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EXOPLANETS AND THE HABITABLE ZONE

Unit 6: Space Exploration

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Exoplanets

Evidence exists for planets around other nearby stars
Exo = outside: these planets are outside our solar system

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Detecting Exoplanets

NASA estimates that the Milky Way Galaxy is home to at least 100 billion planets
Using data from exoplanet-hunting missions such as Kepler, Gaia, and James Webb, we can identify and confirm their existence
NASA confirmed exoplanets: 5,616
Number of confirmed planetary systems: 4,174
NASA exoplanet candidates (unconfirmed): 10,170

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Detecting Exoplanets

Problem: other stars are far away

Out of reach for easy direct detection by present-day instruments
Detection mostly by indirect methods

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Detecting Exoplanets

Transits (most common)
Radial Velocity
Gravitational Microlensing

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Transit Method

The transit method for detection looks for dimming light from the central star
Kepler satellite used this method to discover more than 5000 candidate exoplanets, with more than 4000 confirmed

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Transit: Best for Large Planets

As this method detects the fractional decrease in light from a star, larger planets are easier to find
Terrestrial planets block less than 0.01% of the stars light, so are difficult to detect
Super Earths (larger than Earth, smaller than Neptune) are the most common size

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Radial Velocity

Stars aren’t stationary – they “wobble” around a center of mass

We can determine that a star is moving by using the doppler effect: stars moving away from us are red-shifted and stars moving towards us are blue-shifted
The velocity at which the star is moving can be used to determine the presence of a planet or system of planets

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Gravitational Lensing

Gravity bends the path of light when it passes near a massive object
If that object is a star with planets, we can use the lensing effect to detect them
We have not found many planetary systems using this method, but it has the potential to find low-mass planets!

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Habitable Zone:�The Search for Life

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Requirements for ‘Our Kind’ of Life

Right Elements:

Carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus
Trace elements, like iron (Fe)� & magnesium (Mg)

Stable Environment

Right Temperature
Liquid Solvent (Water)

Energy Source

Light (the Sun)

I. Without a stable environment, no time to develop complexity.

II. Our kind of life is built primarily on chemical compounds of the element carbon (C). It is unique among in its capacity to form complex molecules. 1) Each carbon atom can be attached to 1, 2, 3, or four other atoms. 2) When a carbon atom is bonded to fewer than 4 other atoms (I.e., they have a double or multiple bond), some of its electrons “hang out.”. These outer electrons move easily (from electrical charges, or light), and make the atom (and its molecule) much more responsive to its environment. Tori’s ppt on extremes has much more about this!!!

Simply, carbon provides the backbone for biological molecules, and is a fuel for many life forms. Sulfur can also act a a fuel or oxidant, and can be used to strengthen the backbones of complex molecules. Oxygen and nitrogen can be oxidants, and provide specificity for reactions of chemical compounds (I.e., amino acids have a N-end (amino) and an O-end (acid)). Phosphorus is essential for energy transfer and storage. H is mostly a filler.

Trace elements are also crucial to life as we know it. Iron is the crucial element in hemoglobin (blood), magnesium is crucial in chlorophyll for photosynthesis.

III. Water – life needs some medium or solvent as a home for its chemical reactions. Water also can participate in reactions as a source of oxygen or hydrogen.

IV. Temperature. Even with liquid water, it can be too hot for complex biological molecules to survive.

Image from anonymous website.

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Stable Planetary Environment

Planets have near-circular �orbits

No huge ellipses with hot �and cold times

Sun is stable

Sunspot cycle is really minor

Not many asteroids or comets

Not very many Dinosaur-killers!

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Temperature in a solar system

Heat, mostly from a star, decreases as you get further away from it

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Habitable ZONE

Habitable zone = zone around the star where liquid water can be found

the habitable zone moves based on where a star is in its life cycle
Ideal location: in the continuously habitable zone

Complication by possible greenhouse effect

depends on the planet’s atmosphere

Habitable zone at the beginning of star’s life

Habitable zone at the end of star’s life

Continuously habitable zone

Star

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goldilocks

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Where in the Solar System could we find liquid water?

