Global Warming as a Detectable Thermodynamic Marker of Earth

Global Warming as a Detectable Thermodynamic Marker of
Earth-like Extrasolar Civilizations: The Colossus Telescope
J. R. Kuhn1, Svetlana V. Berdyugina2,3
1
University of Hawaii, Institute for Astronomy, 34 Ohia Ku St, Pukalani, Maui,
HI, 96768, USA
2
NASA Astrobiology Institute, University of Hawaii, Institute for Astronomy,
2680 Woodlawn Dr, Honolulu, HI, 96822, USA
3
Kiepenheuer Institut fuer Sonnenphysik, Schoeneckstr. 6, 79104 Freiburg,
Germany
Address correspondence to:
J.R. Kuhn
University of Hawaii, Institute for Astronomy, 34 Ohia Ku St, Pukalani, Maui,
HI, 96768, USA, Phone: 808-573-9517, FAX: 808 573-9557,
email: [email protected]
Running title:
Detectable Civilization Biomarkers
1
Abstract:
Advanced civilizations must radiate heat while utilizing planetary energy sources.
This waste heat can be a thermodynamic marker of Earth-like civilization. Here
we model such planetary radiation and propose a strategy for detecting an alien
unintentional thermodynamic electromagnetic biomarker. We show that
astronomically observable infrared civilization biomarkers can be detected within
an interestingly large cosmic volume using a next generation large telescope. The
outcome of such a search will be largely independent of alien communication
modes and may have quantifiable statistical completeness as in a true census.
Even a null result will help answer the question, ―why do we appear to be alone?‖
The detection of an alien heat signature in itself will demonstrate that civilization
can achieve a phase of sustainable global-scale power consumption.
Keywords:
extraterrestrial civilizations, global warming, civilization biomarker, Kardashev
Type I civilizations
2
1.
Introduction ..................................................................................................... 4
2.
Modeling Unintentional Heat Signal ............................................................... 6
3.
4.
2.1.
Power Consumption Parameterization ..................................................... 6
2.2.
Model Assumptions and Simulations....................................................... 8
2.3.
Reconstruction Algorithm ...................................................................... 12
Detection Strategy ......................................................................................... 13
3.1.
Earth-like Planets in the Solar Neighborhood ........................................ 13
3.2.
Next Generation Large Telescopes ........................................................ 15
Discussion ...................................................................................................... 18
3
1. Introduction
The detection of intelligent extraterrestrial life will be a great scientific
achievement. Searches for intentional or beaconed alien signals have made great
advances in sensitivity (Backus et al., 2002; Phoenix Project, 2002) but, until we
detect a signal, these results are difficult to interpret. Unambiguous conclusions
from such null results require tenuous assumptions about alien sociology and
―cosmological principles.‖ Furthermore our imaginations simply may not
encompass the alien communication modes we could encounter. Gaining a better
understanding of our evolution, either as one of many realizations of some
biological ―Copernican Universalism,‖ or as a unique ―Anthropic requirement‖
(cf., Michaud, 2007) of physical and biological laws, need more than just a binary
answer to the question ―are we alone in the universe?‖ Search tools whose
completeness can be quantified in terms of the cosmic volume probed versus
civilization type or advancement have the potential for teaching us much about
ourselves. With this aim, we describe here a strategy to find unavoidable
thermodynamic alien biomarkers, that is based on universal physical principles.
The radio or optical signaling power that ―leaks‖ from an advanced
civilization is difficult to predict, but such power estimates are a useful
classification scheme. Kardashev (1964) argued that advanced civilizations could
be distinguished by the radio power they can generate. He called the Earth early
Type I, Type II civilizations harness the full power of their host star, and a Type
4
III can harness the power of its galaxy. The idea of classifying a planetary
civilization by its power generation was further refined by defining the index K =
log10(P)/10 – 0.6, where P is the average power (in Watts) consumed by the
planetary civilization (Shklovskii and Sagan1966). On this scale Earth has K =
0.7, since P was about 15 TW in the year 2010.
