As of now, the scientists associated with NASA's Kepler mission have tallied 2,326 candidate planets around other stars. They were detected by the periodic dips in the light recorded as each planet repeatedly crosses in front of its host star. We science writers have to be careful to mention that only a handful of these candidates have been confirmed by follow-up observations.
But astronomers believe that fewer than 10% (or maybe even 5%) of these candidates will turn out to be "false positives" — stellar systems having mimicked the presence of planets where none actually exist. So I'm not particularly excited by the announcement, earlier this week, that astronomers had confirmed two Earth-size planets identified by the spacecraft. There are already 207 Kepler candidates in this size range, and it's a good bet that most are real.
Moreover, it's not true, as one press release touted, that these widely-trumpeted discoveries, dubbed Kepler-20 e and f, are the "smallest planets found to date." That record is still held by one of the worlds surrounding a pulsar designated B1257+12 — which, I might add, were the first exoplanets to be confirmed, way back in 1992!
Instead, what got my astro-juices flowing this week was a different Kepler-related article, also published by the journal Nature, announcing the likely detection of two Earth-size planets, dubbed KOI 55.01 and 55.02, tightly bound to a giant, hot B subdwarf star known as KIC 05807616. (KOI means "Kepler Object of Interest".) The study, led by Stéphane Charpinet (University of Toulouse, France), is important because KIC 05807616 used to be an enormously swollen red-giant star.
In order for two planets to be circling just 470,000 and 620,000 miles from their star's surface, as Charpinet's team calculates, they must have had a remarkable past. "Our two planets were most probably swallowed by their parent star when it became a red giant,a stage that ended only ~18 million years ago," the researchers write. "They were probably orbiting further away and may have been dragged deep into the red-giant envelope to their current positions."
Astronomers now know of scads of planets being roasted near their host stars, but these two are the ultimate survivors — and they have much to teach us about the ultimate fate of our own solar system. Some 5 billion years from now, the Sun will swell a couple hundred times its present size, engulfing Mercury, Venus, and Earth. Until now, astronomers had assumed that such ill-fated planets would be incinerated.
But KOI 55.01 and 55.02 are telling a radically different story. Perhaps they were initially gas-rich giant planets, and only their rocky cores have survived after their gassy exteriors evaporated away. Or perhaps, as Eliza Kempton suggests in an accompanying Nature commentary, "The two close-in planets formed anew, after the star receded from its red-giant phase, from material that was left behind when the star expelled its outer layers."
Assuming the two really did become engulfed, it's possible that they also hastened the star's mass loss due to their gravitational pull on its outer layers. Most B subdwarfs are thought to be in tight binary star systems, in which close companions help suck away enough mass during their bloated, red-giant phase to expose their cores. But some B subdwarfs appear to be solitary, and the strange case of KIC 05807616 implies that strong tidal tugs from close-in planets might draw away mass just as well as a stellar companion would.
It's important to note that KOI 55.01 and 55.02 weren't found the way most Kepler candidates are (via transits across the host star's disk), nor are they sure things. KIC 05807616 had already been studied closely because it exhibits a wide range of throbbing pulsations that cause its brightness to vary rhythmically. But Charpinet and his team found two weak brightenings with periods too long to be inherent to the star. After ruling out other possibilities, they concluded that Kepler must be detecting starlight reflecting off a pair of planets circling every 5.8 and 8.2 hours. Working the numbers, the researchers concluded that the planets, presumably rocky, would need have diameters just 76% and 87% that of Earth.
"We only dared hope that we'd see reflected light signatures of tiny planets," exclaims Natalie Batalha, Kepler deputy project scientist. "I'll be watching closely to see how this plays out." For more details, check out the press releases here, here, and here.
This is another example of how Kepler is revolutionizing both stellar science and planetary science. My view is that, in the long term, this mission might be remembered as much for what it has revealed about stars as it has about the worlds around them.