By Kate B. Follette
For the first time we've managed to take a baby picture of a planet still in the process of growing. Our team was able to image this so-called “proto"-planet with the Magellan telescope in Chile, taking advantage of the high-speed adaptive optics of the telescope to correct for blurring by the Earth's atmosphere. This allowed us to take a super high-resolution image of the system and, after subtracting the light from the central star, isolate light coming directly from the protoplanet.
More specifically, we isolated light emitted by ultra-hot hydrogen gas falling onto the protoplanet, which is named (systematically, if not super-creatively) LkCa 15 b after its star, LkCa 15 A. LkCa 15 A is a very young Sun-like star which is about 460 light years away in the Taurus-Auriga star-forming region. At the deep-red wavelength we observed with (called “Hydrogen-alpha”), the planet is very bright compared to fully-grown planets that we directly image with instruments like the Gemini Planet Imager (GPI)— it is just hundreds rather than millions of times fainter than its host star. While all previous direct imaging detections have observed the leftover heat from (or starlight scattered off of) already fully-formed exoplanets (i.e., planets around stars other than our Sun), this is the very first time we’ve managed to snap a baby picture of a planet that’s still growing.
Above: A residual image (right) showing that protoplanet LkCa 15 b is brighter in a specific wavelength of light called Hydrogen-alpha (left) when the ordinary visible light (center) is subtracted. Hydrogen-alpha is exactly the wavelength where we expect ultra-hot hydrogen falling onto a forming protoplanet to emit very strongly. (Credit: Kate Follette.)
Like many young stars, LkCa 15 A is surrounded by a pancake-shaped disk of gas and dust made up of leftover material from the star formation process. The disk material is transient, and within a few million years will be either blown away by stellar winds from the star or will fall onto it. Though short-lived (at least astronomically speaking), this disk-bearing epoch is important because we believe that disks provide the raw material (solids and gas) to form planets.
Many young stars have disks, but LkCa 15 A is one of just a handful of systems that host “transition disks” — those with solar-system-sized holes at their centers (making them like pancakes with a giant bite taken out of the center, or a squashed donut.) We think that these holes are carved as newly-formed planets sweep up disk material, and that the outer disks can't survive for long after this happens. That makes gaps in transition disks very popular places to look for actively forming protoplanets. But this is the first time that astronomers have managed to directly image one (at least “non-interferometrically”; see below.)
Although LkCa 15 b is only a few hundred times fainter than its host star, the detection was still very difficult because the planet is about five times closer to its star in angular separation (~0.1 arcseconds) than most of the exoplanets that we image today (~0.5 arcseconds). Like the young planet 51 Eridani b (or “51 Eri”) that we discovered with GPI and announced in 2015, the distance of this newly-discovered protoplanet from its star is tens of AU (where one AU, or astronomical unit, is the distance from the Sun to the Earth.) However, the star itself is much farther away from us than 51 Eri, so that same physical separation of the planet from the star translates to a much tighter angular separation as observed from Earth. This is fundamentally the reason that a planet had not been directly imaged inside of a transitional disk gap before.
The nearest star forming regions in our galaxy (and therefore the nearest transitional disks) are all about 500 light years away, much farther than the region within about 170 light years where we search for young fully-formed planets in the near-infrared with GPI. In fact, in near-infrared light, where young planets glow brightly because of residual heat from their formation process, it’s not even possible to separate the light from two objects that are this close together on the sky. It is only by taking advantage of the Magellan telescope’s ability to correct for the blurring of the atmosphere at visible wavelengths that we were able to separate the planet's light from the starlight.
Our group learned only recently that collaborators of ours had used an interferometric imaging technique to detect the same accreting protoplanet at the same location (see the figure below), and combining the two results allowed us to both confirm and begin to characterize the planet. In particular, we were able to show that the planet is gravitationally bound to the star and is not alone within the disk gap. There is probably at least one more planet in the system—LkCa 15 c—suggesting that this is, in fact, a whole solar system in the process of forming. One of the mysteries that we’re still trying to solve is why the second planet was not also glowing brightly at Hydrogen-alpha when we imaged LkCa 15 b.
Above: These images, taken using the Very Large Array radiotelescope, show two views of the LkCA 15 system: (a) A full view in which the transition disk, with gap, is visible as a whitish ring of hydrogen gas; and (b) a close-up with siblings (both confirmed and suspected) indicated by lower-case letters. The plus sign in the center of the image marks LkCa 15 A, the parent star. (Credit: Steph Sallum.)
This discovery was part of the Giant Accreting Protoplanet Survey (GAPplanetS), and follows on our discovery of an accreting M-dwarf stellar companion inside of the HD142527 disk gap (Close et al. 2014). I have eighteen more disks in my sample, and hope to be able to find a few more of these baby worlds!