Dyson (1960) suggested searching for the thermodynamic signature of aliens
capable of building a star-enclosing ―biosphere‖ at roughly their planet‘s orbit
radius. He argued that they could capture their star‘s luminosity for their uses
while radiating the thermal waste into space with a black-body temperature near
the planet‘s equilibrium temperature. While the last half-century of astronomical
IR surveys have not turned up any such Type II candidates (Carrigan 2009), the
concept of using alien thermal waste is a powerful unintentional ET biomarker.
Waste heat is a nearly unavoidable indicator of biological activity, just as the
energy that civilization consumes is eventually reintroduced into the planetary
environment as heat. On planetary scales biologically produced heat tends to be
spatially clustered, just as an ET civilizations‘ technological heat is unlikely to be
uniformly distributed. Planetary surface topography and the efficient tendency for
population to cluster in agrarian and urban domains leads to heat ―islands‖ (c.f.
Rizwan et al., 2008).The temporal and spatial distribution of this heat can be an
observable ―fingerprint‖ for remote sensing of civilizations. Here we argue that
5
we may soon be in a position to detect a thermodynamic signal from even Type I,
Earth-like civilizations.
2. Modeling Unintentional Heat Signal
2.1. Power Consumption Parameterization
Thriving civilizations evolve toward greater power consumption. Notably,
there is a strong correlation between our power consumption and society‘s
accumulated information content. Humanity collects information with a doubling
time of about three years (Gants et al., 2008), and our power consumption is
increasing faster than the population (IEA, 2008). Even the most efficient
civilization consumes more power eventually because of the fundamental
information-theoretic energy cost to acquire and manipulate its growing
knowledge base (cf., Maruyama et al., 2009).
It is useful to compare civilization‘s energy flux with the host star‘s flux. Thus
we define a quantity
Ω(t)=P(t)/Pstar,
where Pstar is the stellar power intercepted by the planet and P(t) is the
civilization‘s power consumption. This dimensionless number is an
―advancement‖ metric. The Earth currently has ΩE = 0.0004 and 2000 years
earlier had Ω = 10−7. In comparison, present optical light power generation is a
tenth of our power production while human biological heat production
6
corresponds to about Ω=310−5. The power associated with terrestrial plant
photosynthesis, which fuels most terrestrial biology, dominates all of these
sources and is Ω=0.002 (Nealson and Conrad, 1999). It is interesting to note that
human technology generates nearly 20% of the Earth‘s biological heat
production—since the photosynthetic energy consumption which does not get
stored (for example as oil or coal) eventually fuels the net terrestrial heat
production. We expect advanced civilizations to eventually surpass their planetary
biological heat production.
Planetary temperatures are determined by balancing the energy inputs
(stellar, planetary heat sources, etc.) with the thermal energy radiated from the
planet. Since most of the consumed power is eventually returned to the planet
environment as heat, a civilization with growing power needs will eventually
reach a point where they are uncomfortably warm. Perhaps a sufficiently
advanced civilization can engineer the planet‘s albedo and radiative efficiency to
moderate such global warming or could find an energetically favorable way to
radiate this heat away from the planet above the atmosphere (which must also be
detectable remotely). However, we suspect that such measures can help only
temporarily if P continues to grow. Eventually mass-migration to another planet
becomes energetically advantageous. Thus, for somewhat general reasons we can
expect planetary civilizations to evolve toward a maximum Ω(t) = Ω0 ≤ 1 and then
either moderate their power consumption or undergo planetary migration.
