Standing on the Shoulders of Kepler

Johannes Kepler

(Dec 27, 1571 - Nov 15, 1630)

Kepler Satellite

(Mar 7, 2009 - Nov 15, 2018)

Wei Zhu (祝伟)

SHAO Astrophysics Colloquium

2021-5-20

Terrestrial planets

(atmosphere % \(\lesssim10^{-4}\))

Gas giants

(atmosphere % \(\sim 90\%\))

Ice giants

(atmosphere % \(\sim 10\%\))

Nebula model

  • Minimum mass solar nebula (MMSN): A disk model that contains the minimum amount of solid material to form the solar system planets.

Weidenschilling (1977); Hayashi (1981)

$$ \Sigma(r) \propto r^{-3/2} $$

planet #1

planet #2

planet #3

Region of dominance

Murcury

Mars

Asteroid

belt

Distance to Sun (AU)

Surface density (g/cm\(^2\))

Planet formation in one picture

Giant planet formation (\(\lesssim\) 10 Myr)

Terrestrial planet formation (\(\sim\) 100 Myr)

Disk lifetime estimate

  • Age of stellar samples vs. fraction of stars with primordial disks.

Mamajek (2009)

(see also Haisch et al. 2001)

Transit (ground)

Transit (space)

Radial velocity

Microlensing

Imaging

Based on data from NASA Exoplanet Archive.

Hot Jupiters

Cold Jupiters

Super Earths

Cold Neptunes

  • Radial velocity

How to detect exoplanets indirectly?

  • Transit
  • Microlensing

First detection: 1989/1995

# of detections: ~700

Current limit: 1 m/s

First detection: 2000

# of detections: >4000

Current limit: \(R_{\rm p}/R_\star \approx 10^{-2}\)

First detection: 2003

# of detections: ~100

Current limit: \(m_{\rm p}/m_\star \approx 10^{-5}\)

Transit (ground)

Transit (space)

Radial velocity

Microlensing

Imaging

Based on data from NASA Exoplanet Archive.

Hot Jupiters

Cold Jupiters

Super Earths

Cold Neptunes

What You See Is Not What You Get!

Kepler mission (2009-2013)

K2 mission (2014-2019)

\(10^5\) target stars & 4-yr monitoring.

Kepler Mission starts

Radial velocity

Transit

Kepler's "success" in number

Before Kepler   vs.    After Kepler

Kepler detected thousands of extra-solar planets, including zero Earth 2.0.

Figure from Batalha (2014)

  • 1.2 planets (1-20 \(R_\oplus\) & within 400 d) per Sun-like star.
  • Frequency of Sun-like stars with Earth-like planets, \(\eta_\oplus\), is not well constrained.

Zhu & Dong (2021, ARAA)

(see also Fressin et al. 2013, Petigura et al. 2018, Hsu et al. 2019, etc)

How many planets are there?

Habitable Zone (HZ) planets

  • Different studies gave different results: ~1%-1
    • Sample size, sample selection, HZ definition, statistical method, etc.

Figure from Hsu et al. (2019)

Bryson et al (2021)

HZ planet occurrence rate

How many planetary systems (vs. planets) are there?

w/ known companions

w/o known companions

Based on data from NASA Exoplanet Archive.

Hot Jupiters

Cold Jupiters

Super Earths

Cold Neptunes

Multi-planet systems are common.

{\rm \#~of~planetary~systems} = \frac{\rm \#~of~detected~systems}{\rm detectability~of~individual~system}

Planet-planet mutual inclinations affect the frequency of planetary systems

Coplanarity \(\longrightarrow\)>50% of Sun-like stars have Kepler-like planets (e.g., Fressin et al. 2013, Petigura et al. 2013).

mutual inclination

{\rm \#~of~planetary~systems} = \frac{\rm \#~of~detected~systems}{\rm detectability~of~individual~system}

Planet-planet mutual inclinations affect the frequency of planetary systems

  • Fraction of Sun-like stars with Kepler-like planets is ~30% (Zhu et al. 2018).

(Colors mean different multiplicities.)

