitations. He admits that it doesn't affect theories that make more subtle predictions. These are still beyond the reach of today's experiments.

Nonetheless, the work demonstrates astonishing progress. Giovanni Amelino-Camelia, who studies quantum gravity at Sapienza University of Rome, Italy, is thrilled with the quality of observations and the analyses carried out by Laurent's group. He says this kind of work was unthinkable just a decade ago and raises hopes that more subtle effects could soon be tested.

This would rely on astronomers finding ever more distant GRBs and studying photons that are widely separated in energy. "The pace of improvement is well beyond the most optimistic expectations a decade ago," Amelino-Camelia says. Even Rovelli agrees that the future looks promising. "I share this excitement," he says.