7
A planet‘s geography, and the alien sociology and heat tolerance will
determine Ω0 . Since we already have ΩE =0.0004, it is likely that alien
civilizations will achieve Ω0 values larger than ΩE and perhaps close to 1. The
condition Ω0 = 1 defines a natural transition point for Type I civilizations to
become interplanetary (or even interstellar) colonists. The most advanced
civilizations might engineer photonic (starlight) power sources rather than fission,
fusion, or fossil energy that will reduce their optical albedo and direct planetary
heating, while using their technology‘s waste energy to heat their planet. Such a
civilization should have Ω0 ≈ 1. The temperature of the waste heat depends on
the alien technology, but we expect it to be close to the planet‘s radiative
temperature to achieve the highest Carnot efficiency. Thus, we model here an
advanced Earth-like Type I civilization (ΩE<Ω<1) which discards its waste heat at
nearly the same temperature as the planetary environment.
2.2. Model Assumptions and Simulations
In a simple but instructive formulation we describe here the problem in
terms of functions that depend on a visible longitudinal angle, θ, an orbital phase
angle, p, and a planetary rotation angle r (Fig. 1). We simplify the problem to
represent photometric observations of the planet‘s visible and IR flux as integrals
over such distributions from θ=0 to θ=, but with important dependence on the
orbital phase, p, and planet rotation angle r (all measured with respect to the
Earth‘s inertial reference frame). Define S(θ) to be a dimensionless stellar
8
illumination fraction at planetary longitudinal angle θ. We normalize the integral
of S over all planetary longitudes. Thus, for an orbit inclined such that observer,
planet, and star are all in the same plane, S takes values between 0 and 1/, e.g.
S=0 for -/2< θ </2 and S=1/ for /2 < θ <3/2, when p=0 (with an edge-on
orbit). An inclined orbit is approximated by letting 0 < S < 1/.
This simple model incorporates some principal radiation signals we should
find in a civilized alien planet at orbit phase p and planet rotation angle r, as
described in the following:
1) Alien photonic thermal waste power, that varies in antiphase with the
diurnal cycle, i.e., redistributed stellar power which presumably peaks
outside of the regions of direct illumination: P(θ,p,r) = PaP(θ +r)S’(θ+p).
Here Pa is the total alien waste heat production, and P(θ) is the normalized
longitudinal geographic distribution of the planetary waste heat sources.
The function S’ selects the nonilluminated longitudes with weight 1/, i.e.,
S’=1/ - S, and we assume that aliens redistribute heat to non-illuminated
(cold) areas.
2) Alien non-photonic thermal waste power distribution, that is
independent of the planet‘s stellar illumination: G(θ,p,r) = gG(θ+r), i.e.,
alien power generated by fusion, etc. Here G(θ) is integral-normalized
9
over θ, and g is a parameter that describes the total excess thermal power
radiated by the planet from fixed alien sources.
3) Natural thermal heat distribution, that is absorbed stellar flux
modulated by the planet‘s surface albedo, A(θ): T(θ,p,r) = kPstar(1A(θ+r))S(θ+p). Here, k is a dimensionless parameter that describes the
average fraction of the non-reflected starlight that is converted to excess
planetary heat by natural processes during the diurnal cycle.
4) Scattered light, that is also modulated by the planet‘s albedo (mainly in
the optical): V(θ,r,p) = PstarA(θ+r)S(θ+p).
Determining each of the contributions will require several measurements to
over-constrain the model. We assume sensitive observations which sample the
parameter space of wavelength (i.e., planetary temperature), and orbit phase and
planet rotation angles. Note that for simplicity we will neglect heat contribution
from planetary thermal sources such as volcanoes, hot springs, etc., since they are
likely to be hotter than the civilization waste heat and will produce a
distinguishable signal which could be incorporated into the model as needed. In
this analysis we also neglect the biological thermal signal as we expect it to be
smaller than technology‘s thermal waste for civilizations more advanced than the
Earth‘s.
10
Using this simple model we can illustrate the form of the planetary signals and
demonstrate one algorithm for extracting the thermal civilization signal. Of course
there are extrasolar planets where this model is incomplete (for example a planet
covered by clouds or perhaps covered by water) and this detection scheme would
fail. We suggest that additional exoplanetary observations could detect many of
these indeterminate cases and rule them out of any cosmic census.
