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Cleveland State University Press
Preprint typeset using LATEX style emulateapj v. 12/16/11
ON THE DETECTION OF EXOPLANETS VIA RADIAL VELOCITY DOPPLER SPECTROMETRY
Joseph P. Glaser1 and James A. Lock2
Department of Physics, Cleveland State University, Cleveland, OH 44120
Cleveland State University Press
ABSTRACT
Since the discovery of the first exoplanet occurred in the late 1980s, physicists and astronomers have
confirmed the existence of over 1783 planets orbiting other stars. Over the years, several methods
have been developed for detecting these celestial systems, but none has thus far been more successful
than the use of Radial Velocity Doppler Spectrometry. As such, this paper attempts breaks down the
barriers placed before new entries into the field by detailing the theoretical models and experimental
process of this most successful technique at the senior undergraduate level.
Keywords: Exoplanet: General — Exoplanet: Individual(Kepler XXXX) — Planetary Physics: General — Doppler Spectrometry: General — Orbital Mechanics: General
1. INTRODUCTION TO EXOPLANETS
2. OVERVIEW OF TERMINOLOGY
For centuries, some of humanity’s greatest minds have
pondered over the possibility of other worlds orbiting the
uncountable number of stars that exist in the visible universe. The seeds for eventual scientific speculation on the
possibility of these ”exoplanets” began with the works
of a 16th century philosopher, Giordano Bruno. In his
modernly celebrated work, On the Infinite Universe &
Worlds, Bruno states: ”This space we declare to be infinite (...) In it are an infinity of worlds of the same kind
as our own.” By the time of the European Scientific Revolution, Isaac Newton grew fond of the idea and wrote in
his Principia: ”If the fixed stars are the centers of similar systems [when compared to the solar system], they
will all be constructed according to a similar design and
subject to the dominion of One.” Due to limitations on
observational equipment, the field of exoplanetary systems existed primarily in theory until the late 1980s.
The later half of the 20th century brought forward a
number of technological innovations in astronomy. These
improvements on groundbased observation equipment
allowed astrophysicists the ability to revisit the question
of the existence of exoplanets. In 1988, a team of researchers at the University of Victoria and the University of British Colombia discovered the first exoplanet,
Gamma Cephei Ab, through the use of astrometry and
radial velocity techniques similar to that used today.
However, the planet’s existence could not be accurately
confirmed until 2002, leading to the claim of the first
confirmed discovery to be shifted to the discoverers of
PSR B1257+12 B & C in 1992. In the decades since,
astrophysicists and astronomers across the globe have
developed an array of methodologies to discover more
and more of these exoplanets. As these discoveries continue to dot the scientific headlines and push the limits
of observational astronomy, it is important for the nonastrocentric fields of physics to understand the scientific
backbone supporting these discoveries. It is the hope of
the author that the contents of this article will aid in
demystifying this new frontier and encourage others to
join the hunt.
To begin, we will define exactly what it means for an
stellar companion to be considered an exoplanet. In 2003,
the Working Group on Extrasolar Planets (WGESP) of
the International Astronomical Union (IAU) modified its
definition of substellar companions to be defined as:
1
2
Graduating Senior Physics Student.
Professor Emeritus.
Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter
masses for objects of solar metallicity) that
orbit stars or stellar remnants are ”planets”
(no matter how they formed). The minimum
mass/size required for an extrasolar object to
be considered a planet should be the same as
that used in our Solar System.
Thus, when we discuss these discoveries it is important
to keep in mind the maximum size of the substellar companions we consider to be exoplanets. Other objects
commonly discussed by researchers in the field are defined as:
Substellar objects with true masses above
the limiting mass for thermonuclear fusion
of deuterium are ”brown dwarfs”, no matter
how they formed nor where they are located.
Freefloating objects in young star clusters
with masses below the limiting mass for thermonuclear fusion of deuterium are not ”planets”, but are ”subbrown dwarfs” (or whatever name is most appropriate).
