Open access

A sub-Saturn Mass Planet, MOA-2009-BLG-319Lb
N. Miyake1,72, T. Sumi1,72, Subo Dong2,3,73, R. Street4,5,74, L. Mancini6,7,8,75, A. Gould9,73,
D. P. Bennett10,72,76, Y. Tsapras4,11,74, J. C. Yee9,73, M. D. Albrow12,76, I. A. Bond13,72,
P. Fouqu´e14,76, P. Browne15,74,75, C. Han16,73, C. Snodgrass17,18,74, F. Finet19,75,
K. Furusawa1,72, K. Harpsøe20,75, W. Allen21,73, M. Hundertmark22,75, M. Freeman23,72,
D. Suzuki1,72,
and
F. Abe1, C. S. Botzler23, D. Douchin23, A. Fukui1, F. Hayashi1, J. B. Hearnshaw24,
S. Hosaka1, Y. Itow1, K. Kamiya1, P. M. Kilmartin25, A. Korpela26, W. Lin13, C. H. Ling13,
S. Makita1, K. Masuda1, Y. Matsubara1, Y. Muraki27, T. Nagayama28, K. Nishimoto1,
K. Ohnishi29, Y. C. Perrott23, N. Rattenbury23, To. Saito30, L. Skuljan13, D. J. Sullivan26,
W. L. Sweatman13, P. J. Tristram25, K. Wada27, P. C. M. Yock23
(The MOA Collaboration72)
G. Bolt31, M. Bos32, G. W. Christie33, D. L. DePoy34, J. Drummond35, A. Gal-Yam36,
B. S. Gaudi9, E. Gorbikov37, D. Higgins38, K.-H. Hwang16, J. Janczak9, S. Kaspi37,39,
C.-U. Lee40, J.-R. Koo41, S. Koz lowski9, Y. Lee42, F. Mallia43, A. Maury43, D. Maoz37,
J. McCormick44, L. A. G. Monard45, D. Moorhouse46, J. A. Mu˜noz47, T. Natusch48,
E. O. Ofek49,50, R. W. Pogge9, D. Polishook37, R. Santallo51, A. Shporer37, O. Spector37,
G. Thornley46
(The µFUN Collaboration73)
A. Allan52, D. M. Bramich53,76, K. Horne15,76, N. Kains15, I. Steele54
(The RoboNet Collaboration74)
V. Bozza6,7,55, M. J. Burgdorf56,57, S. Calchi Novati6,7, M. Dominik15,58,74, S. Dreizler22,
M. Glitrup59, F. Grundahl59, F. V. Hessman22, T. C. Hinse20,60, U. G. Jørgensen20,61,
C. Liebig15,62, G. Maier62, M. Mathiasen20, S. Rahvar63,64, D. Ricci19, G. Scarpetta6,7,55,
J. Skottfelt20, J. Surdej19, J. Southworth65, J. Wambsganss62, F. Zimmer62
(The MiNDSTEp Consortium75)
V. Batista66,67, J. P. Beaulieu66,67,68, S. Brillant69, A. Cassan62, A. Cole70, E. Corrales66,67,
Ch. Coutures66,67, S. Dieters66,67,70, J. Greenhill70, D. Kubas66,67,69, J. Menzies71
(The PLANET Collaboration76)