We simulate time-series of IR and visible data that measure the normalized
planet scattering and emitted power. Our simulation here assumes an orbit plane
that contains the line-of-sight to the stellar system. The albedo A(θ), civilization
thermal photonic P(θ) and non-photonic G(θ) distributions are taken to be random
―1/f noise‖ functions of the longitude (Fig. 2). We combine those in one orbital
period which we have taken to be 10 planet rotation periods. The normalized
functions V(t), T(t), P(t), G(t) are computed by integrating the corresponding
functions of the angles p and r (at time t) over θ (Fig. 3). Note that this example
represents a civilization on a planet where natural heating sources dominate the
total flux.
These time series light curves illustrate some important characteristics that
the data processing algorithms may use to separate the physical contributions to
the total flux:

The long period (orbital) modulation is due to the dark/light side of the
planet as it becomes visible to the Earth.
11




The higher frequency modulation shows the effect of planetary surface
features rotating across the visible hemisphere as characterized by the
albedo and heat functions G and P.
The visible light flux, V(t), and star-heated thermal flux T(t) are antiphased
at high (rotation) frequencies, because the scattered light is largest when
the albedo is highest and the absorbed energy is less.
One of our civilization signals is anticorrelated on orbital times because
the night-time civilization power dissipation is greater.
The non-photonic, geographically fixed planetary thermal emission, G(t),
has no orbital phase dependence and must be dominated by harmonics of
the planet rotation signal.
2.3. Reconstruction Algorithm
Now, is it possible to detect an alien heat signal in the presence of the natural
heat distribution? An analysis of the simulated curves shows that this is possible
without prior knowledge of the planet albedo and heating functions. Here we
demonstrate how one can determine the amplitudes of functions T(t), G(t), and
P(t) from the variations in the visible and IR fluxes over an orbital period. The
first step is to use the modulation of the visible light flux function V(t) to
determine the form for T(t). This is done by extracting the low frequency (orbital)
contributions to V and then inverting the rotational signal and harmonics. The
orbital component is restored back to this rotation function and this signal is leastsquares fit to the IR flux time-series. This works because the scattered visible
light anti-correlates with the absorbed stellar heating function. The derived
amplitude represents the star-heated, thermally modulated, planet signal. The
residual of this fit is then analyzed for the rotational amplitude and the orbital
12
amplitude signals. These amplitudes measure, respectively, the civilization orbitphased photonic heat signal (P) and the planetary rotation-phased non-photonic
heat (G).
This algorithm provides generally successful results of a simulated search for
a set of given input amplitudes. Table 1 shows examples of our derived output
signals for the simulated alien planet presented in Figures 2 and 3. Although we
find somewhat higher heat amplitudes for an Ω=P+G=0.02 civilization and lower
heat for a more advanced Ω=0.2 planet, it does illustrate that without prior
knowledge of the planet‘s albedo and heat distributions it is possible in principle
to extract an alien civilization signature from sensitive visible and IR flux
measurements, even when natural heating sources dominate the total planetary
flux. By constraining the planet‘s albedo from other astronomical observations
(e.g., Berdyugina et al., 2011; Demory et al., 2011) the robustness of the solution
could be further improved subject to other observational constraints or aprior
assumptions.
3. Detection Strategy
3.1. Earth-like Planets in the Solar Neighborhood
While our primary goal is to detect advanced alien civilizations, we believe
the incentive to communicate with neighbors, if they are found, will be
irresistible. Consequently we have great interest in finding life which is within
13
communication light-travel times of Earth, say 20 pc, and life which has a
chemical basis similar to humanity‘s that makes intelligible communication a
more likely possibility. Therefore, here we focus our estimates on water-based
biology and, for simplicity, consider Earth-like exoplanets in the habitable zone
(HZ) around their host stars.
