Spotting a planet that transits in front of its host star is old news thanks to the Kepler probe—Kepler has probably spotted over 2,000 of them. But this week, scientists announced the first time a planetary transit was observed using X-rays. Although the data from X-rays is harder to interpret than that of a visible-light transit, it has also told us more: specifically, that the planet may be in the process of evaporating away.

The planet in question, HD 189733b, is a hot Jupiter (a gas giant that orbits close to its host star) that had previously been identified by other observatories. This is a good thing, since it turns out that X-rays would be lousy at planet identification. In many cases, X-rays are produced by individual features on a star's surface, and these features are both transient and rotate along with the star, meaning that they come and go from our field of view. This makes the X-ray background from a star very noisy, which makes picking out the loss of some of those X-rays (caused by the planet blocking them) rather challenging.

Spotting the HD 189733b's transit required seven separate observations, six from NASA's Chandra Observatory and one from the ESA's XMM-Newton. Even though the researchers knew there was a planet there and when it was going to be orbiting, they were only 98 percent sure they were detecting the planet's transit. After they inspected the data visually for signs of a stellar flare and eliminated some of the observations that seemed to involve such a flare, they were able to boost their certainty to 99.8 percent.

Once they had the transits, however, there was a bit of disagreement between the amount of light obscured by the planet at visible wavelengths (2.4 percent) and the amount of X-ray light obscured (six to eight percent). After considering other explanations for this discrepancy, the authors concluded that the most likely reason for this is that the planet's close proximity to its host star was causing its atmosphere to heat up and puff out. A thin cloud of hydrogen surrounding the planet would largely be transparent to visible light, but it would absorb significant X-rays. This would make the planet's X-ray radius larger than its visible light radius, accounting for the difference between the figures.

The problem with this is that the hydrogen would be so far from the planet's core and so energetic that it wouldn't remain bound to the planet by gravity. In effect, it would evaporate away—the authors suggest that the giant planet may look a bit like a comet, with a tail of material flowing away from the star. They calculate that it's probably losing mass at a rate of about 109 kilograms every second. That may sound horribly unstable, but given the planet's mass, it would take about 70 billion years for the planet to evaporate entirely.

The planet, in turn, seems to be influencing the behavior of the star. The activity level of the star would indicate that it's about 1.2 billion years old. But a companion star, which should have formed at about the same time, appears to be over four billion years old. The authors suggest that the apparent youth of HD 189733 may be a product of the tidal influence on the star's interior exerted by the nearby planet.

Although the results of the paper are interesting in their own right, they are a further indication that we've moved past the discovery phase of exoplanets into the an era where we can perform detailed observations of them and learn a bit about their properties and history. All of which will ultimately give us a much clearer picture of the Universe we inhabit.

The arXiv. Abstract number: 1306.2311 (About the arXiv). To be published in The Astrophysical Journal.