1. Stellar Classification & Planet Detection
Meteo 466

2. 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))

3. 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)

4. Color- (apparent) magnitude
Apparent Magnitude: Brightness Measure as seen from Earth
Color: The difference in apparent magnitudes in different filters

5. Color- (absolute) magnitude
If the distance ‘d’ to the star is known:
Absolute magnitude = app. Magnitude – 5 Log(d)
Giants
Main
sequence
White dwarfs

6. Hertzsprung-Russell (HR) Diagram
O and B
stars
Main sequence
Sun
G stars
M-stars
See also The Earth System,
p. 194
O B A F G K M

8. Evolution of
Sun like star
Is white-dwarf
“The” end ??

9. Evolution of Sun like star
Center for Interdisciplinary Exploration and
Research in Astrophysics (CIERA),
Northwestern University

10. Evolution of 10 MSun star
Fission
Fusion

11. Evolution of 10 MSun star Center for Interdisciplinary Exploration and
Research in Astrophysics (CIERA),
Northwestern University

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

13. Ultimate goal: To find Earth-like planets, if they exist, and to search for evidence of life
So, how do we do that?

14. 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

15. Pulsar planets (Doppler technique in time)
Alex Wolszczan (1991): 3 planets around pulsar PSR B
Arecibo radio telescope
pulsar_planets_text.html

16. 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

17. Radial (Doppler) Velocity

18. Radial (Doppler) Velocity
Jupiter: K = 12.6 m/s
Earth : K = 0.1 m/s

19. First detection around sun like star :
51 Pegasi b ~ 0.5 Jupiter (Mayor & Queloz, 1995)

20. 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)

21. Radial (Doppler) Velocity elliptical orbit

22. Velocity curve for HD66428  orbit is eccentric (e = 0.5 in this case)
More often than not, the velocity curves are not symmetric
 orbit is eccentric (e = 0.5 in this case)

23. RV around M-dwarfs
Mahadevan et al.(2011)

24. Currently known exoplanets
exoplanet.eu J E V

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

26. 88 Known Multi - Planet Systems
Kepler - 11

27. Planetary systems allow for more detailed analysis
2 massive planets orbiting HD
Planetary masses
8 MJ
18 MJ
HD 69830
b : 0.61 neptune
c : neptune
d : neptune

28. Gliese 581 system Spectral type: M3V (0.31 M, 0.0135 L)
4 planets discovered by radial velocity:
a (AU) Mass (M)
b >15.6
c >5.06
d >8.3
e >1.7
Ref.: S. Udry et al., A&A (2007)
(Image from Wikkipedia)

29. 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)

30. 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 !

31. Planet Mass Distribution
Occurrence rate α M-0.48
(for periods < 50 days)
Howard et al. (2011), Science

32. Packed Planetary systems
Planetary 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)
Barnes et al.(2005)
Kopparapu et al.
(2009)
HD (Barnes et al.2005)
HD (Kopparapu et al. 2009)

33. Gravitational microlensingdL ds
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)

34. A microlensing event
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

35. 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 to be detected
Typically, the ring radius is outside of the habitable zone, so this technique is not that useful for finding habitable planets

36. Planet Mass Distribution (Microlensing)
Sensitivity:
0.5 to 10 AU
5 Earth to 10 Jup
The majority of all
detected planets have
masses below that of
Saturn, though the survey
sensitivity is much lower
for those planets
Low-mass planets are
thus found to be much more
common than giant planets.
Cassan et al.(2012)

37. 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.

38. 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 ( )
Studied hard by Peter van de Kamp from 1938 until his death in Thought to have a planet, but this hypothesis was later proved to be incorrect
1985
1990
1995
2000
2005

39. 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
µrad
arcsecond
1/60 arcminute
″ (double prime)
arcsec
µrad
milliarcsecond
1/1000 arcsecond
mas
4.848 nrad
Equivalently, there are 1,296,000 arcsec in a circle

40. 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
= ×1013 km
= light years

41. 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 
material/sim_material.cfm

42. 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 (ESA)
Parallax for 1 billion stars
(20 micro-arc sec)
3-D map of our Galaxy

43. SIM – Space Interferometry Mission
This mission will do extremely accurate astrometry from space