A new cutting-edge planet-imaging system on the Subaru Telescope on Maunakea has now provided a first-ever direct, sharp look at a distant planetary system long thought to resemble the early solar system. Previous studies of this system inferred the existence of up to three candidate planets that are more massive than Jupiter and still growing. Now, a new result obtained with Subaru's system – the SCExAO (Subaru Coronagraphic Extreme Adaptive Optics) adaptive optics system coupled to the CHARIS integral field spectrograph -- shows that most of the light thought to come from planets originates from a disk of gas and dust: thus, planets have to be fainter and likely lower in mass.
Figure 1: Image of the Sun-like star LkCa 15 obtained with SCExAO/CHARIS on September 7, 2017 (left panel), showing two arcs of light consistent with two components of LkCa 15's circumstellar disk. The middle panel shows a predicted image for a two-component disk from a theoretical model to explain the arcs of light; the right panel shows a predicted image if instead the innermost arc of light were three planets around LkCa 15. North is up and east is left in the images. The star is about 500 light-years from the Earth. Light around LkCa 15 can be seen as close as ~9 au from the star (dashed circle; Sun-to-Saturn distance); the innermost arc of light extends out to ~0.2 arcsec or ~30 au (Sun-to-Pluto distance). Analysis of SCExAO/CHARIS data shows that most of the light around LkCa 15 comes from disk material instead of from planets. (Credit: NAOJ/SCExAO Team)
"SCExAO's unprecedented sharp images show that the planets in this infant solar system could actually be a lot more like our own solar system than previously thought," said Thayne Currie, lead author of the study and astrophysicist at NASA-Ames Research Center and the Subaru Telescope.
The infant Sun-like star, named LkCa 15, is surrounded by a massive protoplanetary disk made of gas and dust– the building blocks of planets. Early analysis of LkCa 15's disk showed it has a large cavity depleted of dust. This is tell-tale sign that much of the disk material has already been incorporated into massive, still-forming "protoplanets".
However, directly detecting these protoplanets around LkCa 15 on solar system-like scales is exceptionally challenging even for large telescopes like Subaru with adaptive optics systems. This is because LkCa 15 is over 500 light-years away and faint in visible light where such systems work to sharpen images of stars and allow orbiting protoplanets to be seen in infrared light. Instead, the three candidate protoplanets around LkCa 15 were inferred at Saturn-to-Neptune orbital separations using an advanced interferometry technique called "sparse aperture masking" (SAM). These were the first protoplanets ever identified. For SAM, though, determining exactly how much light comes from a planet versus other sources like a disk can be particularly difficult.
SCExAO achieves far sharper images of faint stars than typical adaptive optics systems thanks to a fast, ultra-sensitive camera that determines how the atmosphere blurs starlight and a deformable mirror with 2000 actuators that corrects for this blurring. Light is then sent to the CHARIS integral field spectrograph, which can reveal direct light from (proto-)planets and determine the shape of their spectrum, constraining the object's total brightness and atmospheric properties (Note 1).
"SCExAO/CHARIS allows us to directly look for and characterize faint planets around distant stars on solar system scales better than ever before," noted Olivier Guyon, the Principal Investigator of SCExAO and astrophysicist at the Subaru Telescope.
SCExAO/CHARIS data showed that most of the light surrounding LkCa 15 originates from an extended arc-like structure: the visible edge of another component of LkCa 15's disk. This arc has the same brightness previously attributed to planets around LkCa 15. Complementary data taken with the Keck Observatory helped establish that this arc-like structure is static over time and therefore better consistent with a fixed structure like a disk than orbiting planets.
"LkCa 15 is a highly complex system. Prior to SCExAO/CHARIS and given the same prior aperture masking data, we also would have concluded that LkCa 15 has three detected superjovian planets. While our data show that these signals come from a disk, planets are certainly there somewhere, possibly embedded in the disk. We will keep trying to find them," said Currie.
Decisively finding these obscured planets and separating their light from LkCa 15's disk will be challenging. However, there is a clear path forward. SCExAO will undergo upgrades in the near future which may allow it to see faint Jupiter-like planets on Saturn-like orbits moving against the background of LkCa 15’s disk. Further in the future, a successor to SCExAO on the upcoming Thirty Meter Telescope – the Planetary Systems Imager – could image even lower-mass, fainter planets around LkCa 15 on Mars-like orbits.
"State-of-the-art planet imagers like SCExAO open the door to better understanding the origin and evolution of planetary systems and thus whether own solar system's history is common or rare," said Motohide Tamura, the Director of the Astrobiology Center of National Institutes of Natural Sciences in Japan and coauthor of this study.
These results are soon to be published in the Astrophysical Journal Letters (Currie et al., "NO CLEAR, DIRECT EVIDENCE FOR MULTIPLE PROTOPLANETS ORBITING LKCA 15: LKCA 15 bcd ARE LIKELY INNER DISK SIGNALS"). A preprint is available here.
(Note 1) Prior to targeting LkCa 15, SCExAO/CHARIS data for another system – kappa Andromedae – provided new constraints on the temperature, gravity, and mass of its planetary companion: kappa Andromedae b (Currie, Brandt, and Uyama, et al. 2018, Astronomical Journal, 156, 291).
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