[Paper Review] ET White Paper: To Find the First Earth 2.0

Jian Ge, Hui Zhang+165 more|arXiv (Cornell University)|Jun 14, 2022

Astronomy and Astrophysical Research17 citations

TL;DR

The ET mission proposes a space-based, wide-field photometric survey using seven 30 cm telescopes at Earth-Sun L2 to detect Earth-sized planets, particularly Earth 2.0s, via transit and microlensing methods. It enables high-precision radial velocity follow-up with 30-meter ground telescopes and atmospheric characterization of habitable-zone planets using JWST and TMT, significantly advancing the search for Earth-like worlds.

ABSTRACT

We propose to develop a wide-field and ultra-high-precision photometric survey mission, temporarily named "Earth 2.0 (ET)". This mission is designed to measure, for the first time, the occurrence rate and the orbital distributions of Earth-sized planets. ET consists of seven 30cm telescopes, to be launched to the Earth-Sun's L2 point. Six of these are transit telescopes with a field of view of 500 square degrees. Staring in the direction that encompasses the original Kepler field for four continuous years, this monitoring will return tens of thousands of transiting planets, including the elusive Earth twins orbiting solar-type stars. The seventh telescope is a 30cm microlensing telescope that will monitor an area of 4 square degrees toward the galactic bulge. This, combined with simultaneous ground-based KMTNet observations, will measure masses for hundreds of long-period and free-floating planets. Together, the transit and the microlensing telescopes will revolutionize our understandings of terrestrial planets across a large swath of orbital distances and free space. In addition, the survey data will also facilitate studies in the fields of asteroseismology, Galactic archeology, time-domain sciences, and black holes in binaries.

Motivation & Objective

• To measure the occurrence rate and orbital distribution of Earth-sized planets around solar-type stars.
• To detect long-period and free-floating planets using microlensing, especially in the galactic bulge.
• To identify nearby, bright, transiting Earth-like planets suitable for atmospheric characterization.
• To enable high-precision radial velocity follow-up of Earth twins using 30-meter class ground telescopes.
• To fill critical gaps in exoplanet surveys by targeting habitable-zone planets missed by Kepler and TESS.

Proposed method

• Deploy seven 30 cm telescopes at Earth-Sun L2: six transit telescopes with 500 deg² field of view, one microlensing telescope with 4 deg² field of view.
• Conduct a four-year continuous monitoring of the original Kepler field to detect transiting planets.
• Use simultaneous ground-based KMTNet observations to enhance microlensing event detection and mass measurement.
• Simulate radial velocity measurements using ESPRESSO-type instruments on 30-meter class telescopes (TMT) to detect Earth-mass planets.
• Model transmission spectra of Earth-like planets around G and K-type stars using JWST and TMT to assess signal-to-noise ratios (SNR).
• Integrate data for asteroseismology, Galactic archaeology, time-domain studies, and binary black hole detection.

Experimental results

Research questions

• RQ1 What is the occurrence rate of Earth-sized planets in the habitable zone around solar-type stars?
• RQ2 Can microlensing from a space-based telescope detect long-period and free-floating planets with high mass sensitivity?
• RQ3 Can 30-meter class ground telescopes achieve 3-σ detection of Earth-mass planets via radial velocity measurements around V ~ 15 stars?
• RQ4 What is the detectability of atmospheric absorption features in Earth-like planets around bright G and K-type stars using JWST and TMT?
• RQ5 How do the photometric and spectroscopic capabilities of ET enable improved constraints on planetary system architecture and habitability?

Key findings

• With 300 nights of observation using a TMT-like instrument, a 3-σ detection of an Earth-mass planet around a V ~ 15 solar-type star is achievable via radial velocity measurements.
• JWST can detect atmospheric absorption features in Earth-like planets around G-type stars with V = 8 and K-type stars with V = 8, assuming 100 hours of total exposure time.
• TMT can detect the same atmospheric features around G and K-type stars with V = 10 and 100 hours of exposure, demonstrating high sensitivity to nearby, bright targets.
• The simulated transmission spectra show detectable SNR for molecular features (e.g., O₂, H₂O, CO₂) in Earth-like atmospheres under realistic observing conditions.
• ET’s transit survey is expected to discover tens of thousands of transiting planets, including many Earth twins around solar-type stars.
• Combined transit and microlensing observations will enable mass measurements for hundreds of long-period and free-floating planets, filling a key gap in exoplanet demographics.