CE31 - Physique Subatomique, Sciences de l'Univers, Structure et Histoire de la Terre

demographics of planets close to their birth place & their occurrence in the disk and the bar of our Galaxy. – COLD-WORLDS

COLD-WORLDS: Precise measurement of the masses of cold planets discovered by microlensing

Cold Worlds uses the gravitational microlensing technique to explore a unique niche: cold planets with masses as small as the Earth orbiting all types of stars (even stellar remnants), old planets in large orbits, free planets (i.e. not bound to a star) and even exolunes orbiting extra-solar planets.

Statistics on cold planets

The objectives of the project are to answer the following questions:<br />1/ What is the mass distribution of cold planets up to ~1 Earth mass at the ice line where most planets form?<br />2/ What is the spatial distribution and abundance of cold planets towards the centre of our Galaxy?<br />3/ How can we routinely reach an accuracy of better than 10% in determining the mass of planets by microlensing with the next generation of satellites (Euclid and WFIRST)?

To achieve these goals, a number of steps are required. First, we need to produce a photometric archive in the near-infrared (JHK) from dedicated surveys of the Galactic bulge by the ESO VISTA telescope. Then, we will make observations with the KECK 10m telescope and the Hubble Space Telescope of the planets discovered by microlensing to date and included in our statistical sample. Then we will have to re-analyse, one by one, all the planets detected by microlensing that are part of our statistical sample.
We have set up a server containing all the infrared images obtained over the centre of our Galaxy by ESO's VISTA telescope. The 10 Tb of images are accessible and extractable in 'calibrated data cubes'. These data are being used as the basis for the large survey starting with the PRIME telescope in South Africa, have been used to estimate the optical gravitational depths towards the centre of the galaxy to choose the lines of sight for NASA's Roman satellite.
We have developed open source codes for photometry with subsampled spatial data, and for microlensing modelling. These will be very useful for the upcoming projects, Euclid and Roman.
We have measured with ~10% accuracy the masses of more than 30 planets by combining KECK, HST observations. This has been one of the big jobs of the project.

Blackman et al. 2021 Nature, presents the first detection of a planet orbiting a dead white dwarf star. Published in Nature with excellent media coverage.
Cassan et al. 2022 Nature Astronomy, presents the first interferometric measurement of the arc rotation of gravitational lenses, and opens a new method for measuring the masses of stars and planets.
Bachelet et al. 2022 shows that it is possible to measure the masses of planets detected by the Roman satellite with a 7h survey performed with the ESA satellite Euclid. This survey is currently being studied by ESA and the Euclid Science Team.
Beaulieu and Ranc are the two Frenchmen invited to join the NASA PIT proposal for the exploitation of the large microlensing survey to be carried out by the Roman satellite (January 2023).
Sahu et al. 2022, Lam et al. 2022, Bachelet & Beaulieu 2023 (in prep) present the discovery of an isolated 7 solar mass black hole in the disk of our Galaxy. This is the first black hole detected by microlensing, after 30 years of research.
We have measured the masses of 30 cold planets with an accuracy of 10-20%. This work is also the basis of the method that will be routinely applied to the analysis of data from the Euclid and Roman satellites.

We will publish the mass function of the cold planets, as well as a first indication of their spatial distribution towards the centre of our Galaxy before autumn 2023. The work done in COLD-WORLDS also shows the power of using high angular resolution to constrain the masses of planets detected by microlensing. What we have done with KECK and HST will be done routinely for planets discovered by the NASA Roman satellite combined with a precursor field survey by Euclid.

