Is There Life Beyond Earth?

Philip Hughes

Department of Astronomy

University of Michigan

phughes@umich.edu

www-personal.umich.edu/~phughes/

(Material For Download)

Life Beyond Earth

➢ Some important distinctions:➢ Simple (single-celled), Complex (multi-celled)

➢ Intelligent (dolphins....), Technological (humans)

➢ Current or Past

➢ Finding evidence of current or past, simple or complex, life beyond Earth would be remarkable

➢ Finding evidence of current or past, intelligent/technological, ditto would be more socially significant

➢ Where do we look?

Carbon & Water??

➢ What seems to matter for life are (as judged by life on Earth):➢ organic chemistry – involving simple compounds of

Carbon

➢ H20

➢ Is this universal?

Carbon-based Life

➢ Carbon is versatile; H has one bond, O two, C four: can get a vast array of C-based scaffoldings, to which other simple atomic groupings may attach, making complex organic molecules such as➢ Lipids which store energy & form membranes

➢ Carbohydrates which provide cell energy & structures (cellulose)

➢ Amino acids which are the building blocks of proteins,catalyzing reactions & forming structures

Carbon-based Life contd.

➢ Is there a substitute for Carbon?

➢ Silicon has four bonds, but...➢ the bonds are weak

➢ Si-based molecules will not survive long in water

➢ only single, not double bonds, so➢ fewer chemical reactions than for C-molecules

➢ less rich set of molecular structures

➢ There is a high probability that “life elsewhere” is Carbon-based

The Importance of H2O

➢ Liquid water is an invaluable solvent:➢ it facilitates reactions by bringing together the

chemical components➢ in ice, no transport occurs

➢ in vapor the chemicals are dispersed

➢ it transports chemicals to and from cells

➢ Water actually participates in key reactions

➢ Water is a polar molecule (see later), that can pass through cell boundaries

The Importance of H2O contd.

➢ Is there a substitute for H2O?

➢ A substitute must be common, and liquid, over a wide range of temperature and pressure

➢ Organics like ethane and methane can be liquid but are less plentiful, and liquid only when the temperature is so low that chemical reactions will be very slow – maybe 10-20 x slower than in Earth's primeval oceans

The Importance of H2O contd.

➢ Is there a substitute for H2O?

➢ Water is unique in that ice is lighter than liquid water, so floats: under cold conditions, an ice layer forms an insulating sheet above a body of liquid water (freezing throughout occurs only under the most extreme conditions)

The Importance of H2O contd.➢ Is there a substitute for H2O?

➢ Water is a polar molecule, with + and – charge at either end:

hydrogen bond, a key feature in the organic chemistry of life

The Importance of H2O contd.

➢ water dissolves other polar molecules easily (some organic compounds & salts)

➢ but does not dissolve non-polar molecules, such as the stuff of cell walls

➢ water is one of the few simple molecules that can cross a cell membrane allowing osmosis – critical in living organisms

➢ There is a high probability that “life elsewhere” will not develop in the absence of H2O

Warning: Extremophiles

➢ Don’t exclude acidic, alkali, salty, hot….

➢ or lithophiles….

WithinThe Solar System

Mars: Past

“Blueberries”:hematite concretions maybe formed under water (Opportunity, 2004)

Mars: Past (Speculative)

Mars: Search For Past Life

➢ ExoMars – astrobiology project (ESA/Roscosmos)

➢ Part II launch 2020, ExoMars Rover deploys 2021

➢ Pasteur Analytical Laboratory – biosignatures of past life

➢ Oxia Planum most favored site

Europa

Enceladus

Outside The Solar System

Exoplanets!

Total 18 June, 2019: 4086 planets in 3048 systems

Discovery: We Can’t Just “Look”

➢ From beyond the Solar System, the Sun outshines Jupiter by a billion, and the Earth by 10 billion

➢ We have the sensitivity, but....➢ compare viewing a firefly next to a search-light

Transit Method

Kepler Mission

➢ Launched by NASA, 2009

➢ Photometer monitored brightness of >145,000 stars

➢ Periodic dimming reveals planet(s)

Stellar Habitable Zone

➢ Simple definition (liquid water) is naïve but practical

But Are The Exoplanets Habitable?

