Herald of the Change: A microlensing Jupiter-analogue spotted in K2 data portends Roman’s yield of new planets

Title: Kepler K2 Campaign 9: II. First space-based discovery of an exoplanet using microlensing

Authors: D. Specht, R. Poleski, M.T. Penny, E. Kerins, I. McDonald, Chung-Uk Lee, A. Udalski, I.A. Bond, Y. Shvartzvald, Weicheng Zang, R.A. Street, D.W. Hogg, B.S. Gaudi, T. Barclay, G. Barentsen, S.B. Howell, F. Mullally, C.B. Henderson, S.T. Bryson, D.A. Caldwell, M.R. Haas, J.E. Van Cleve, K. Larson, K. McCalmont, C. Peterson, D. Putnam, S. Ross, M. Packard, L. Reedy, Michael D. Albrow, Sun-Ju Chung, Youn Kil Jung, Andrew Gould, Cheongho Han, Kyu-Ha Hwang, Yoon-Hyun Ryu, In-Gu Shin, Hongjing Yang, Jennifer C. Yee, Sang-Mok Cha, Dong-Jin Kim, Seung-Lee Kim, Dong-Joo Lee, Yongseok Lee, Byeong-Gon Park, Richard W. Pogge, M.K. Szymański, I. Soszyński, K. Ulaczyk, P. Pietrukowicz, Sz. Kozlowski, J. Skowron, P. Mróz, Shude Mao, Pascal Fouqué, Wei Zhu, F. Abe, R. Barry, D.P. Bennett, A. Bhattacharya, A. Fukui, H. Fujii, Y. Hirao, Y. Itow, R. Kirikawa, I. Kondo, N. Koshimoto, Y. Matsubara, S. Matsumoto, S. Miyazaki, Y. Muraki, G. Olmschenk, C. Ranc, A. Okamura, N.J. Rattenbury, Y. Satoh, T. Sumi, D. Suzuki, S.I. Silva, T. Toda, P.J. Tristram, A. Vandorou, H. Yama, C. Beichman, G. Bryden, S. Calchi Novati

First Author’s Institution: Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK

Status: Submitted to the Monthly Notices of the Royal Astronomical Society (MNRAS) [closed access, preprint on arXiv]

In the next 10 years, an observational technique termed “microlensing” might become the key to understanding the demographics of planets orbiting distant stars. Many of the current techniques for studying so called “exoplanets” have been unable to observe planets with orbital separations and masses that are comparable to those in our own Solar System, but microlensing is sensitive to these planets, given enough time and effort, and can therefore tell us about the frequency of Solar System-like exoplanets. That is why NASA’s next space telescope, the Nancy Grace Roman Space Telescope (Roman for short), is devoting a significant portion of its observations to a microlensing survey that is projected to detect thousands of planets, many comparable to those in our own backyard. A recent discovery of a Jupiter-analog exoplanet in space-based data from the Kepler K2 mission is the first drop in potential downpour that could revolutionize our understanding of exoplanets.

What is Microlensing?

One of the revelations of Einstein’s theory of general relativity is that massive objects can bend spacetime and curve the path of light itself in an effect called gravitational lensing. Gravitational lensing of galaxies have produced spectacular images of distant galaxies, and recently revealed the most distant star we currently know of. When a massive object, called a lens, passes in front of a more distant source of light, the light from the source is bent around the lens, leading to an increase in brightness (Figure 1).

Figure 1: Schematic of a planetary microlensing event. Light from a distant source star is “lensed” by a foreground star that is orbited by a planet. The magnification from the foreground star takes place over a longer period of time than the magnification from the planet. These chance alignments of foreground and background stars allow astronomers to study the exoplanet population throughout the galaxy. Image credit: Physics World

While the most intense gravitational lensing occurs via chance alignments of the brightest and most massive objects in the Universe, such as quasars, galaxies and clusters of stars, it can also occur when humble, Sun-like stars overlap in the night sky. These events are referred to as microlensing events, because they occur over short periods of time and are caused by (relatively) small objects in the universe. If a lens star travels in between our line of sight on the Earth and a background source, the background will brighten. If the lens star is orbited by the planet, and the planet is aligned such that it also lenses the lensed star, a second smaller brightening event can be observed. This is explained more in detail in this astrobite. You can also explore these cool microlensing animations, courtesy of Scott Gaudi. 