Selection of articles
1. Blackman J., Beaulieu J.P., et al., 2021, “A Jovian Analog Orbiting a White Dwarf Star”, Nature 14 octobre 2021.
2. Cassan, A., et al. 2022, «Microlensing mass measurement from images of rotating gravitational arcs«, Nature Astronomy, 6, 121.
3. Bachelet E., et al. 2022, «Euclid-Roman joint microlensing survey: early mass measurement, free floating planets and exomoons«, Astronomy and Astrophysics 664, 136.
4. Sahu K., et al., 2022, “An Isolated Stellar-mass Black Hole Detected through Astrometric Microlensing”, The Astrophysical Journal 933, 83
5. Beaulieu, J.-P., Batista, V., Bennett, D. P., et al. 2018, «Combining Spitzer Parallax and Keck II Adaptive Optics Imaging to Measure the Mass of a Solar-like Star Orbited by a Cold Gaseous Planet Discovered by Microlensing«, The Astronomical Journal, 155, 78.
6. Beaulieu J.P., 2018, “Accurate Mass Measurements for Planetary Microlensing Events Using High Angular Resolution Observations”, Universe, vol. 4, issue 4, p. 61
7. Bennett, D. P., Bhattacharya, A., Beaulieu, J.-P., et al. 2020, «Keck Observations Confirm a Super-Jupiter Planet Orbiting M Dwarf OGLE-2005-BLG-071L«, The Astronomical Journal, 159, 68.
8. Bennett, D. P., et al. 2021, «No Sub-Saturn-mass Planet Desert in the CORALIE/HARPS Radial-velocity Sample«, The Astronomical Journal, 162, 243.
9. Fukui, A., et al., 2019, “Kojima-1Lb Is a Mildly Cold Neptune around the Brightest Microlensing Host Star”, The Astronomical Journal, Volume 158, 206.
10. Lam, C. Y., et al. 2022, «An Isolated Mass-gap Black Hole or Neutron Star Detected with Astrometric Microlensing«, The Astrophysical Journal, 933, L23.
11. Lam, C. Y., et al. 2022, «Supplement: «An Isolated Mass-gap Black Hole or Neutron Star Detected with Astrometric Microlensing« (2022, ApJL, 933, L23)«, The Astrophysical Journal Supplement Series, 260, 55.
12. Miyazaki, S., et al. 2022, «OGLE-2014-BLG-0319: A Sub-Jupiter-mass Planetary Event Encountered Degeneracy with Different Mass Ratios and Lens-source Relative Proper Motions«, The Astronomical Journal, 163, 123.
13. Poleski, R., et al., 2020, “A Wide-orbit Exoplanet OGLE-2012-BLG-0838Lb”, The Astronomical Journal, 159, 261
14. Nagakane, M., et al., 2019, “OGLE-2015-BLG-1649Lb: A Gas Giant Planet around a Low-mass Dwarf”, The Astronomical Journal, 158, 212.
15. Olmschenk, Greg, et al., 2022, «MOA-2020-BLG-208: Cool Sub-Saturn Planet Within Predicted Desert«, submitted to The Astronomical Journal.
16. Ranc, C., et al. 2021, «New giant planet beyond the snow line for an extended MOA exoplanet microlens sample«, Monthly Notices of the Royal Astronomical Society, 506, 1498.
17. Specht, D., et al., 2022, «Kepler K2 Campaign 9: II. First space-based discovery of an exoplanet using microlensing«, Submitted to Monthly Notices of the Royal Astronomical Society.
18. Terry S., et al., 2021, “MOA-2009-BLG-319Lb: A Sub-Saturn Planet inside the Predicted Mass Desert”, The Astronomical Journal, 161, 54.

COLD-WORLDS aims to use gravitational microlensing to explore a unique niche, cold planets down to Earth mass orbiting around any kind of star, at any distance towards the Galactic center, rogue planets and moons orbiting exoplanets (exomoons). These are in very different environments from most known exoplanets, allowing key tests of planet formation theory. Indeed, the maximum sensitivity is for planets at the snow line, close their formation location. To date, 55 microlensing planets have been published and these results challenge theories of planet formation. The core accretion population synthesis predictions by Ida’s and Bern’s groups are quite similar and both under-predict the number of observed cold planets at a mass ratio of q =2E-4) by a factor of ~25. It might be due to the run-away gas accretion phase of planet formation, which is a basic feature of the core accretion theory. Alternatively, it could be that there is some host star mass dependence of this run-away gas accretion gap that smooths out this feature when plotted as a function of mass ratio. So, it is important to accurately determine the individual masses for the planets and host stars.

Microlensing provides precise mass-ratio and projected separations in units of the Einstein ring radius. In order to obtain the physical parameters (mass, distance, orbital separation) of the system, it is necessary to combine the result of light curve modeling with lens mass-distance relations and/or perform a Bayesian analysis with a galactic model. Often, physical parameters are determined to 30-50 %, or even worse. However, we have shown that a tight constraint can be obtained on the lens mass-distance, thanks to detection or upper limits on its luminosity using high angular resolution observations with 8m class telescopes or HST. The pioneering work by our team shows that we can derive physical parameters on known systems to 10 % or better with Keck adaptive optics for instance. In the uncertainty budget, we would then be dominated by extinction correction, distance to the source, calibrating luminosity function of main sequence stars and our understanding of the galactic structure.
COLD-WORLDS will use infrared wide field imagers (public surveys from VISTA, UKIRT, and dedicated observations) and operate adaptive optics on 10m class telescopes. We obtained data already on 30 systems (Keck, VLT, SUBARU, HST) and we have 10 nights approved as Key Strategic Mission Support to WFIRST with Keck for the years 2018-2019. By 2019, we will have observations of the host stars of ~100+ systems with cold planets. We will use Gaia DR2 to measure the source distances, revisit the galactic disk, bar, bulge. Combining Gaia with the multiband observations, we will revisit our model of extinction. It will also give the kinematics on the line of sights to the Bulge and will allow to revise our Bayesian modeling of microlensing plane.
We will then perform demographics of the Disk and Bulge cold planet populations and address the following science objectives.

1/ What is the mass distribution of cold planets down to ~1 Earth mass at the snow line, where most planets are formed?
2/ What is the spatial distribution and abundance of cold planets towards the centre of our Galaxy?
3/ How to routinely achieve better than 10% accuracy in microlensing planet mass determinations with the next generation of satellites (Euclid and WFIRST)?
4/ Producing a near-infrared (JHK) photometric archive from ESO dedicated surveys of the Galactic bulge, a lasting resource for broad areas of stellar and galactic astronomy.
Our highly dedicated team covers all the range of needed expertise. It is also using public data and has all the needed allocated telescope time. It is a risk free, high impact project, giving roots to the Euclid and WFIRST microlensing surveys, while providing important results and useful legacy products.

Project coordination

Jean-Philippe BEAULIEU (Institut d'astrophysique de Paris)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.


LAB Laboratoire d'astrophysique de Bordeaux
IAP Institut d'astrophysique de Paris

Help of the ANR 417,969 euros
Beginning and duration of the scientific project: September 2018 - 48 Months

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