Spectral class A, F, G, K, and M main-sequence stars live longer than a few
100 Myr, and may be old enough to spawn advanced civilizations, or have
sufficient lifetimes to justify colonizing their planets. There are about 650 such
stars brighter than V-magnitude 13 within 20 pc of the Sun (SIMBAD), and at
least 30% of them should have terrestrial planets (Pepe et al., 2011). The habitable
zone distance, dHZ, for Earth twins orbiting main-sequence stars is scaled by the
stellar effective temperature and varies between 0.07AU (for a 3000K M star) up
to 10AU at A star temperatures of 11,000K.
We argued in Sect. 2 that finding unintentional ETC heat footprints requires
sensitive time-series in both the visible and IR over at least an orbital period. It is
necessary therefore to estimate the optical and IR contrast of the planet with
respect to the direct stellar light. The relative optical flux of the light scattered
from a HZ Earth (HZE) compared to the direct stellar flux depends only on the
star temperature and the planet radius, R, and albedo, A. It equals AR2/4dHZ2 and is
plotted in Figure 4 (blue curves). The increase of the reflective contrast toward
cooler stars is promising, but for a HZE around the Sun it is 10−10, and no
14
telescope has yet achieved this contrast sensitivity to detect optical light from
such planets. The thermal emission from HZEs has higher contrast because the
stellar IR flux is relatively fainter than the cooler planet emission (Fig. 4, green
and red curves). It is at wavelengths between 5 and 10 μm that we have the
greatest sensitivity for detecting ETC waste heat flux. We conclude that many
terrestrial HZ planets are potentially detectable if we can achieve IR contrast
sensitivity of at least 0.510−7 or visible reflected light contrast sensitivity of
about 5 times better.
3.2. Next Generation Large Telescopes
Since the number of possibly observable HZ planets increases approximately as
the cube of the telescope diameter, a large telescope can have enormously larger
detection sensitivity. The telescope must satisfy three stringent optical
requirements:
1) high level of scattered light suppression in order to see the faint terrestrial
planet against the optical ―glare‖ of the nearby star,
2) sufficient sensitivity for detecting enough photons from the planet to allow
statistical analysis,
3) low-enough thermal emission that the planetary IR flux is not lost in the
terrestrial thermal background.
The first requirement depends on excellent adaptive optics performance and
coronagraphic scattered light suppression. The second relies on having a large
15
telescope aperture, and the third goal is attained with careful control of the
primary mirror and secondary optics emissivity.
Glare from the central star is due in part to limitations in the telescope
adaptive optics that do not completely correct the atmosphere-scattered light.
Additional scattered-light is caused by diffraction from the telescope. To
suppress this background requires both a coronagraph and adaptive optic systems.
Instruments for ground-based extrasolar planet detection are currently being built
to yield contrast sensitivity of 10−8 at angles larger than 6λ/D on 8m telescopes,
i.e., angles larger than 0.12 arcsec at λ=800nm, (D is the telescope diameter)
(Macintosh et al., 2007; Roelfsema et al., 2011). It is possible that higher contrast
will be achieved from space and with more advanced coronagraphs (Guyon et al.,
2006), but 10−8 at 6λ/D is a practical goal for future ground telescopes.
Here the advantage of a larger telescope for detecting ETCs is
overwhelming. Figure 5 shows that simultaneous visible (0.5μm) and IR (5μm)
measurements of HZEs are possible for a number of stars with a telescope larger
than about 70m. There are possibly ~30 nearby HZ bright systems detectable with
a 75m telescope with intermediate contrast having a median distance of 5.9pc and
V magnitude of 8.6. All currently known Earth-like planets in HZ (Table 2)
within 20pc will be resolved by such a telescope. On the contrary, the currently
planned ―World‘s Largest Telescopes‖ (WLTs) with apertures of 39m (TMT
16
30m, E-ELT 39m, and GMT 24m) will not reach a significant number of ETC
candidates. None of the known HZ super-Earths could be resolved by them.
