Authors: Markus Janson, Joe Carson, David Lafreniere, David Spiegel, John Bent, Palmer Wong
First Author’s Institution: Princeton
I’m sure many of you have seen the shirts that say, “When I was your age, Pluto was a planet.” Although we might feel sympathy for Pluto, which was demoted to a dwarf planet due to a change in naming convention, a far more serious fate has hit the Formalhaut star system. Undergrads: when I was your age, Formalhaut b was a planet. Now, it is unclear what it is, or whether it is anything at all.
Kalas et al. first identified Formalhaut b as a planet candidate in 2008 when they detected optical emission from a point-source at the inner edge of Formalhaut’s impressive dust ring. (Check out the Science paper.) Although a stray point of light is not much evidence on its own, the selling point for this planet candidate was that the source of light appeared to move between 2004 and 2006, mimicking orbital motion around the star. Before I tell you why the existence of Formalhaut b is in question, consider the image below, which was publicized by NASA. Do you think there’s a planet here?
If you’re still not convinced, consider this: the geometry of Formalhaut’s dust ring led theorists to predict a planet at its inner edge, which prompted Kalas et al. to look for Formalhaut b with the Hubble Space Telescope. This was no accidental discovery.
So where did this seeming success story of a theoretical prediction followed by an observational result take a wrong turn?
If Formalhaut b exists, it is a giant planet, like Jupiter. Planets of that size glow brightest at infrared wavelengths due to thermal (or blackbody) radiation. The authors of this paper used the Spitzer Space Telescope‘s Infrared Array Camera (IRAC) to observe the Formalhaut system at 4.5 microns. They took 48 frames on each of 8 nights between August 2010 and July 2011. They aligned and stacked their images and subtracted the background. And what did they find? Nothing. They ruled out a 38 micro-Jansky (μJ) source to 5 standard deviations, an upper limit one tenth of the previous limit. To test that their reduction process was not accidentally removing a true planet signature, they injected a 57 μJ bright spot in their data where the planet should have been and recovered it after the reduction process. This is shown in the first figure of their paper, presented below.
However, the new result does not rule out the possibility of a planet; rather, it sets new limits on the planet’s mass and age. Smaller planets and older planets that have cooled since their formation are fainter than larger, younger planets. Whereas before this result, Formalhaut b could have been a planet 2-3 times the mass of Jupiter as young as 200 million years old, it now must be less than the mass of Jupiter and about 400 million years old, if it is a planet at all. A planet this small could still have formed Formalhaut’s dust ring, so if the planet is really there, the theory still holds together.
Yet, how could a planet so small leave such a bright signature in the optical Hubble images? In the 2008 paper, Kalas et al. favored a hypothesis that this planet could have an extensive ring system, like Saturn. To explain the discrepancy between the planet’s optical brightness and infrared dimness, the ring system would need to extend about 20 planetary radii. Sound crazy? Janson et al. discarded this possibility. However, Kalas pointed out to me today that Saturn’s Phoebe Ring is about 5-6 times larger than the extent of the ring system needed to explain Formalhaut b. And with the recent claim of a ring system that spans a tenth of the Earth-sun distance, who knows what kind of ring systems are possible?
Although the possibility of planethood remains, Janson et al. have substantially narrowed the window on the size and age of the putative Formalhaut b, rings or no rings. In their paper, they say, “[It] is highly unlikely that the observed ﬂux at visible wavelengths has any direct connection to the suspected giant planet that might shepherd the debris disk of Fomalhaut.” Though Janson et al. cannot prove that Formalhaut b is not a planet, their paper has sown doubt among the community.
Janson et al. favor the interpretation that Formalhaut b was a transient dust cloud and not a planet. Between 2004 and 2006, Formalhaut b dimmed by a factor of 2. They argue that dust, which scatters light, could explain such drastic variability. They also note that the spectrum of Formalhaut b is much like the spectrum of its host star, indicating that the light is probably reflected rather than thermally emitted. In the discovery paper, Kalas et al. modeled how such dust clouds would produce the observed optical signature, but they concluded that a planet with rings, which would also scatter and reflect starlight, was more likely. However, Janson et al. argue that a ringed planet cannot explain the factor of 2 dimming.
What is the lesson for us young scientists, graduate students and undergrads alike? Science is falsifiable. When new evidence disagrees with our previous understanding, we must reinterpret everything. What we hold dear as fact today might be proven false tomorrow, so we must check our dreams with skepticism. We must be prepared to see treasured entities – Pluto, Formalhaut b, our own hard-earned discoveries – diminished. We must move on. While Pluto and Formalhaut b are not what they originally seemed to be, we must remember that there are plenty of planets out there. We must keep reaching for them.