Earth: Yep!
Mars: Definitely in the past, maybe now
Europa (Jupiter’s moon): Liquid water ocean (blue) under ice; rocky mantle, metal core
Venus: Possibly, in its distant past

Europa

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Ceres: Dwarf Planet, Ex-asteroid

Avg. diameter 942 km

Equatorial - 975 km
Polar - 909 km

Density = 2.07 gm/cm³ - mix rock + water
Polar flattening suggests rotating ‘fluid’ glob (ice or water shell over rock.
Maybe liquid water, depending on how much internal heat from radioactivity.

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Enceladus: Moon of Saturn with water vapor plumes

Plumes erupting from Enceladus.
Known to be water vapor by its temperature, just about 0°C

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Titan: Saturn’s Moon with Riverbeds

Dense atmosphere, thick cloud cover. Orange color is hydrocarbon smog
RADAR imager on Cassini spacecraft shows river beds and lakes on surface
Lakes probably made of liquid hydrocarbons, like methane (CH₄) and ethane (C₂H₆)

lake

land

Top image is Voyager ‘real’ color image.

http://photojournal.jpl.nasa.gov/catalog/PIA09211.Image ~ 160 km by 270 km. On May 12, 2007, Cassini completed its 31st flyby of Saturn's moon Titan, which the team calls T30. The radar instrument obtained this image showing the coastline and numerous island groups of a portion of a large sea, consistent with the larger sea seen by the Cassini imaging instrument Like other bodies of liquid seen on Titan, this feature reveals channels, islands, bays, and other features typical of terrestrial coastlines, and the liquid, most likely a combination of methane and ethane, appears very dark to the radar instrument. What is striking about this portion of the sea compared to other liquid bodies on Titan is the relative absence of brighter regions within it, suggesting that the depth of the liquid here exceeds tens of meters (tens of yards). Of particular note is the presence of isolated islands, which follow the same direction as the peninsula to their lower right, suggesting that they may be part of a mountain ridgeline that has been flooded. This is analogous to, for example, Catalina Island off the coast of Southern California.

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WHAT ABOUT EXOPLANETS?

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Planets around Other Stars:�Do they have Stable Environments?

Planets are common - thousands now known!
Most known are not good

Hot gas giants, or highly elliptical orbits (unstable environments)

Very tough to detect Earth-like planets - small and far from star

Until a decade ago, planets were thought to be very rare. But many have been found by several different methods, so it seems that a great proportion of stars have planets of some sort.

It is difficult to detect small rocky planets relatively far from the star (like Earth): 1) they are small, so they cause very little wiggle in the position and speed of their parent star; 2) their year is fairly long, so detection requires long long observation times; So, no one knows how common Earth-like planets are. But still, if one finds lots of solar systems with Jupiter-sized planets, its reasonable to think that they might have smaller planets too.

Top photo shows geometry of a planet detection as it passes between us and its star. The measured brightness of the star decreases slightly as the planet goes by. Bottom graph is the actual detection of a planet.

The measured brightness of the star drops by only 0.3%, which gives the diameter of the planet compared to the star (0.3% of the area of the star’s disk). The time it takes to pass in front of the star gives how long the planet’s ‘year’ is, which gives how close it is to the star. From both the kind of star and its distance to the planet, they calculate the planet’s temperature.

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What sort of Star?

Only some main sequence stars are suitable

Hot, large stars (O, B, A) explode too soon
Hot stars (O, B) make too�much deadly ultraviolet radiation
Variable stars don’t provide stable environments

Giants, supergiants

White dwarfs are remnants �after star explosions

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Where in the galaxy �can life survive?

Far from the core

Intense radiation from �its huge black hole
Too many stars, will disrupt planets’ orbits

In from the rim

Rim stars tend to be older, poor in necessary elements

Outside galaxy arms

Too many stars, will disrupt planets’ orbits

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