Once discovered and classified into one of the three
above categories, the exoplanets must be named. Before
confirmation by other research teams, exoplanets retain
the name designated by the nomenclature of the experiment (e.g. Keppler19c). After confirmation, the creation of a naming standard is the subject of current debate within the IAU. For simplicity’s sake, the reader
should recognize the following naming guidelines presented by Hessman et al. as an acceptable standard:
1. The formal name of an exoplanet is obtained by
appending the appropriate suffixes to the formal
2
Glaser et al.
name of the host star or stellar system. The upper hierarchy is defined by uppercase letters, followed by lowercase letters, followed by numbers,
etc. The naming order within a hierarchical level
is for the order of discovery only.
2. Whenever the leading capital letter designation is
missing, this is interpreted as being an informal
form with an implicit A unless otherwise explicitly
stated.
3. As an alternative to the nomenclature standard in
#1, a hierarchical relationship can be expressed by
concatenating the names of the higher order system
and placing them in parentheses, after which the
suffix for a lower order system is added.
4. When in doubt (i.e. if a different name has not
been clearly set in the literature), the hierarchy
expressed by the nomenclature should correspond
to dynamically distinct (sub)systems in order of
their dynamical relevance. The choice of hierarchical levels should be made to emphasize dynamical
relationships, if known.
For more information regarding coreterminology or
conventions central to astronomy, it is suggested for the
reader to review astronomical introductory texts, such
as Frank Shu’s The Physical Universe: An Introduction
to Astronomy or Robert Baker’s Astronomy. For an informative read on detection methods and their own terminology other than the Idoine Cell technique described
later in this article, the author suggests the review Sara
Seager’s Exoplanets and related texts by the University
of Arizona Press.
3. THE TWO BODY PROBLEM
A fundamental part of constructing our detection
method is ability to translate the ”wobble” of a star into
the orbital information of its substellar companion(s).
For simplicity’s sake, we will only consider the case of a
singular companion for the entirety of this paper. For
a more detailed description of multiplanet systems, the
author suggests the review of the numerical methodology
employed by Beaug´e et al. (2012).
Consider a star, Ms , with a planet, Mp , in orbit around
it. It is well known to any undergraduate in physics that
by converting coordinate systems to that of the reference frame of the stationary center of mass, the twobody
problem reduces to that of a single body moving within
a potential well. We do this by stating:
~rcm =
Mp~rp + Ms~rs
Ms + Mp
~r = ~rs − ~rp
(1)
(2)
with ~rs being the position vector of star from the origin,
~rp being the position vector of the planet, ~r being the
difference in position vectors, and ~rcm being the vector
pointing to the center of mass from the origin. Without
loss of generality, we may set ~rcm = 0. Combining this
fact with Eq. 1 & Eq. 2 yields the following relationships:
~rs =
Mp
~r
Ms + M p
(3)
~rp = −
Ms
~r
Ms + M p
(4)
Thus, if we know ~r, which is the solution to the equivalent Lagrange onebody problem, we can solve for the
individual motions of the star and the planet.
When considering a planet orbiting in a plane parallel
to the XYplane, we express ~r in the following manner:
!
cos f
a(1 − e2 )
sin f
~r =
(5)
1 + e cos f
0
where a is the semimajor axis, e is the eccentricity, and
f = θ − $ is the true anomaly3 . However, this implies
that the observation axis is that of the orbital plane’s
Zaxis. In most cases, the plane of the orbit has been rotated about each of the observation axes individually. We
can represent this coordinate transformation under rotations about an axis by the following linear transforms:
"
#
1 0
0
(6)
P x (φ) = 0 cos φ − sin φ
0 sin φ cos φ
"
#
cos φ − sin φ 0
P z (φ) = sin φ cos φ 0
(7)
0
0
1
Allowing our orientation angles to be i, Ω, & ω, we can
summarize the coordinate transformation from the orbital plane (x, y, z) to the observer’s plane (xob , yob , zob )
as the product of three rotations:
1. The rotation about the zaxis through the angle ω
to align the periapse with the ascending node.
2. The rotation about the x0 axis through the angle i
to allow the orbital plane and the observer’s plane
to be parallel.
3. The rotation about the z 00 axis through the angle
Ω to align the periapse with the xob axis.
