Astronomers have tended to search for extrasolar planets around stars that are roughly the mass of our Sun. There are practical reasons for this. Lower-mass stars are faint and therefore harder to study. Higher-mass stars, such as Vega and Sirius, are hotter than the Sun, spin faster, and tend to pulsate slightly — which means that their spectral lines are fewer, more smeared out, and show changing velocities. Sharp, stable spectral lines are critical for measuring the tiny wobble in a star’s radial velocity caused by an orbiting planet.
According to John A. Johnson (University of California, Berkeley), stellar evolution offers a way to sneak around this problem. At the American Astronomical Society meeting in Honolulu, Johnson described observing former A-type stars that have evolved into subgiants. “We have a star that’s roughly the same mass as the normal A star, but has a much larger radius, much cooler temperature, and a significantly lower rotational velocity,” he explained. “So we can get roughly the same radial-velocity precision around such a subject as we can for around a solar-mass G star.” He has nicknamed these evolved suns, which have nearly completed their hydrogen-burning phase, “retired A stars.”
For the past three years Johnson and his team have used the Lick and Keck Observatories to examine 150 subgiants. They've discovered Jupiter-size planets around three of them so far; the host stars are between 1.6 and 1.9 times the mass of our Sun. This brings the number of retired A stars with planets to nine.
This is enough to start showing an intriguing trend. According to Johnson, none of these planets orbits its host star closer than 0.8 astronomical units; all are in long-period orbits, very unlike the planets around solar-mass suns. “About 45% of all known planets orbiting Sun-like stars are within 1 a.u., so the gap is striking for A stars,” he noted.
One possible explanation is that close-in planets existed but were destroyed when the host star evolved and expanded. But Johnson argues that these particular subgiants haven’t expanded enough to do this. Instead, he thinks the gap is probably telling us something about how newborn planets migrate inward and how stellar mass affects the migration process.
Johnson hopes to investigate how stellar mass influences planet formation in general. The star's mass should be important. The larger a newborn star the more massive the gas-and-dust disk around it. The core-accretion model of planet formation predicts that a heavier disk will tend to spawn heavier worlds. So A stars should indeed have more giant planets.
According to Johnson's preliminary findings, the chance of having a Jupiter-like giant orbiting within 2 a.u. of its host is about 1% for small M dwarfs, 4% for Sun-like stars, and nearly 9% for stars weighing 1.3 to 2 solar masses.
Johnson is expanding his search to include 450 subgiants. During the next three years he hopes to add 20 to 30 new planets to the list and determine whether the lack of planets inward of 0.8 a.u. is real or simply a statistical fluke in the small number of discoveries so far.