A promising WLT-concept with D=74m uses a scalable system of 8m offaxis telescopes to create a large, nearly filled-aperture interferometer-style
instrument. Such a system is being studied in detail and it could be technically
and economically feasible (Colossus, 2012). A telescope of this aperture at a
mountain site could collect about 103 photons/s in the 4.5-5.5μm band from an
Earth-like planet at a distance of 5pc. With realistic telescope emissivity of 0.05
(e.g., Gemini), a 3h HZE observation at wavelengths near 5μm could measure an
Ω≈0.05 thermal flux. Nearer stars, larger-than-Earth planets (Fig. 5), or hotter
ETC heat-dumps will be more sensitively measured. Thus several nights of
observations distributed over the planet‘s orbital and rotation periods could detect
ETCs with Ω>0.05 (Sect. 2).
In the visible the larger star-HZ separation angle (in units of λ/D) and
possibility to observe a planet‘s polarized scattered light to further suppress
starlight (e.g., Berdyugina et al., 2011) should allow even better contrast
sensitivity. In this case it will be possible to obtain the planet‘s rotationally
modulated albedo and thermal signatures from many ETC candidates.
17
4. Discussion
We parameterized Kardashev Type I ETCs with a dimensionless quantity, Ω,
and considered their detection based on their unintentional waste heat. Our
approach is to look for civilization‘s geographically clumpy thermal excess by
analyzing the time dependence of a planet‘s thermal radiation and reflected light.
A low-temperature (≥300K) general excess power could also be a civilization
marker if it is distinguishable from the background thermal radiation of the planet.
As civilizations are pushed toward utilizing photonic stellar power to avoid global
warming, planets with unusually low albedo can be prime advanced civilization
candidates.
The IR flux from the Earth-facing exoplanet hemisphere will vary due to
the planet‘s rotation and its orbital motion but, in general, its time variation will
be distinct from the natural stellar planetary heating effects. We have built simple
model distribution functions in terms of the planet longitude angle that allow us to
simulate visible and thermal exoplanet flux observations. This analysis shows that
the longitudinal distribution of planetary waste heat is distinguishable from time
variable geographic stellar heating caused by longitudinal variation in the planet‘s
albedo. We found that alien heat sources with 1% of the stellar illumination power
(Ω≥0.01) may be identified with our approach. In principle, planets with large
natural geothermal sources could also be distinguished spectroscopically by their
higher radiative temperature. Pervasive clouds or alien power distribution
18
networks that mimic natural stellar planetary heat could make some ETCs
difficult to find.
The technology of more primitive civilizations (but still more advanced
than the Earth‘s) may be difficult to distinguish from a planetary biological waste
heat signature. A civilization‘s advancement might not always be measured by
the energy it consumes, although the manipulation of a civilization‘s growing
knowledge-base can eventually require more power than all other needs. An
interesting example is social insects which by some measures could be considered
‗intelligent‘ but whose energy footprint is dominated by biological heat (c.f. Korb
2003; Gould and Gould 2007).
We have argued that a large telescope operating from the ground with a
powerful adaptive optics system and coronagraph can measure interesting visible
and IR flux levels from Earth-like planets around a significant sample of nearby
stars, including already known HZ super-Earths. Most of these stars within 20pc
are likely to be older than the Sun, and most of them will have at least one planet.
The practicality of building a telescope of large enough aperture is, naturally, a
subject of debate. Currently planned WLTs appear to be just out of reach of the
performance described here. A 74m optical system will require new thinking
about how to manufacture and control large, precise optical structures. A
promising approach is to use what could be described as a scalable system of 8m
19
off-axis telescopes to create a nearly filled-aperture interferometer (Colossus,
2012).
Regardless of the details of the next WLT, it seems that thermodynamic
signals of moderately advanced Earth-like alien civilizations are detectible with
current technologies. With such an instrument we could achieve a complete
neighborhood cosmic survey for Type I extraterrestrial civilizations that are
within 6pc of the Sun over a period of about 2 years. This will be the first
quantitative estimate of just how alone we are in our stellar neighborhood.
