Astronomers had already detected individual starspots by watching exoplanets pass in front of them. But recent advances may allow for extensive mapping of spots on distant suns, complementing other techniques that are restricted in other ways.
A team led by Juergen Schmitt at Hamburg Observatory in Germany analyzed fine-brightness measurements of the Sun-like star Corot-2a. This is a G-type star like ours, but it's much younger. It hasn't had time to lose much of its spin to magnetic stellar-wind braking, as happens to Sun-like stars as they age, so it still rotates with a period of around 4½ days (compared to the Sun's rotation period of about 27 days).
As seen from Earth, a very close-in planet named Corot-2b passes in front of the star every 1.74 days. ESA's orbiting Corot telescope measured the light of the star during 80 or so of these transits. But the Hamburg group realized that no two light curves were quite the same.
For one thing, the star’s overall brightness varies slightly from day to day. This was already known; the reason is that spots on the star’s surface rotate in and out of view, and other spots form or disappear while in view. Fast-rotating Sun-like stars (stars with convective zones in their interiors) have extensive spots and other signs of strong magnetic activity.
In addition, during a transit the planet may directly cross a spot. In at least one instance, Uwe Wolter (lead author of the group's paper) found that the transit's light curve wasn't a smooth dip but displayed a brief bump when the planet crossed a small area that wasn't as bright as the rest of the star's surface.
This is not the first starspot detected this way, but it's a particularly interesting one. “This is the closest analogy to sunspots that we have yet on another star,” says John Thomas (University of Rochester). “The spot they detect lies within 20° of the equator, as do most sunspots, and the spot size that they measure is comparable to a large sunspot group on the Sun.”
Patient work by the Hamburg group with more Corot data might tell much more, Thomas explains: “They can potentially use their technique over a longer term to follow spot evolution on this star and hence study its activity patterns, maybe even producing the analog of the solar 'butterfly diagram' for this star. That would be wonderful!” (The butterfly diagram is the graph of sunspots' changing latitudes over the course of the Sun's 11-year activity cycle).
Schmitt asks the question, “How many of those spots are at low latitude?” To address it the Hamburg group has written another paper looking at the statistics of the 80 transits of Corot-2b. The group found something unexpected. “The depth of the transit depends on the [overall] spot coverage,” says lead author Stefan Czesla. That is, the dimmer the star appears out of transit (presumably because it's covered with more spots), the smaller the fraction of its light that gets blocked by the planet during a transit, on average.
This seems to mean that the extra spots dimming the star lie at low latitudes, under the planet’s trajectory — so it eclipses more spots and less of the bright surface. But Schmitt remains cautious: “We’re actually working to try and quantify that.”
This wouldn’t mean all spots on CoRoT-2a lie at low latitudes. Sun- and starspots appear when buoyant material with an embedded magnetic field rises up to the surface through the deep convective layer. Rotation deflects this process via the Coriolis force, so in fast-rotating stars, the material should rise mostly in the polar regions. This means CoRoT-2a’s poles are likely to have even more spots than its law latitudes. The poles may even be continually covered in spots.
This method for identifying starspots complements the established one that relies on the various Doppler shifts of different parts of a star's surface as it rotates. But that only works on stars that spin even faster than CoRoT-2a, and even then it tends to be restricted to high latitudes.
The Hamburg group also determined that starspots affect measurements of a transiting exoplanet's diameter. In the case of CoRoT-2b, the astronomers determined that the planet is about 3% larger than was found without considering the starspots. The effect is small but worth taking into account when possible.