(Conservative sample: 13; potential: 52)

The Rise Of Oxygen

➢ In early, Oxygen-free atmosphere, simple organisms would have been anaerobic; they were probably➢ chemoautotrophs – getting energy from inorganic

compounds➢ modern Archaea in hot springs get their energy from H/S/Fe

compound reactions

➢ Photosynthesis evolved from light absorbing pigments, that eventually allowed➢ photohetero(auto)trophs – getting energy from

sunlight➢ release oxygen

Oxygen contd.

➢ Oxygen is highlyreactive:➢ Oxidizes surface rock &

Iron minerals in oceans

➢ Rocks > 2 billion years old have 1% modern oxygen levels

➢ No more than 10% current until about 1 billion years ago

➢ Then reaches current level

➢Look for oxygen in exoplanet atmospheres!

Aside: Spectra

The Sun’s Spectrum

Detecting Exoplanet Atmospheres I

➢ First direct detection: David Charbonneau et al. in 2002, using Hubble Space Telecope

Detecting Exoplanet Atmospheres II

➢ To date, more than 50 atmospheres have been studied

➢ We are just beginning to probe structure, as for planets in our Solar System:➢ day/night temperature differences & winds

➢ We are beginning to probe composition:➢ TiO, CO, C02, H2O, CH4

…no oxygen as yet!

Beware False Positives

➢ NASA Astrobiology Institute: Virtual Planetary Lab has simulated thousands of “atmospheres”, allowing for many reactions

➢ O2/O3 could come from CO2 – broken down by ultraviolet light

➢ Most terrestrial methane is biological but could come from volcanic activity

➢ Both ozone & methane would be a good indicator of life, because a burst of methane from volcanism doesn’t last long in atmosphere with oxygen

Kepler Shadowgrams

➢ Recall: the Kepler satellite monitored stars for the telltale periodic dimming of starlight as a planet transits

➢ Suppose an alien civilization has constructed a light weight “gossamer” billboard orbiting their star; it's shape would be evident to us in the shadowgram:

(rotating triangle)

Shadowgrams contd.➢ This could be used passively – for generations

➢ Or actively, like semaphore, sending information as binary digits

(multiple transits by groups)

Civilizations Need Energy

➢ A data center can use as much as medium sized town

➢ Globally, “data warehouses” use 30 billion watts –30 nuclear power plants; the USA accounts for about 1/4-1/3 of that

➢ Up to 70% of the power is used for cooling/air handling

➢ This is just one example of how an advancing civilization's energy use rises dramatically as technology develops

We Borrow Energy

➢ Energy is not created or destroyed, it just changes form

➢ eg, Potential ⇒ Kinetic ⇒ Heat (falling object)

➢ eg, Electrical ⇒ Heat (electronics)

➢ An advanced civilization with vast energy needs will generate a vast amount of waste energy

➢ Use 'degrades' energy; we can expect the waste energy to show up as heat – radiation in infra-red

Dyson Sphere/Swarm

➢ Freeman Dyson, British/American physicist, b. 1923

A Search For Waste Energy

➢ Jason Wright at Penn State

➢ Funded by the John Templeton Foundation

➢ Using WISE (Wide-field Infrared Survey Explorer): radiation from solar-system sized objects at -100 oF to +100 oF, in infrared

➢ Search for ‘astronomically anomalous’ infrared emission from the vicinity of (unseen?) stars oreven from whole galaxies – a web of stars enshrouded in ‘industrial megastructures’

Essential Points:➢ Does life exist beyond Earth? We don’t know.

But…

➢ Mars and moons of the gas giants will be explored

➢ Exoplanets are common and today we could detect

➢ Oxygen in exoplanet atmospheres, an almost certainindicator of (maybe only primitive) life

➢ Signals from “billboards” orbiting stars – an indicator of life a little more advanced than us

➢ Evidence of vastly more advanced civilization via their waste energy

We don’t have to wait for ET to visit!