  • Each system has on average 3 planets (average multiplicity).
    • Kepler observed 1.5.
  • Dynamical evolution may have reshaped the architecture.

Zhu et al. (2018), Zhu & Dong (2021, ARAA)

(see also Xie et al. 2016, He et al. 2020)

Intrinsic architecture of Kepler planetary systems

\( \sigma_i,~\sigma_e \propto k^\zeta \)

Figure adapted from Penny et al. (2019)

Kepler systems vs. Solar system

  • Typically super Earths & ~3 per system \(\longrightarrow\) Kepler planetary systems are massive.
  • Atmosphere fraction of a few % \(\longrightarrow\) Early formation.

Chiang & Laughlin (2013)

Mimimum mass extra-solar nebula

Mimimum mass solar nebula

Forming Kepler planetary systems

Image credit: P. Armitage

  • Disk-driven planet migration
  • Pebble accretion model

Pebble (\(\sim\) cm)

Pebble isolation mass (\(\sim10\,M_\oplus\))

Implication to the cold (~1-10 AU) planet companion?

Cold Jupiters

Super Earths

22 from Kepler (triangles) + 39 from RV (squares)

Inner-outer correlation

Kepler-48 as an example

\(M_{\rm b}=3.9\pm2.1\,M_\oplus\)

\(P_{\rm b}=4.78\) d

\(M_{\rm c}=14.6\pm2.3\,M_\oplus\)

\(P_{\rm c}=9.67\) d

\(M_{\rm d}=7.9\pm4.6\,M_\oplus\)

\(P_{\rm d}=42.90\) d

\(M_{\rm e}=2.1\pm0.1\,M_{\rm J}\)

\(P_{\rm d}=982\) d

Cold Jupiters

Super Earths

P({\rm CJ}|{\rm SE}) \approx 33\% {\rm ~vs.~} P({\rm CJ})=10\%

22 from Kepler (triangles) + 39 from RV (squares)

  • 1/3 of Kepler systems have cold Jupiter companions.
    • >50%, if [Fe/H]>0.

Inner-outer correlation

Cold Jupiters

Super Earths

P({\rm CJ}|{\rm SE}) \approx 33\% {\rm ~vs.~} P({\rm CJ})=10\%

22 from Kepler (triangles) + 39 from RV (squares)

  • 1/3 of Kepler systems have cold Jupiter companions.
    • >50%, if [Fe/H]>0.
  • Cold Jupiters (almost) always have inner super Earth companions!

Inner-outer correlation

\( P({\rm SE}|{\rm CJ}) \cdot P({\rm CJ}) = P({\rm CJ}|{\rm SE}) \cdot P({\rm SE}) \)

\( \rightarrow P({\rm SE}|{\rm CJ})=100\% \)

(Un)Popularity of Solar system-like architecture

  • Solar system has no super Earth (70%).
  • Solar system has a cold Jupiter (10%).
  • A pathway toward finding another Solar System?

Summary

  • Extra-solar planets are ubiquitous in our Galaxy.
    • They occur around \(\sim 30\%\) stars and usually have other planetary companions.
  • Many close-in planets have cold planetary companions.
    • Contradictory to leading theories of planet formation.
    • Solar System-like architecture is a common type?
  • What's next?

Transit (ground)

Transit (space)

Radial velocity

Microlensing

Imaging

Based on data from NASA Exoplanet Archive.

Hot Jupiters

Cold Jupiters

Super Earths

Cold Neptunes

(Sub-)Earths

The Earth 2.0 Space Mission will complete the exoplanet census.

Earth 2.0 Microlensing

ET Science Products:

  1. ~10-30 Earth2.0s.
  2. ~5,000 new planets (~1000 Earth-sized planets).
  3. ~500 planets with masses (including Earth2.0s).
  4. ~500 microlensing planets, including ~100 with \(<3 M_\oplus\), ~100 free-floating planets, with masses down to moon-mass.

Images & numbers from Jian Ge, Hui Zhang, and Weicheng Zang (Tsinghua).

Standing on the shoulders of Kepler

By Wei Zhu(祝伟)

Standing on the shoulders of Kepler

SHAO Astrophysics Colloquium, 2021 May 20

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