Thus, our transform is of the form:
~rob = P z (Ω)P x (i)P z (ω)~r
(8)
And we arrive with:
~rob
a(1 − e2 )
=
1 + e cos f
cos Ω cos ψ − sin Ω sin ψ cos i
sin Ω cos ψ + cos Ω sin ψ cos i
sin ψ sin i
!
(9)
with ψ = ω + f . This is useful because we can now
accurately describe any planetary system’s orientation
towards our line of sight and the sky’s reference frame.
With the ability to describe an orbit about the center
of mass of either body, we can now continue towards
deriving the radial velocity equation. This will allow us
to relate our observations to the parameters of the substellar companion’s orbit. The radial velocity of the star
can be expressed as:
vr = ~r˙s · zˆob = Vcm,z +
Mp
r˙ob,z
Ms + Mp
(10)
3 It should be noted that the true anomaly is being expressed
here as a function of the current angular position, θ, and the longitude of periapse, $. There are other methods of expression, but
this is the most common, regardless of its difficulty of measurement.
3
where Vcm,z is the magnitude of the velocity vector of the
barycenter relative to the observer and r˙ob,z = ~r˙ob · zˆob is
the velocity of ~rob projected onto the zob axis. We now
obtain that:
r˙ob,z = r˙ sin ψ sin i + rf˙ cos ψ sin i
(11)
Next, we obtain r˙ by taking the derivative with respect
to time:
a(1 − e2 )
f˙e sin f
rf˙e sin f
r˙ =
·
=
(12)
1 + e cos f 1 + e cos f
1 + e cos f
Noting Kepler’s Second and Third Laws, we know that:
2π 2 p
L=
a 1 − e2 = r2 f˙
(13)
T
After some algebra is done, we obtain:
2π
a
√
rf˙ =
(1 + e cos f )
T 1 − e2
r˙ob,z
where:
K=
2π Mp sin i
a
√
T Ms + Mp 1 − e 2
(15)
(16)
(17)
Thus, we now have a model which relates the measurable
radial velocity and mass of the star to the parameters
of its substellar companion’s orbit: the period T ; the
minimum mass Mp sin i; the semimajor axis a; and the
eccentricity e.
4. THE RELATIVISTIC DOPPLER SHIFT
It is a great triumph of physics to be able to utilize
the radial velocity of a star to detect the presence of
substellar companions. However, to have the precision
needed to measure the effects on the parent star, we need
a technique stronger than that of astrometry. The way
we retrieve this radial velocity is through noneother than
the relativistic Doppler effect.
We know that the general Lorentz transforms for a
~ with respect to the frame
frame S 0 moving at a velocity V
S is given by:
~ ~ ~
~
~0 = R
~ + (γ − 1) (R · β)β − γ βct
R
β2
for the spacial part and:
~ + 1)]
−t [2πν 0 γ((kˆ0 · β)

{z
}
(22)
2πν
We can now finally write our final form of the radial
velocity equation as:
vr = Vcm,z + K(cos ψ + e cos ω)
0
~
k
(14)
Making use of Eq. 12 & 14, we are left with:
2π a sin i
√
=
(cos ψ + e cos ω)
T 1 − e2
~0 ~0
having the form ei(k ·R −2πν t) . Thus, we can substitute
the Lorentz transforms into the phase equation of the
light and retrieve:
"
#
i
~ β~
~ · β)
(
R
2πν 0 h
~k 0 · R
~
~ + (γ − 1)
~
− γ βct
−
γ(ct − β~ · R)
~ 2
c
β
(20)
Note that when the plane wave is viewed in the source’s S
~ ~
frame, it has the form ei(k·R−2πνt) . Thus, we can separate
the spacial and temporal parts of the equation to find:
"
#
β~
0
0 ~
0 ~
~
~
~
~
R · k + γ(k · β)
+ γk β
(21)
~ 2
β

{z
}
(18)
~ · R)
~
ct0 = γ(ct − β
(19)
q
−1
~ 2 , as per the usual
and γ = 1 − β
~ c−1
with β~ = V
convention.