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1Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya 464-8601, Japan;
nmiyake,sumi,furusawa,dsuke,abe,afukui,fhayashi,hosaka,itow,kkamiya,makita,kmasuda,ymatsu,[email protected]ya-
u.ac.jp
2Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540, USA; [email protected]
3Sagan Fellow
4Las Cumbres Observatory Global Telescope Network, 6740 Cortona Dr., Suite 102, Goleta, CA 93117
5Dept. of Physics, Broida Hall, University of California, Santa Barbara CA 93106-9530, USA
6Universit`a degli Studi di Salerno, Dipartimento di Fisica ”E.R. Caianiello”, Via Ponte Don Melillo,
84085 Fisciano (SA), Italy
7Istituto Internazionale per gli Alti Studi Scientifici (IIASS), Via G. Pellegrino 19, 84019 Vietri sul Mare
(SA), Italy
8Dipartimento di Ingegneria, Universit`a del Sannio, Corso Garibaldi 107, 82100 Benevento, Italy
9Department of Astronomy, Ohio State University, 140 W. 18th Ave., Columbus, OH 43210, USA;
gould,gaudi,jyee,pogge,[e-mail]
10Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA; b[email protected]
11Astronomy Unit, School of Mathematical Sciences, Queen Mary, University of London, London E1 4NS
12Department of Physics and Astronomy, University of Canterbury, Private Bag 4800, Christchurch, New
Zealand
13Institute of Information and Mathematical Sciences, Massey University, Private Bag 102-904, North
Shore Mail Centre, Auckland, New Zealand; [e-mail]
14LATT, Universit´e de Toulouse, CNRS, 14 Avenue Edouard Belin, 31400 Toulouse, France
15SUPA, University of St Andrews, School of Physics & Astronomy, North Haugh, St Andrews, KY16
9SS, Scotland, United Kingdom
16Department of Physics, Institute for Basic Science Research, Chungbuk National University, Chongju
361-763, Korea; c[email protected]ungbuk.ac.kr
17European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Santiago 19, Chile
18Max Planck Institute for Solar System Research, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
19Institut d’Astrophysique et de G´eophysique, All´ee du 6 Aoˆut 17, Sart Tilman, Bˆat. B5c, 4000 Li`ege,
Belgium
20Niels Bohr Institutet, Københavns Universitet, Juliane Maries Vej 30, 2100 København Ø, Denmark
21Vintage Lane Observatory, Blenheim, New Zealand; [email protected]
22Institut f¨ur Astrophysik, Georg-August-Universit¨at, Friedrich-Hund-Platz 1, 37077 G¨ottingen, Germany
23Department of Physics, University of Auckland, Auckland, New Zealand

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24Department of Physics and Astronomy, University of Canterbury, Private Bag 4800, Christchurch, New
Zealand
25Mt. John University Observatory, University of Canterbury, P.O. Box 56, Lake Tekapo 8770, New
Zealand
26School of Chemical and Physical Sciences, Victoria University, Wellington, New Zealand
27Konan University, Kobe, Japan
28Faculty of Science, Department of Physics and Astrophysics, Nagoya University, Nagoya 464-8602, Japan
29Nagano National College of Technology, Nagano 381-8550, Japan
30Tokyo Metropolitan College of Aeronautics, Tokyo 116-8523, Japan
31Perth, Australia; [email protected]
32Molehill Astronomical Observatory, Auckland, New Zealand; [e-mail]
33Auckland Observatory, Auckland, New Zealand; gwc[email protected]
34Department of Physics and Astronomy, Texas A&M University, College Station, TX, USA; de-
po[email protected]u.edu
35Possum Observatory, Patutahi, New Zealand; john [email protected]
36Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel;
avishay.gal-y[email protected]
37School of Physics and Astronomy and Wise Observatory, Tel-Aviv University, Tel-Aviv 69978, Israel;
shai,dani,david,shporer,odedspec,[email protected]
38Hunters Hill Observatory, Canberra, Australia; [e-mail]
39Department of Physics, Technion, Haifa 32000, Israel
40Korea Astronomy and Space Science Institute, Daejon, Korea
41Department of Astronomy and Space Science, Chungnam National University, Daejeon, Korea and Korea
Astronomy and Space Science Institute, Daejeon, Korea
42Department of Astronomy and Space Science, Chungnam National University, Korea
43Campo Catino Austral Observatory, San Pedro de Atacama, Chile; francoma-
[e-mail], [email protected]
44Farm Cove Observatory, Centre for Backyard Astrophysics, Pakuranga, Auckland, New Zealand; farm-
co[email protected]
45Bronberg Observatory, Centre for Backyard Astrophysics, Pretoria, South Africa; [email protected]
46Kumeu Observatory, Kumeu, New Zealand; [email protected], guy[email protected]
47Departamento de Astronom´ıa y Astrof´ısica, Universidad de Valencia, E-46100 Burjassot, Valencia, Spain
48AUT University, Auckland, New Zealand; [email protected]

