Stellar Classification & Planet Detection

Meteo 466

Reading for this week

• “How to Find a Habitable Planet”, James Kasting. Chapter 10, 11 & 12.

• For this lecture, part of Chapter 10 & all of Chapter 11

• Cassan et al., Nature (2012)

• Udry & Santos, Ann. Rev. Astron. Astrophys. (2007))

How bright is a star ?• The brightness of a star is specified in magnitudes.

• Hipparchus (190 B.C – 120 B.C) based it on how bright a star appeared to the unaided eye.

• Brightest stars are Magnitude 1 & dimmest stars are Magnitude 6 (barely visible)

• Refined definition: Difference of 5 magnitudes (1 to 6) corresponds to a factor of 100 times in intensity (flux).

m1 – m2 = -2.5 log(flux2 / flux1)

Color- (apparent) magnitude

Apparent Magnitude: Brightness Measure as seen from Earth Color: The difference in apparent magnitudes in different filters

Color- (absolute) magnitudeIf the distance ‘d’ to the star is known: Absolute magnitude = app. Magnitude – 5 Log(d)

Mainsequence

White dwarfs

Giants

Hertzsprung-Russell (HR) Diagram

http://www.physics.howard.edu/students/Beth/bh_stellar.html

See also The Earth System,p. 194

G stars

O and Bstars

Main sequence

M-stars

Sun

O B A F G K M

Evolution of Sun like star

Is white-dwarf“The” end ??

Evolution of Sun like star

Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA),Northwestern University

Evolution of 10 MSun star

FusionFission

Evolution of 10 MSun star

Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA),Northwestern University

Stars in the solar neighborhood Within 12.5 light years, there are 33 stars. Most of themRed dwarfs.

• Ultimate goal: To find Earth-like planets, if they exist, and to search for evidence of life

– So, how do we do that?

Exoplanet detection methods(ordered by the number of detections )

Indirect• Radial velocity (Doppler method)• Transits• Gravitational microlensing• Pulsar planets• Astrometric

Direct• Optical imaging• Infrared interferometry

Pulsar planets (Doppler technique in time)

• Alex Wolszczan (1991): 3 planets around pulsar PSR PSR B1257+12B1257+12

• Arecibo radio telescope

http://www.astro.psu.edu/users/alex/pulsar_planets_text.html

Extrasolar planet geometry

• One must think about geometry at which the system is being observed• Let i = inclination of the planet’s orbital plane with respect to the plane of the sky = angle of the planet’s orbital planet with respect to the observer

Radial (Doppler) Velocity

http://www.eso.org/public/videos/eso1035g/

Radial (Doppler) Velocity

Jupiter: K = 12.6 m/sEarth : K = 0.1 m/s

http://www.astro.sunysb.edu/mzingale/software/astro/

First detection around sun like star :

51 Pegasi b ~ 0.5 Jupiter (Mayor & Queloz, 1995)

Velocity curve for 51 Pegasus(Mayor & Queloz, 1996)

• Mass of the planet is only a lower limit because the plane of the planet’s orbit is uncertain (Msin I = 0.47 MJ in this case)

http://obswww.unige.ch/~udry/planet/51peg.html

Radial (Doppler) Velocityelliptical orbit

Velocity curve for HD66428

• More often than not, the velocity curves are not symmetric orbit is eccentric (e = 0.5 in this case)

http://exoplanets.org/figures.html

RV around M-dwarfs

Mahadevan et al.(2011)

Currently known exoplanets

V

E

J

exoplanet.eu

Planet eccentricity vs. semi-major axis(Jan 27, 2012)

Extrasolar Planet Encyclopedia

88 Known Multi - Planet Systems

Kepler - 11

Planetary systems allow for more detailed analysis

• 2 massive planets orbiting HD 168443

• Planetary masses– 8 MJ

– 18 MJ

HD 69830b : 0.61 neptunec : 0.70 neptuned : 1.07 neptune

Gliese 581 system• Spectral type: M3V

(0.31 M, 0.0135 L)

• 4 planets discovered by radial velocity:

a (AU) Mass (M)

b 0.041 >15.6

c 0.073 >5.06d 0.253 >8.3e 0.028 >1.7

(Image from Wikkipedia)Ref.: S. Udry et al., A&A (2007)

Tentative conclusions for the Gliese 581 system*

• Gliese 581c (> 5.1 M) is probably not habitable– Stellar flux is 30% higher than that for Venus

• Gliese 581d (>8.3 M) could conceivably be habitable, but it is probably an ice giant– Near the (poorly determined) outer edge of the HZ

*Selsis et al., A&A (2007)*von Bloh et al., A&A (2007)

Gliese 581g ?• Gliese 581g (~ 3

M), “Zarmina’s world”, apparently exists in the HZ (Vogt et al 2010)

• The Swiss group with HARPS instrument found it doesn’t exist !