Let us now imagine a plane wave of light is emitted in
~ ~
the S rest frame, denoted as ei(k·R−2πνt) , at the position
~ Now, allow an observer in the moving S 0 frame to
R.
be looking at the light along the kˆ0 direction in his rest
frame. The observer would measure the emitted wave as
for the temporal part. Therefor, an observer in the S 0
frame, which sees the light source in the S frame moving
~ , will measure the Doppler shift
at a relative velocity of V
to be:
ν
λ0
~
=
= γ(1 + kˆ0 · β)
(23)
ν0
λ
For the case related to exoplanets, we have the ability
to approximate kˆ0 · β~ as equal to vr . This is because if
the distance to the source’s barycenter is on the order
of light years and the orbit’s semimajor axis is on the
order of AU, then ~kz0 0 >> ~kx0 0 , ~ky0 0 and thus kˆ0 ≈ zˆob . In
addition, if one is interested in lowamplitute variations
in vr of more than 0.1 m s− 1, the relativistic term can be
dropped. This allows us to neglect the need to measure
the transverse velocity of the star, which would require
direct observation methods. Thus, we find that the radial
velocity of a star is related to the Doppler shift of its
received spectrum by:
vr = c
where z =
λ0
λ
λ0 − λ
= c(z − 1)
λ
(24)
is the Doppler parameter.
5. INSTRUMENTATION
Depending on the precision needed with measurements
to break noise threshold in the radial velocity measurements, the design of instrumentation is key to detecting
lesser massive exoplanets. To give the reader an understanding of the precision needed to discover exoplanets,
consider the following:
For an observer on Earth to detect an exoplanet with the mass and orbit of Jupiter
around a sunlike star, they would need to
resolve a radial velocity semiamplitude of
K = 12.7 m s1 . For a Neptunian planet
orbiting at a = 0.1 AU, one would need to
resolve K = 4.8 m s1 . Finally, to detect an
exact duplicate of Earth around another sunlike star, the observer would need to resolve
K = 0.09 m s1 .
4
Glaser et al.
In order to achieve a precision of just 3 m s1 , a measurement of wavelength shifts on the order of femtometers is required. Measurements at the scale must be done
through the use of multiple intense spectral lines with
low amounts of line broadening. Since most are comprised of many elements at high temperatures and pressures, significant broadening and mixing of the specrtum
occurs. To combat this, a method proposed by Butler
et al. (1996) allows for the observed starlight to pass
through an iodine cell before entering the optics of the
telescope. By doing so, the stellar spectrum is multiplied by the intense spectrum of iodine, allowing for fine
Doppler shifts in the peak wavelengths to be measured.
It is important that the line broadening of the iodine
spectrum be kept at a minimum, which is successfully
achieved by constructing a cell similar to that described
by Marcy & Butler (1992).
In addition, the method requires high resolution spectrometers (R ≥ 50, 000) that also allow for high intensity
efficiency and coverage over a large spectral range. This
requirement has lead to the wide use of echelle spectrometers in experiments requiring high velocity precision. The detailed description of this is detailed in the
following subsection.
With regards to groundbased telescope observations,
such as those done during the Lick Iodine Planet Search,
other considerations much be taken into consideration
to lower the noise threshold of the observed stellar spectrum. The effects of an change in air temperature of 0.1
K is sufficient to introduce an error of 1 m s1 into the
experiment (Lovis et al. 2010). Telescope optics must be
chosen in such a way as to reduce thermal expansion variations during exposures. The CCD array being used to
record the output of the spectrometer must also maintain thermal stability as well as pixelpixel uniformity.
Ultimately, these and other errors alter the pointspread
function (PSF) of the instrument during an exposure,
which can lead to disastrous effects during analysis.
5.1. The Echelle Grating
The center piece of the echelle spectrometer is the
echelle grating: a type of blazed reflection grating. A
diagram of this type of grating can be found in Figure
??. Since knowing the pointspread function (PSF) of the
instrumentation will become vital to precisely measuring
the Doppler parameter, it is worth deriving the contribution of the echelle grating itself. Not only will this
showcase why the grating is used in the spectrograph,
but also allow us to understand exactly how it works.
