OGLE-2018-BLG-1185b : A Low-Mass Microlensing Planet Orbiting a Low-Mass Dwarf (Planetary Science)

A team of international astronomers reported on the analysis of planetary microlensing event OGLE-2018-BLG-1185, which was observed by a large number of ground-based telescopes and by the Spitzer Space Telescope. They discovered a low-mass planet OGLE-2018-BLG-1185b around the star. The ground-based light curve indicates a low planet-host star mass ratio of q = (6.9 ± 0.2) × 10¯5, which is near the peak of the wide-orbit exoplanet mass-ratio distribution. Their study recently appeared on Journal ArXiv.

The most common method for measuring the microlens parallax has been via the effects of the motion of the observer, which is called the “orbital parallax effect.” In order to detect the orbital parallax, the ratio of tE (typically tE is ∼ 30 days) to Earth’s orbital period (365 days) should be significant. Thus, we only measure the orbital parallax effect for microlensing events with long durations and/or with relatively nearby lens systems, yielding mass measurements in less than half of published microlensing planetary systems.

The most effective method for routinely obtaining a microlens parallax measurement is via the “satellite parallax effect”, which is caused by the separation between two observers. Because the typical Einstein radius projected onto the observer plane, ˜rE, is about 10 au, the satellite parallax effect can be measured for a wide range of microlenses provided the separation between Earth and the satellite is about 1 au (as was the case for Spitzer).

Now, astronomers estimated the masses of the host star, OGLE-2018-BLG-1185 and planet, OGLE-2018-BLG-1185b with a Bayesian analysis using the measured angular Einstein radius under the assumption that stars of all masses have an equal probability to host this planet. The flux variation observed by Spitzer was marginal, but still places a constraint on the microlens parallax. Imposing a conservative constraint that this flux variation should be ∆fSpz < 4 instrumental flux units indicated a host mass (star) (Mhost) of 0.37 M and a planet mass (Mp) of 8.4 M located at 7.4 Kpc.

A Bayesian analysis including the full parallax constraint from Spitzer suggested, smaller host star and planet masses of Mhost = 0.091 M and mp = 2.1 M, respectively, that is either in the disk at DL ∼ 5 kpc or in the bulge at DL ∼ 7.5 kpc.

Figure 1. The mass distribution of the detected exoplanets as of 2021 February 25 from http://exoplanetarchive.ipac.caltech.edu. The purple stars indicate OGLE-2018-BLG-1185. The pink circles show the microlens planets without mass measurements, and the red circles show the microlens planets with mass measurements from ground-based orbital parallax effects and/or the detection of the lens flux by the high resolution follow-up observations. The red squares represent the microlens planets with mass measurements from satellite parallax effects by Spitzer. The blue, yellow, and black dots indicate planets found by the transit, direct imaging, and radial velocity methods, respectively. © Kondo et al.

They also compared the Bayesian estimates from the conservative Spitzer flux constraint and the full Spitzer parallax measurement of the host and planet mass for OGLE-2018-BLG-1185 to those of other planetary systems (as shown in Fig 1 above) . The pink circles shows the microlens planets without mass measurements, and the red circles shows the microlens planets with mass measurements from ground-based orbital parallax effects and/or the detection of the lens flux by high resolution follow-up observations. The red squares represent microlens planets with mass measurements from the satellite parallax effect observed by Spitzer. It indicates that if the Spitzer parallax is correct, this is one of the lowest mass planets discovered by microlensing.

“If the Spitzer parallax is correct, this is one of the lowest mass planets discovered by microlensing.”

— told Kondo, lead author of the study

Astronomers concluded that future high-resolution imaging observations with HST or ELTs could distinguish between these two scenarios and help to reveal the planetary system properties in more detail.


Reference: Iona Kondo, Jennifer C. Yee, David P. Bennett, Takahiro Sumi, Naoki Koshimoto, Ian A. Bond, Andrew Gould, Andrzej Udalski, Yossi Shvartzvald, Youn Kil Jung, Weicheng Zang, Valerio Bozza, Etienne Bachelet, Markus P.G. Hundertmark, Nicholas J. Rattenbury, F. Abe, R. Barry, A. Bhattacharya, M. Donachie, A. Fukui, H. Fujii, Y. Hirao, S. Ishitani Silva, Y. Itow, R. Kirikawa, M. C. A. Li, Y. Matsubara, S. Miyazaki, Y. Muraki, G. Olmschenk, C. Ranc, Y. Satoh, H. Shoji, D. Suzuki, Y. Tanaka, P. J. Tristram, T. Yamawaki, A. Yonehara, P. Mróz, R. Poleski, J. Skowron, M. K. Szymański, I. Soszyński, S. Kozłowski, P. Pietrukowicz K. Ulaczyk, K. A. Rybicki, P. Iwanek, M. Wrona, M. D. Albrow, S.-J. Chung, C. Han, K.-H. Hwang, H.-W. Kim, I.-G. Shin, S.-M. Cha, D.-J. Kim, S.-L. Kim, C.-U. Lee, D.-J. Lee, Y. Lee, B.-G. Park, R. W. Pogge, Y.-H. Ryu, C. A. Beichman, G. Bryden, S. Calchi Novati, S. Carey, B. S. Gaudi, C. B. Henderson, W. Zhu, D. Maoz, M. T. Penny, M. Dominik, U. G. Jørgensen, P. Longa-Pe{~{ n}}a, N. Peixinho, S. Sajadian, J. Skottfelt, C. Snodgrass, J. Tregloan-Reed, M. J. Burgdorf, J. Campbell-White, S. Dib, Y. I. Fujii, T. C. Hinse, E. Khalouei, S. Rahvar, M. Rabus, J. Southworth, Y. Tsapras, R. A. Street, D. M. Bramich, A. Cassan, K. Horne, J. Wambsganss, S. Mao, A. Saha, “OGLE-2018-BLG-1185b : A Low-Mass Microlensing Planet Orbiting a Low-Mass Dwarf”, ArXiv, pp. 1-30, 2021. https://arxiv.org/abs/2104.02157


Copyright of this article totally belongs to our author S. Aman. One is allowed to reuse it only by giving proper credit either to him or to us

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s