Exoplanets:

Theories vs. Observations

Wei Zhu (祝伟)

SURP Lecture

2019 June 12th

A simple picture of

planet formation & evolution

Snow line

Minimum-mass solar/extra-solar nebula:

Weidenschilling (1977); Hayashi (1981)

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

The outer region dominates the mass and angular momentum budget

Nebula model \(-\) initial condition

Mimimum mass extra-solar nebula

Mimimum mass solar nebula

Chiang & Laughlin (2013)

Murcury

Mars

Asteroid

belt

Planet-disk interaction \(\rightarrow\) planet migration

Image credit: P. Armitage

  • Transfers mass
  • Changes orbital configuration: more mean-motion resonance

Image credit: M. Rex

Dynamical evolution \(\rightarrow\) chaos

Image credit: Raymond, Izidoro, & Morbidelli (2018)

 

  • Modifies orbital spacing, eccentricity, inclinations

 

  • Giant planets, if exists, play dominant roles

small planets alone

w/ outer Giants

Outline

  • Exoplanet detection
    • Different techniques
    • Representative discoveries
  • Exoplanet demographics
    • Occurrence rate of individual populations
    • mass-radius relation
  • Summary & future

What does solar system look like from outside?

'Pale Blue Dot' by Voyager 1 from 40 AU

Solar system seen from alpha centauri (1.3 pc)

Earth: R_Earth, 300 K

100xR_Earth, 6000 K

10xR_Earth, 150 K

L \propto R^2 T^4
\frac{L_{\rm Jupiter}}{L_{\rm Sun}} = 4 \times 10^{-9} \\ \frac{L_{\rm Earth}}{L_{\rm Sun}} = 6 \times 10^{-10}

(In addition, planets are very close to the star.)

Can we see planets directly?

  • Yes, but only hot (young) and distant planets
  • Star light must be suppressed
    • atmosphere turbulence \( \rightarrow \) Adaptive optics (AO)

HR 8799: planet brightness ~\(10^{-5}\) star

Can we see planets directly?

https://www.dcsc.tudelft.nl/~mverhaegen/n4ci/gallery.htm

So far, <10 detections

  • Radial velocity

How to detect planets indirectly?

  • Transit
  • Astrometry
  • Microlensing

First detection: 1989/1995

# of detections: ~700

First detection: 2000

# of detections: >4000

# of detections: 0

(Gaia ~2021)

First detection: 2003

# of detections: ~50

Exoplanet detections

Ground-based transit

Radial velocity survey

Global microlensing survey

Kepler

Mayor & Queloz (1995)

Charbonneau et al. (2000)

Gillon et al. (2017)

Bond et al. (2003)

Hot Jupiters

Cold Jupiters

Cold Neptunes

Super Earths

Data from NASA Exoplanet Archive

Warm Jupiters

Sub-Saturns

Radial velocity

K \propto M_{\rm p} \sin{i}
  • RV measures \(m\sin{i}\) of the planet.
  • Current limit: \( K \geq 1 \) m/s.
  • <1 m/s is limited by stellar noise.

RV

Transit

Microlensing

Figure from Bouchy et al. (2009)

Hot Jupiter puzzle

Minimum-mass solar/extra-solar nebula:

Weidenschilling (1977); Hayashi (1981)

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

Why is this a puzzle?

Mimimum mass extra-solar nebula

Mimimum mass solar nebula

Chiang & Laughlin (2013)

Murcury

Mars

Asteroid

belt

Dynamical channel

Hot Jupiter puzzle

Disk migration

  • Dynamical channel:
    • Planet-planet scattering; Kozai-Lidov cycle; secular chaos;...
  • Disk migration:
    • Planet-disk interaction
  • In situ formation
  • Light curve shape: depth & duration

 

 

 

 

 

 

  • Transit condition:

Transit

{\rm transit~depth} = \frac{R_{\rm p}^2}{R_\star^2} \\ {\rm duration} = \frac{2R_\star}{v_{\rm p}}
a \cdot \sin{i} < R_\star \rightarrow \sin{i} < R_\star/a \sim 3\%
  • Earth: depth=\(10^{-4}\), duration=12 hr (vs. period=1 yr).
  • Jupiter: depth=1%, duration=1 d (vs. period=11 yr).