Let us imagine shining monochromatic light emanating
from a pointsource at the echelle grating. The length of
the grating faces will be d and the blaze angle will be γ.
Allow the light rays to hit the grating at the center of
each face and at the angle θi ≈ γ. Then the problem of
finding the intensity pattern of the reflected light (in this
case, the PSF) breaks down into two parts: the interference of the light waves and the diffraction of the light
waves around the blazed edges of the grating faces.
First, we shall attack the interference problem. For an
echelle grating, the derivation is that of an nslit transmission grating of ”slit width” of d. The total electric
field at a screen in the far field is:
E = E0
N
X
ei(2jαI ) = E0
j=1
sin(N αI )
sin αI
(25)
where αI = πλ d(sin θi + sin θo ) is the phase difference due
to different path lengths. Thus, the normalized intensity
due to interference is:
II (λ, θo ) =
sin2 [ πλ L(sin θi + sin θo )]
sin2 [ πλ d(sin θi + sin θo )]
(26)
Now we will tackle the problem of diffraction of the
light rays around the blazed edges of the grating. As
before we will only be allowing the angle θi ≈ γ. Given
this condition, the electric field at a screen in the far field
is:
sin αB
EB = E0
(27)
αB
where αB = πλ (d cos γ)(sin θi0 + sin θo0 ) is the phase difference, θi0 = θi − γ and θo0 = θo − γ. Therefor, the
normalized intensity due to diffraction is:
IB (λ, θo ) =
sin2 [ πλ (d cos γ)(sin θi0 + sin θo0 )]
[ πλ (d cos γ)(sin θi0 + sin θo0 )]2
(28)
Equation 28 is referred to as the ”blaze function” in
most literature and allows for the central maximum to
be shifted to a higher order based on the blaze angle, γ.
We now combine the two products through an algebra
trick involving the diffraction grating equation. The goal
is to find a relationship λd in the interference case and
substitute it into αB to find the total intensity. We do
this by:
nλ = d(sin θi + sin θo )
= d(sin(θi0 + γ) + sin(θo0 + γ))
= d(2 sin γ cos θi0 )
when θi0 = −θo0
d
n
=
λ
2 sin γ cos θi0
(29)
Substituting Equation 29 into our relationship for αB , we
arrive at final form for the PSF due to an echelle grating:
0
0
2 nπ(sin θi + sin θo )
0
I(n, θo ) = sinc
(30)
2 tan γ cos θo0
This final form allows us to see that the echelle grating’s
primary purpose. For each wavelength, the grating shifts
the central maximum out from the first order and into to
another higher order. The order to which it then resides
is dependent on the physical characteristics of the grating and how it is placed into the spectrograph. Thus, a
spectrometer can be constructed where each wavelength
band to can lie on a single pixel of a CCD array.4 This
allows the maximum intensity to be measured for the
band and analysed as a discrete spectrum.
4 The reader should note that since the wavelength dependence
is continuous, overlapping of the orders occur. When transferred
to a discrete grid, like that or a CCD array, this overlapping is seen
as a band of wavelengths at a single intensity per pixel. The range
of this band depends both on the grating used and the size/spacing
of each pixel on the CCD.
5
7. CONCLUDING THOUGHTS
6. EXPERIMENTAL PROCEDURE
6.1. Measuring the Spectrum of the Gas Cell
6.2. Calculating the Instrumental PSF
6.3. Modeling the Observations
6.4. Correcting for the Relative Motion of the Observer
6.5. Finding Exoplanets
6.5.1. HD 72659b
We are grateful to: the Department of Physics at
Cleveland State University for facilitating the course
which allowed this study to be done; the Honors Scholarship which funded J. Glaser during his studies; and J.
Lock for his guidance and support throughout the project
and the years previous to its completion.
APPENDIX
APPENDIX MATERIAL
REFERENCES
Auri`
ere, M. 1982, A&A, 109, 301
6
Glaser et al.
Figure 1. Derived spectra for 3C138 (see ?). Plots for all sources are available in the electronic edition of The Astrophysical Journal.
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