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2011 got off to an exciting start with the announcement of Kepler-10b, the first rocky planet ever discovered outside our solar system. The announcement was widely expected. Last August, a NASA scientist referred to 140 candidates for rocky planet status and now, it appears, they have a confirmed result. The discovery was announced at the a meeting of the American Astronomical Society by Nasa's Kepler team, accompanied by a press release. Richard Kerr, writing in Science, pointed to the significance of the find:

"Astronomers have announced the discovery of an extrasolar planet not much larger than Earth - the smallest exoplanet yet found. Although the world orbits too close to its sun to sustain life, the finding is a milestone in the quest to find out how common Earth-sized, habitable planets really are. It also shows that, with some luck and some innovative new technology, astronomers could be announcing the discovery of a habitable Earth-like exoplanet within a few years."

Artist concept of Kepler-10b (Credit NASA, source here)

According to NASA, the planet has a diameter 1.4 times that of Earth, a mass 4.6 times that of Earth, and an average density of 8.8 grams per cubic centimetre. In the words of Dimitar Sasselov, one of the scientists leading the project, "This planet is unequivocally rocky, with a surface you could stand on". However, according to Nasa, Kepler-10b orbits once every 0.84 days, and is more than 20 times closer to its star than Mercury is to our sun. The daytime surface temperature is about 1500 degrees C, making it likely that if anyone tried to stand on the irradiated surface, there would be a danger of sinking into molten lava. Furthermore, at these temperatures, there is no prospect of the planet retaining an atmosphere.

What then is the significance of this discovery? Several reasons have been adduced. First, it is a rocky planet. All the previous planets discovered (about 500 of them) are giant - Jupiter-like bodies. Astronomers have always expected rocky planets to be discovered when the instruments became sufficiently sensitive - but this only became possible with the Kepler instrument. Second, it demonstrates that Kepler can sense planets of comparable magnitude to the Earth. This gives encouragement that other finds will be forthcoming. "All of our very best capabilities have converged on this one result and they all converge to form a picture of this planet," said Natalie Batalha, an astrophysicist who helped lead the Kepler mission.

The other reasons offered go beyond the science and reveal the aspirations of researchers and the significance of this find for them. For these, we must look at the reports carried by the popular media. BBC News carried some informative comments:

"[. . .] as Professor Batalha explained, it is a significant step in Kepler's mission. "We want to know if we're alone in the galaxy, simply put - and this is one link in the chain toward getting to that objective. First we need to know if planets that could potentially harbour life are common, and we don't know if that's true - that's what Kepler is aiming to do."
A pioneer of the hunt for exoplanets, Geoffrey Marcy, from the University of California Berkeley, said that Kepler 10b represented "a planetary missing link, a bridge between the gas giant planets we've been finding and the Earth itself, a transition... between what we've been finding and what we're hoping to find". "This report... will be marked as among the most profound scientific discoveries in human history," he said.

These comments show that a major driver for these scientists is the quest to find planets in the habitable zone that could potentially harbour life. They are excited because here is the first rocky planet. It does not matter to them that it is not in the habitable zone, that its day side is a furnace, nor that it has lost all atmosphere and water. Finding this hostile rocky planet is nevertheless a milestone for these researchers that they are determined to celebrate. It is perhaps significant that in September 2010 another object was claimed as the first rocky planet: Gliese 581g. This was said to lie in the habitable zone of its star and have a 37-day orbit. Its mass was said to be between 3.1 and 4.3 times that of Earth. One astronomer commented: "That's the most exciting exoplanet I've seen yet". Significantly, Prof Steven Vogt, who led the team that discovered this planet, said that he was 100% convinced it would contain life:

"Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say, my own personal feeling is that the chances of life on this planet are 100 percent," he said during a press briefing. "I have almost no doubt about it." [. . .] Prof Vogt estimates that as many as one in five to 10 stars in the universe have planets that are Earth-sized and in the habitable zone. With an estimated 200 billion stars in the galaxy, that means that around 40 billion planets could have the potential for life, he said.

However, excitement about this find was short-lived, as another research team reported their failure to find the signal diagnostic of Gliese 581g. This provides the context for Nasa reporting the "first solid evidence of a rocky planet". They did not have a habitable zone planet, but they are convinced that will come in the not too distant future. However, the evidence on which this hope is based is actually rather tenuous. All the extrasolar planets discovered before this first rocky Earth-like planet are either gas giants, hot-super-Earths in short period orbits, or ice giants. The presumption that there are millions of Earth-like planets in habitable zones is based on theory that is not supported by evidence. This point has recently been made by Howard Smith, a senior astrophysicist at Harvard. He has made the claim that "we are alone in the universe" after an analysis of the 500 planets discovered so far showed all were hostile to life.

"Dr Smith said the extreme conditions found so far on planets discovered outside out Solar System are likely to be the norm, and that the hospitable conditions on Earth could be unique.
"We have found that most other planets and solar systems are wildly different from our own. They are very hostile to life as we know it," he said."

This argument for extrasolar planets is essentially the same as it is for planets in our solar system. The Earth is unique in the mix of characteristics it has: size, distance from the sun, oxygenated atmosphere, substantial coverage by oceans, and so on. Based on evidence, some argue that the Earth is a Privileged Planet. The basic approach of that book is being vindicated as research discovers just how extraordinary the Earth is.

"Gonzalez and Richards counter the prevailing notion among scientists that Earth is merely an average rocky planet revolving around an ordinary star on the outskirts of an undistinguished galaxy. The authors present evidence that suggests life in the cosmos is a rarity due to a variety of prerequisite conditions, such as the unique properties of water, the peculiarities of the Earth-moon system, the sheltering effects of Jupiter and Saturn, and the fine-tuned nature of the universe. The authors maintain that these same conditions allow mankind's significant discovery of the structure of physical laws and the universe. The appendices examine a revised Drake Equation and tackle the idea of "panspermia" - the seeding of life on Earth." (Source here)

Kepler's First Rocky Planet: Kepler-10b
Natalie M. Batalha et al.
Astrophysical Journal, 2011, 729(1), 27 | doi: 10.1088/0004-637X/729/1/27

NASA's Kepler Mission uses transit photometry to determine the frequency of Earth-size planets in or near the habitable zone of Sun-like stars. The mission reached a milestone towards meeting that goal: the discovery of its first rocky planet, Kepler-10b. Two distinct sets of transit events were detected: 1) a 152 +/- 4 ppm dimming lasting 1.811 +/- 0.024 hours [. . .] and 2) a 376+/-9 ppm dimming lasting 6.86+/-0.07 hours [. . .] Statistical tests on the photometric and pixel flux time series established the viability of the planet candidates triggering ground-based follow-up observations. Forty precision Doppler measurements were used to confirm that the short-period transit event is due to a planetary companion. The parent star is bright enough for asteroseismic analysis. Photometry was collected at 1-minute cadence for > 4 months from which we detected 19 distinct pulsation frequencies. Modeling the frequencies resulted in precise knowledge of the fundamental stellar properties. Kepler-10 is a relatively old (11.9+/-4.5 Gyr) but otherwise Sun-like Main Sequence star [. . .] Physical models simultaneously fit to the transit light curves and the precision Doppler measurements yielded tight constraints on the properties of Kepler-10b that speak to its rocky composition: [. . .] Kepler-10b is the smallest transiting exoplanet discovered to date.

See also:

Kerr, R.A. First Earth-Sized Exoplanet Discovered, ScienceNow, 10 January 2011

Mann, A. Space scope finds scorched super-Earth, Nature, 469, 143-144 (10 January 2011) | doi:10.1038/469143a