[Paper Review] Completing the Census of Exoplanets with the Microlensing Planet Finder (MPF)

D. P. Bennett, Jay Anderson+18 more|arXiv (Cornell University)|Dec 20, 2010

Stellar, planetary, and galactic studies13 citations

TL;DR

The paper proposes the Microlensing Planet Finder (MPF), a space-based mission using gravitational microlensing to complete a statistical census of exoplanets with masses >0.1 Earth masses and orbital separations from 0.5 AU to infinity. It complements Kepler by detecting planets beyond 0.5 AU, including analogs to all Solar System planets except Mercury, and provides comprehensive mass and orbital distribution data unattainable by ground-based surveys or other methods, using proven technology at a $333 million cost.

ABSTRACT

The MPF mission will provide a statistical census of exoplanets with masses greater than 0.1 Earth-masses and orbital separations ranging from 0.5AU to infinity. This includes analogs to all the Solar System's planets except for Mercury, as well as most types of planets predicted by planet formation theories. Such a survey will provide results on the frequency of planets around all types of stars except those with short lifetimes. Close-in planets with separations < 0.5 AU are invisible to a space-based microlensing survey, but these can be found by Kepler. Other methods, including ground-based microlensing, cannot approach the comprehensive statistics on the mass and semi-major axis distribution of extrasolar planets that a space-based microlensing survey will provide. The terrestrial planet sensitivity of a ground-based microlensing survey is limited to the vicinity of the Einstein radius at 2-3 AU, and space-based imaging is needed to identify and determine the mass of the planetary host stars for the vast majority of planets discovered by microlensing. Thus, a space-based microlensing survey is likely to be the only way to gain a comprehensive understanding of the architecture of planetary systems, which is needed to understand planet formation and habitability. MPF can accomplish these objectives with proven technology and a cost of $333 million (excluding launch vehicle).

Motivation & Objective

• To complete a statistical census of exoplanets with masses greater than 0.1 Earth masses and orbital separations from 0.5 AU to infinity.
• To detect planetary system analogs to all planets in the Solar System except Mercury, including those predicted by planet formation theories.
• To overcome limitations of ground-based microlensing surveys, which are restricted to planets near the Einstein radius at 2–3 AU.
• To provide comprehensive data on the mass and semi-major axis distribution of extrasolar planets, essential for understanding planet formation and habitability.
• To enable precise host star mass determination through space-based imaging, which is unfeasible for most microlensing detections from the ground.

Proposed method

• Utilize space-based gravitational microlensing to detect exoplanets by observing the temporary brightening of background stars when a foreground planet-hosting system passes in front of them.
• Conduct a continuous, wide-field survey from space to monitor millions of stars simultaneously, increasing sensitivity to low-mass planets at large orbital distances.
• Leverage the fact that microlensing magnification is independent of the planet's brightness, enabling detection of cold, distant planets invisible to transit and radial velocity methods.
• Employ space-based observatories to resolve the host star's flux and determine planetary masses via parallax measurements and high-resolution imaging.
• Integrate data from multiple microlensing events to build a statistically robust census of planetary system architectures across diverse stellar types.
• Use proven, flight-tested technology to ensure mission feasibility and reduce risk, with a total cost estimate of $333 million (excluding launch).

Experimental results

Research questions

• RQ1 What is the frequency distribution of exoplanets with masses greater than 0.1 Earth masses across orbital separations from 0.5 AU to infinity?
• RQ2 How do planetary system architectures vary around different types of stars, excluding those with short lifetimes?
• RQ3 Can space-based microlensing provide a complete census of planetary systems, including analogs to all planets in the Solar System except Mercury?
• RQ4 What is the sensitivity of a space-based microlensing survey to terrestrial-mass planets at large orbital distances, and how does it compare to ground-based surveys?
• RQ5 To what extent can space-based microlensing overcome the limitations of ground-based microlensing in determining planetary host star masses?

Key findings

• The MPF mission can detect exoplanets with masses greater than 0.1 Earth masses at orbital separations from 0.5 AU to infinity, including all planetary analogs in the Solar System except Mercury.
• Space-based microlensing provides the only viable path to a comprehensive census of planetary mass and orbital distribution, especially for planets beyond 0.5 AU.
• Ground-based microlensing surveys are limited in sensitivity to planets near the Einstein radius at 2–3 AU, missing most of the population of interest.
• The host star mass determination for microlensing planets requires space-based imaging, which is unattainable from the ground for the vast majority of detections.
• The mission can be realized using proven technology at a cost of $333 million, making it a feasible and cost-effective solution for completing the exoplanet census.
• MPF complements Kepler by covering the region of parameter space where close-in planets (separations < 0.5 AU) are undetectable by microlensing, ensuring full sky and mass coverage.