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49Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA
91125, USA
50Einstein Fellow
51Southern Stars Observatory, Faaa, Tahiti, French Polynesia; [e-mail]
52School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
53European Southern Observatory, Karl-Schwarzschild-Straβe 2, 85748 Garching bei M¨unchen, Germany
54Astrophysics Research Institute, Liverpool John Moores University, Liverpool CH41 1LD, UK
55INFN, Gruppo Collegato di Salerno, Sezione di Napoli, Italy
56Deutsches SOFIA Institut, Universit¨at Stuttgart, Pfaffenwaldring 31, 70569 Stuttgart, Germany
57SOFIA Science Center, NASA Ames Research Center, Mail Stop N211-3, Moffett Field CA 94035, United
States of America
58Royal Society University Research Fellow
59Department of Physics & Astronomy, Aarhus University, Ny Munkegade 120, 8000 ˚
Arhus C, Denmark
60Armagh Observatory, College Hill, Armagh, BT61 9DG, United Kingdom
61Centre for Star and Planet Formation, Københavns Universitet, Øster Voldgade 5-7, 1350 København
Ø, Denmark
62Astronomisches Rechen-Institut, Zentrum f¨ur Astronomie der Universit¨at Heidelberg (ZAH),
M¨onchhofstr. 12-14, 69120 Heidelberg, Germany
63Department of Physics, Sharif University of Technology, P.O. Box 11365–9161, Tehran, Iran
64School of Astronomy, IPM (Institute for Studies in Theoretical Physics and Mathematics), P.O. Box
19395-5531, Tehran, Iran
65Astrophysics Group, Keele University, Staffordshire, ST5 5BG, United Kingdom
66Institut d’Astrophysique de Paris, UPMC Univ Paris 06, UMR7095, F-75014, Paris, France
67CNRS, UMR7095, Institut d’Astrophysique de Paris, F-75014, Paris, France
68University College London, Gower Street, London WC1E 6BT, UK
69European Southern Observatory, Casilla 19001, Vitacura 19, Santiago, Chile
70School of Math and Physics, University of Tasmania, Private Bag 37, GPO Hobart, Tasmania 7001,
Australia
71South African Astronomical Observatory, P.O. Box 9 Observatory 7925, South Africa
72Microlensing Observations in Astrophysics (MOA) Collaboration, http://www.phys.canterbury.ac.
nz/moa
73Microlensing Follow Up Network (µFUN), http://www.astronomy.ohio-state.edu/∼microfun

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ABSTRACT
We report the gravitational microlensing discovery of a sub-Saturn mass
planet, MOA-2009-BLG-319Lb, orbiting a K or M-dwarf star in the inner Galac-
tic disk or Galactic bulge. The high cadence observations of the MOA-II survey
discovered this microlensing event and enabled its identification as a high mag-
nification event approximately 24 hours prior to peak magnification. As a result,
the planetary signal at the peak of this light curve was observed by 20 different
telescopes, which is the largest number of telescopes to contribute to a planetary
discovery to date. The microlensing model for this event indicates a planet-star
mass ratio of q= (3.95±0.02)×10−4and a separation of d= 0.97537±0.00007 in
units of the Einstein radius. A Bayesian analysis based on the measured Einstein
radius crossing time, tE, and angular Einstein radius, θE, along with a standard
Galactic model indicates a host star mass of ML= 0.38+0.34
−0.18 Mand a planet
mass of Mp= 50+44
−24 M⊕, which is half the mass of Saturn. This analysis also
yields a planet-star three-dimensional separation of a= 2.4+1.2
−0.6AU and a dis-
tance to the planetary system of DL= 6.1+1.1
−1.2kpc. This separation is ∼2 times
the distance of the snow line, a separation similar to most of the other planets
discovered by microlensing.
Subject headings: gravitational lensing: micro - planetary systems
1. Introduction
We present the eleventh microlensing planet, following ten previous discoveries (Bond
et al. 2004;Udalski et al. 2005;Beaulieu et al. 2006;Gould et al. 2006;Gaudi et al. 2008;
Bennett et al. 2008;Dong et al. 2009;Sumi et al. 2010;Janczak et al. 2010). Microlensing
is unique among exoplanet detection methods in that it is sensitive to planets with masses
down to 1M⊕(Bennett & Rhie 1996) at relatively large separations, typically between 1
AU and 6 AU, depending on the mass of the host star. These separations are generally
beyond the “snow line” at ∼2.7 AU M/M(Ida & Lin 2004;Lecar et al. 2006;Kennedy
74RoboNet, http://robonet.lcogt.net
75Microlensing Network for the Detection of Small Terrestrial Exoplanets (MiNDSTEp), http://www.
mindstep-science.org
76Probing Lensing Anomalies Network (PLANET), http://planet.iap.fr