It started over coffee in China.
Last month, a group of visiting astronomers at the Kavli Institute for Astronomy and Astrophysics at Peking University were talking about the latest “hot” topic: the curious case of two small but high temperature objects found orbiting around a pair of distant stars.
The discovery was made by Kepler — NASA’s planet-hunting satellite launched one year ago this week. Kepler’s approach is brute force; it continuously monitors a field of over 150,000 stars looking for those that periodically dim when orbiting planets block a tiny fraction of their light.
In January, when mission scientists unveiled their first discoveries with Kepler, a couple of strange stars turned up. Each has a light curve that is the reverse of what's expected from Kepler. The signal is dimmest when a planet-size companion passes behind the star rather that in front of it. This means the companions are brighter per unit surface area (and therefore hotter) than the stars they circle.
Jason Rowe, a NASA postdoctoral fellow based at the Ames Research Center in Moffett Field California, was puzzled when he first saw these light curves. “They stuck out like sore thumbs,” he says. The objects were dubbed KOI-81 and KOI-74 (mission-speak for “Kepler Object of Interest”).
It’s hard to imagine planets that are hotter than stars, so the Kepler team considered an alternative. Might they be white dwarfs? The degenerate cores of dead stars that have blown off their outer layers, white dwarfs are squeezed by their powerful self-gravity into small, dense, and very hot objects. The trouble was KOI-81 and KOI-74 didn’t seem small enough to be white dwarfs.
Clearly, the most relevant information would be the masses of the two hot companions. Were they heavy, like white dwarfs, or lightweights, like planets?
Kepler can’t measure an orbiting companion’s mass directly. But in a paper discussing the objects Rowe and his coauthors estimate the masses by measuring how much the hot companions seems to physically deform the stars they orbit. (The companion’s gravity squishes the star slightly, turning its spherical shape into something that is slightly more like a football). Such deformations produce slight variations in the light curves, which Kepler can measure.
These results point to objects that are less massive than a typical white dwarf star. KOI-74, in particular, seems to weigh in at no more than one tenth the Sun’s mass. Could it be a planet after all — a planet heated to more than 12,000 kelvins (21,600°F)?
The search for a resolution to this paradox is what brings us back to the Kavli Institute in China. The astronomers sipping coffee there and chewing over Kepler’s findings in mid-January included Marten Van Kerkwijk and Stephen Justham along with Rene Breton, a visiting postdoc from the University of Toronto. Their discussions were prompted by an email from MIT astronomer Saul Rappaport, and remotely included Philipp Podsiadlowski of Oxford University and Zhanwen Han of Yunnan Observatory.
The group realized there might be another way to measure the masses of the hot companions using the Kepler data. This is because Kepler is tailor-made not only to spot slight changes in the brightness of a star but also to do so at very well-defined wavelengths. This means it’s perfectly set up to detect “Doppler boosting”.
Doppler boosting means an approaching light source looks brighter because more of its photons are arriving at your detector per unit of time compared to when the object is stationary. The faster the approach the bigger the boost. Conversely, a receding source looks dimmer because you’re getting fewer photons per unit time. (The effect can be diminished depending on the colour of the source, but for the Kepler stars with the hot companions, it’s the photon count that dominates.)
Van Kerkwijk obtained the raw Kepler data for the two stars with the mysterious hot companions and found that, indeed, there was Doppler boosting in the signal. In each case, the strength of the boost is directly related to how much the orbiting companion is pulling on the primary star. The more massive the companion, the stronger the gravitational pull.
The method allowed the China group to recalculate the masses of the hot companions. Their results neatly resolve the paradox. The objects are light but still fall into the white-dwarf mass range, especially white dwarfs that form in binary systems where some transfer of mass onto the neighboring star is likely. After the recalculation, KOI-74 ends up being more like 20% of the Sun’s mass. This fits with the larger than standard radius detected by Kepler, since lower-mass white dwarfs have less self-gravity and so are less compact.
In another recent paper, Rosanne DiStefano of the Harvard-Smithsonian Center for Astrophysics has calculated that Kepler should detect about 1,000 transiting white dwarfs that are the products of mass-transfer binaries. If so, KOI-74 and KOI-81 are simply the first two entries in what could be quite a long list.
Finding a couple of white dwarfs is not exactly a home run for Kepler (it's supposed to be finding Earth-mass planets). On the other hand, showing that these objects are massive enough to be white dwarfs by using Doppler boosting is the equivalent of a nicely executed catch. It’s the kind of everyday science that doesn’t attract headlines, but that strengthens our understanding of the universe in a way that makes major discoveries possible down the road.
I see two encouraging signs from this little saga. The first is that Kepler is performing with exquisite precision. The effective use of Doppler boosting could not have been accomplished (at least not yet) from the ground. “The effect is very small,” says van Kerkwejk, “which means it can only be used with a space-based instrument.”
The second is that the excitement generated by this new piece of hardware is bringing out the best in the available software — namely, the collective brainpower of the astronomical community. Researchers are stimulated by the possibilities Kepler has opened up. They’re talking about it over coffee in China and in places around the globe where astronomers gather. And whenever there’s talk of this kind, innovations follow.
Ivan Semeniuk is host of the podcast The Universe in Mind and a science journalist in residence at the Dunlap Institute for Astronomy and Astrophysics, University of Toronto.