We’re still arguing about how giant planets form around Sun-like stars, but terrestrial planets seem to be less controversial. Assuming the model is right, we start with a swarm of planetesimals in the range of one kilometer in size. As these objects grow, out to a range of at least 2 AU, the largest bodies at some point go through a runaway period of chaotic growth marked by collisions. Emerging from the debris should be terrestrial worlds, some in Earth-like orbits. Add to this the fact that gas and dust disks seem to be relatively routine outcomes of star formation and you have an indication that small rocky planets may be widespread.
The problem with all this is that theory has to be matched with observation. On that score, new work by Mike Meyer (University of Arizona) and colleagues Lynne Hillenbrand and John Carpenter (California Institute of Technology) is instructive. The researchers chose to look at mid-range infrared emissions at the 24 micron level, a range chosen because it originates between 1 and 10 AU from the parent star. As targets, they chose 328 Sun-like stars in spectral types F5-K3, with masses generally not dissimilar from the Sun (though in some cases ranging as high as 2.2 solar masses). A finding of excess emissions at 24 µm was taken to be evidence of dust debris from planetesimal collisions.
The conclusions from this work are absorbing indeed. From the paper (internal references omitted for brevity):
We suggest that SST observations at 24 µm can be interpreted as evidence for terrestrial planet formation occurring around many (19–32 %), if not most (62 %), sun–like stars. This range is higher than the observed frequency of gas giant planets (6.6–12 %) within 5–20 AU…but comparable to the inference that cool dust debris beyond 10 AU might be very common…Radial velocity monitoring of low mass stars, micro-lensing surveys, as well as transit surveys such as COROT and Kepler, will provide critical tests of our interpretation.
And so we test theory with observation (and note the reference to the doughty COROT mission, gathering key data at unprecedented rates as its work continues, and doubtless setting us up for its share of surprises). But the broader picture growing out of the work of Meyer’s team is that terrestrial planets may be common around Sun-like stars, an assumption most everyone connected with the exoplanet hunt would be delighted to see confirmed. Not only would it be striking evidence that the formation mechanisms for Earth-like planets are becoming better understood, but it would strengthen the hope for living worlds around stars for which the conditions of life may not be so rare after all.
The paper is Meyer et al., “Evolution of Mid‐Infrared Excess around Sun‐like Stars: Constraints on Models of Terrestrial Planet Formation,” Astrophysical Journal Letters 673 (February 1, 2008), pp. L-181-L184 (abstract); also available in full text here. I notice that Science News has picked up on this team’s work as well, with a story quoting Caltech astronomer Charles Beichman (not a member of Meyer’s group):
“Meyer’s result is exciting confirmation that around many other stars like our sun, the region analogous to our own asteroid belt is full of solid material, possibly related to past or present planet formation.”
Note the ‘possibly’ in that sentence, a reminder of how much work remains to be done. Beichman goes on to call the work “…a good sign that the basic stuff of planetary systems is widespread.” All of which gibes with current thinking, but there is no substitute for getting the right hardware into space (think Kepler and beyond) to verify the existence of those tantalizing worlds. Kepler is scheduled for launch next February.