Acknowledgements
JRK was supported by a senior research award from the Alexander von Humboldt
Foundation at the Kiepenheuer Institut fuer Sonnenphysik, Freiburg, Germany.
SVB was supported by the NASA Astrobiology Institute senior fellowship at the
University of Hawaii, Honolulu, USA. We thank the Searchlight Observatory
Network, the Colossus Corporation, and Dynamic Structures Ltd. for their support
of the large telescope design and engineering efforts that would enable this search
for extraterrestrial intelligence.
Abbreviations
ETC, extraterrestrial civilization; WLT, world-largest telescope; IEA,
International Energy Agency; HZ, habitable zone; HZE, habitable zone Earth; IR,
infra-red.
20
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FIG. 1. Exoplanet geometry. Distance of star-planet system from Earth is d, the
alien planet has radius R at a distance in the watery habitable zone of a. The
orbital phase angle p and observing longitude θ are measured with respect to the
horizontal line.
FIG. 2. A realization of simulated planetary power distribution functions:
albedo/100 (dashed), civilization photonic thermal P(θ) (solid), and non-photonic
thermal G(θ) (dotted) fluxes.
26
FIG. 3. One orbital period corresponding to 10 rotation periods of simulated
planet power signals. The dark solid curve shows the (normalized) visible flux
(V), red dashed curve shows the non-photonic alien thermal flux (G), dark dashed
shows the star-heated planetary flux (T), and solid red shows the photonic thermal
dissipation signal (P). All curves are normalized to unity amplitude using the
simulated rotational curves shown in Fig. 2.
27
FIG. 4. Flux contrast for a planet in the habitable zone versus star temperature in
scattered stellar light (blue), in planet emission at the wavelength of 5μm (green)
and in emission at 10μm (red). Solid lines show contrast of Earth-radius planets
and dashed lines correspond to 5 Earth-radius planets. The Earth-like geometrical
albedo 0.3 was assumed.
FIG. 5. Maximum number of detectable Earth-size HZ planets (assuming 1 per
star and Ω≈1) versus telescope size. 'Star' symbols show number detectable at
5μm due to thermal emission assuming 5x10-8 contrast at an angle of 2λ/D from
the host star and at 500nm with five times smaller contrast at 20λ/D. 'Diamond'
symbols show detectable number with corresponding IR contrast at 2x10-8 and
‗plus‘ symbols show number at 10-8. Up arrow shows the increase in detection
numbers if HZ planets have a radius twice the Earth‘s.
28
Table 1: Simulated input alien civilization heat signatures and derived output
amplitudes.
Alien photonic
heat amplitude
(P)
Alien nonphotonic heat
amplitude (G)
Natural heat
amplitude (T)
Model 1: in/out
0.01 / 0.03
0.01 / 0.08
1.0 / 0.8
Model 2: in/out
0.1 / 0.07
0.1 / 0.18
1.0 / 0.8
Models
Table 2: Known Earth-like planets in habitable zones.
Planet
Stellar
D
Ang. sep.
Ref.
Sp
[pc]
[mas]
Gleise 581g
>3.1
M2.5V
6.2
24
1,2,3
Kepler-22b
>3.5
2.3
G5
190
4.5
4
HD85512b
>3.5
K5V
11.15
23
5
Gliese 667C c
>4.3
M1.5V
6.8
18
6,7
Gliese 581d
>9.2
M2.5V
6.2
35
8
References: [1] Vogt et al. (2010); [2] Vogt et al. (2012); [3] Forveille et al.
M/M
R/R
(2011); [4] Borucki et al. (2012); [5] Pepe et al. (2011); [6] Delfosse et al. (2011);
[7] Anglada-Escude et a. (2012); [8] Udry et al. (2007)
29

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