The chance of observing a planetary microlensing event is a function of the density of stars along your line of sight, and the chance orientation of the star and planet system with the background star. When compared to other techniques of studying exoplanets, like the transit, radial velocity, or imaging techniques, microlensing is much less biased towards discovering planets of a specific type, and is therefore very useful when trying to understand the occurrence of planets of different sizes and orbits in our galaxy (as covered in this astrobite).

This strength of microlensing is the reason why the upcoming Roman Space Telescope will dedicate a significant portion of its observations to it’s Galactic Bulge Time Domain Survey, which is predicted to yield many planetary microlensing detections, and dramatically shape our view of the population of exoplanets in our galaxy.

So far, though, all microlensing surveys have been conducted from the ground. While some recent publications have detected microlensing events from the ground, and subsequently found the signal of such an event in space-based data, today’s paper is the first example of a planetary microlensing event discovered in space-based data. It heralds the next step in microlensing surveys, the transition from the ground into outer space.

Finding K2-2016-BLG-0005L b

The authors of today’s paper followed up on a paper by McDonald et al. (2021) that searched data obtained by the Kepler Space Telescope’s K2 mission, and reported finding 22 previously known events and five candidate events. Today’s paper took one of those five candidate events, and after a lot of data cleaning and modeling, confirmed the discovery of a planet from the signal.

Figure 2: The planetary microlensing event K2-2016-BLG-0005L b. The x-axis plots time, and the y-axis plots brightness. The data is plotted as red x’s with error bars, and the best fitting model of a planetary microlensing event is plotted as a black line. The planet passes in front of the image of the source star, magnifying the light from the source in two brightening peaks that look like a “rock-on” hand sign. Figure 6 in the paper.

One of the difficulties in extracting a microlensing event from Kepler data is that many of the sources in Kepler’s field-of-view are blended together, and after a failure in 2014 in the reaction wheels (components that stabilize the telescope), the telescope’s data were no longer stable. This means that detecting brightening events is made difficult by stars being blended together and moving across the camera. Using a model for extracting precise measurements of brightness versus time, the authors were able to then model their Kepler observations (Figure 2) with a microlensing scenario. 

After fitting different models of microlensing events, varying the orientation and lens mass, the authors found a best fitting scenario, and used an ensemble of ground and space-based data to determine the mass and orbital separation of the planet that caused the brightening event. They determined that the brightening event, referred to as K2 (for Kepler’s 2nd mission) – 2016 (the year of the event) – BLG (because Kepler was pointed towards the “bulge” of our galaxy) – 0005L (remember those 5 candidate events from the previous paper?) was caused by a planet with a mass 1.1 times that of Jupiter, that orbits its host at a separation of 4.4 astronomical units (the average distance between the Earth and the Sun; for example, Jupiter orbits at a separation of 5.2 a.u.). They dubbed this planet K2-2016-BLG-0005L “b” (for context on the commonly accepted exoplanet naming scheme, see this article).

Microlensing in the Future

While K2-2016-BLG-0005L b is just one of the many exoplanets discovered in the last year, it’s unique because it occupies the edge of the population of planets currently detectable by other discovery techniques, it is so similar to Jupiter (Figure 3a), and because it is the first of many exoplanets that will be discovered by space-based microlensing observations (Figure 3b). Be on the lookout for many new discoveries of exoplanets that will sound far more familiar than those found so far…

Astrobite edited by Maryum Sayeed

Featured image credit: Dune (2021) concept art by Deak Ferrand, edited by William Balmer

About William Balmer

William Balmer (they/them) is a PhD student at JHU/STScI studying the formation, evolution, and composition of giant planets, brown dwarfs, and very low mass stars. They enjoy reading, tabletop games, cycling, and astrophotography.

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