Kepler mission (2009-2013)

\(10^5\) target stars & 4-yr observations, how many Earth-like planets do we expect to detect?

0 Earth-like planets, but 1000s of exoplanets!

K2 mission (2013-2019)

TESS: Transiting Exoplanet Survey Satellite (2018-now)

  • All-sky survey, bright stars
  • 27 days per sector
  • Looking for close-in planets around nearby stars

Kepler transiting planets (tranets)

  • Planets are ubiquitous in the Galaxy.
  • More small planets than big ones.
  • Interesting features:
    • HJs are lonely;
    • Hot Neptune desert;
    • Ultra-short-period planets (USPs);
    • Photo-evaporation gap;

Saturn

Neptune

Earth

Hot Jupiters

Sub-Earths

  • Exoplanets show a great diversity in composition.

Mass-radius relation

Gravitational microlensing

Brightness

Gravitational microlensing

Brightness

Brightness

Gravitational microlensing

Einstein note, ~1912

Einstein (1936)

Planet 2

Planet 1

Sabrina Madsen

A pair of planets likely in mean-motion resonance from gravitational microlensing

Madsen & Zhu, ApJL in press

Light curve from Han et al. (2013)

Multi-planet microlensing systems

Ground-based transit

Radial velocity survey

Global microlensing survey

Kepler

Mayor & Queloz (1995)

Charbonneau et al. (2000)

Gillon et al. (2017)

Bond et al. (2003)

Hot Jupiters

Cold Jupiters

Cold Neptunes

Super Earths

Data from NASA Exoplanet Archive

(1%)

(10%)

(30%)

(tens of %)

Warm Jupiters

(~3%)

Sub-Saturns

(~5%)

  • Planets are found around all types of stars

Planets are common

Neutron Star Planets

Binaries

  • Different techniques probe different parameter space:
    • Radial velocity \(-\) relatively massive, close-in planets;
    • Transit \(-\) relatively large, close-in planets;
    • Microlensing \(-\) intermediate-separation planets;
    • Direct imaging \(-\) distant, young, massive planets.
  • Planets are ubiquitous in the Galaxy:
    • Some are like ours, but many don't.
  • We still don't understand how planets form.

Where we are now...

Future direction:

A global view of planetary systems

Hot Jupiters

Cold Jupiters

Cold Neptunes

Super Earths

(1%)

(10%)

(30%)

(tens of %)

Warm Jupiters

(~3%)

Sub-Saturns

(~5%)

  • Can these different planet populations co-exist in the same system?
  • How often do they do so?

WFIRST

  • Wide-Field InfaRed Survey Telescope

 

  • Planet mass-radius relation \(\rightarrow\) constraint on composition;
  • Exoplanet atmosphere observations.

Future direction:

Better characterization of known planets

James Webb Space Telescope (~2021)

  • Directly detect exoplanet and measure the atmosphere
  • Are we alone?

Star shade concept

Future direction:

Direct imaging Earth-like planets

  • Different techniques probe different parameter space:
    • Radial velocity \(-\) relatively massive, close-in planets;
    • Transit \(-\) relatively large, close-in planets;
    • Microlensing \(-\) intermediate-separation planets;
    • Direct imaging \(-\) distant, young, massive planets.
  • Planets are ubiquitous in the Galaxy:
    • Some are like ours, but many don't.
  • We still don't understand how planets form:
    • Future observations will answer this question (and many others).

Summary

Back-up slides

Radial velocity

RV also measures the orbital eccentricity and orientation

e=0

e=0.3

e=0.6

e=0.9

to observer

\( \omega \)

Eccentric giant planets

Solar system giant planets: \(e<0.06\)

Extra-solar giant planets: \(e\sim0.3\)

Microlensing trajectory

Microlensing planet perturbations

Major image perturbation

Minor image perturbation

Extra-Solar Planets: Theories vs. Observations

By Wei Zhu(祝伟)

Extra-Solar Planets: Theories vs. Observations

Lecture given to the 2019 UofT SURP students

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