Planet Mass Distribution

Howard et al. (2011), Science

Occurrence rate α M-0.48

(for periods < 50 days)

Packed Planetary systemsPlanetary systems form in such a way that the system could not support additional planets between the orbits of the existing ones (gaps with stable orbits contain an unseen planet)

HD 74156 (Barnes et al.2005)HD 47186 (Kopparapu et al. 2009)

Barnes et al.(2005)Kopparapu et al. (2009)

Gravitational microlensing

• Planets can also be detected by gravitational microlensing• This method takes advantage of the fact that, according to general relativity, light rays are bent by a gravitational field -- or, equivalently, space-time is distorted and light travels along straight paths in the distorted reference frame)

http://www.astro.cornell.edu/academics/courses/astro201/microlensing.htm

dL

ds

A microlensing event

http://www.nasa.gov/topics/universe/features/planet20110518-video.html

• When the lensing star passes in front of the source star, the light from the source star is amplified by a factor of as much as 10-20

• The typical duration of a microlensing event is minutes to hours

An event with planets

• If the lensing star has planets, then the light curve can be distorted (i.e., you get spikes)

• The planets must be near the Einstein ring radius Einstein ring radius to be detected– Typically, the ring radius is

outside of the habitable zone, so this technique is not that useful for finding habitable planets

http://exoplanet.eu/catalog-microlensing.php

Planet Mass Distribution (Microlensing)

Cassan et al.(2012)

Sensitivity:0.5 to 10 AU5 Earth to 10 Jup

• The majority of all detected planets havemasses below that ofSaturn, though the surveysensitivity is much lowerfor those planets

• Low-mass planets arethus found to be much morecommon than giant planets.

Planet Mass Distribution (Microlensing)

• 17% of stars host Jupiter mass planets

• 52% of stars host Neptune mass

• 62% of stars host Super-Earths • On average, every star in the

Milkyway has 1.6 planetstwithin 0.5 to 10 AU !!

• Planets around stars in our Galaxy thus seem to be the rule rather than the exception.

Historical astrometry: Barnard’s star

• Second closest star to Earth (6 light yrs), in Ophiucus

• Red dwarf (M3.8)• Largest stellar proper motion

(10.3”/yr)– Moving towards us. Will be

closest star (3.8 l.y.) in about 12,000 yrs

• Discovered by Edward Emerson Barnard (1957-1923)

• Studied hard by Peter van de Kamp from 1938 until his death in 1995. Thought to have a planet, but this hypothesis was later proved to be incorrect

1985

1990

1995

20002005

The sexagesimal system of angular measurement

unit value symbol abbreviations conversion

degree 1/360 circle ° deg 17.45 mrad

arcminute 1/60 degree ′ (prime)arcmin, amin, , MOA

290.89 µrad

arcsecond 1/60 arcminute″ (double prime)

arcsec 4.8481 µrad

milliarcsecond 1/1000 arcsecond

mas 4.848 nrad

http://en.wikipedia.org/wiki/Arcsecond

• Equivalently, there are 1,296,000 arcsec in a circle

Determination of parallax

• A star’s parallax, p, is the angle by which it appears to move as the Earth moves around the Sun

• A star that moves by 1 arcsecond when Earth moves by 1 AU relative to the Sun is defined to be at at distance of 1 parsec

• 1 pc = 1 AU/sin p = 3.0857×1013 km

= 3.262 light years

http://en.wikipedia.org/wiki/Parsec

Astrometric method

• Calculated motion of the Sun from 1960 to 2025, as viewed from a distance of 10 pc, or about 32 light years above the plane of the Solar System, i.e., at i = 0o

• Scale is in arcseconds• You get the actual mass of the

planet because the plane of the planet’s orbit can be determined

• Can do astrometry from the ground, but the best place to do it is in space

http://planetquest.jpl.nasa.gov/Navigator/material/sim_material.cfm

Astrometric missions

• Hipparcos 1989 – 1993 (ESA)

• Precise proper motion & parallax for 118,000 stars (1 milli-arc sec)

Sun-Earth 0.3 micro-arc sec

• Gaia 2013 (ESA)• Parallax for 1 billion stars (20 micro-arc sec)• 3-D map of our Galaxy

SIM – Space Interferometry Mission

http://planetquest.jpl.nasa.gov/SIM/simImageGallery.cfm

• This mission will do extremely accurate astrometry from space