National Aeronautics and Space Administration

www.nasa.gov

Produced by the NASA 

Astrobiology Program to 

commemorate 50 years of 

Exobiology and Astrobiology 

at NASA.

#6

Issue
Astrobiology

A History of Exobiology and Astrobiology at NASA

This is the story of life in the Universe—or at least the story as we know it so far. As 

scientists, we strive to understand the environment in which we live and how life relates to this environment. As astrobiologists, we study an environment that includes 

not just the Earth, but the entire Universe in which we live.

The year 2010 marked 50 years of Exobiology and Astrobiology research at the 

National Aeronautics and Space Administration (NASA). To celebrate, the Astrobiology Program commissioned this graphic history. It tells the story of some of the 

most important people and events that have shaped the science of Exobiology and 

Astrobiology. At only 50 years old, this field is relatively young. However, as you will 

see, the questions that astrobiologists are trying to answer are as old as humankind.

Concept & Story

Mary Voytek

Linda Billings

Aaron L. Gronstal

Artwork

Aaron L. Gronstal

Script

Aaron L. Gronstal

Jordan Rizzieri

Editor

Linda Billings

Layout

Aaron L. Gronstal

Jenny Mottar

Copyright 2017, NASA Astrobiology Program
Issue #6—Living Beyond the Solar System

The year 2010 marked the 50th anniversary of NASA’s Exobiology Program, established in 1960 and expanded into a broader Astrobiology Program in the 1990s. To 

commemorate the past half century of research, we are telling the story of how this 

field developed and how the search for life elsewhere became a key component 

of NASA’s science strategy for exploring space. This issue is the sixth in what we 

intend to be a series of graphic history books. Though not comprehensive, the 

series has been conceived to highlight key moments and key people in the field as 

it explains how Astrobiology came to be.

-Linda Billings, Editor

1
Earth and the planets of our solar 

system* are not the only locations of 

interest for NASA or for astrobiology.

Astrobiology is the study of 

life’s origins and potential in 

the Universe...

...and the Universe 

is a very big place.

Astronomers have now discovered 

thousands upon thousands of 

planets orbiting distant stars.

We call these worlds extrasolar 

planets, or exoplanets.

But could any of these planets out 

among the stars be habitable for 

life as we know it?

Issue 6—Living Beyond 

*Check out the Solar System Issues 1-5!

2
Right now, we are 

at a critical point in the 

history of our search 

for exoplanets.

But how do we 

choose just a handful 

of planets when there 

are thousands of 

possibilities?

*James Webb Space 

Telescope (JWST)

So we must 

carefully pick 

the right ones.

So far, we haven’t 

had telescopes powerful 

enough to let us look for 

planets with life.

In its lifetime, 

JWST will only be 

able to examine a select 

number of targets in 

enough detail to answer 

questions about 

habitability.

Shawn Domagal 

Goldman, NASA 

Goddard Space 

Flight Center 

(GSFC)

Victoria Meadows, Principal 

Investigator for the Virtual 

Planetary Laboratory at the 

University of Washington (UW)

Mary Voytek, Director of 

the NASA Astrobiology 

Program, NASA HQ (2)

Dawn Gelino, NASA 

Exoplanet Science 

Institute, California 

Institute of Technology

But new telescopes 

being designed and 

built right now might be 

able to do the job.

JWST* launches 

soon, and could be our 

first real chance if we pick 

just the right planet 

to look at. (1) 

Studying habitability 

on exoplanets requires the 

expertise of scientists from 

many disciplines at NASA. 

It’s the perfect challenge 

for astrobiology.

That’s a big 

question for today’s 

astrobiologists.

3
...that is a 

lot of stars.

Whoa...

I believe 

in a ‘plurality 

of worlds.’

There are 

countless Earths 

rotating around their 

suns in exactly the same 

way as the planets of 

our system.

Before NASA even existed, 

humankind dreamt of other 

worlds out among the stars.*

*see Issue #1

For centuries, scientists 

and philosophers argued 

about the possibility of 

extrasolar planets.

As time went on, technology 

improved and changed our 

view of the Universe.

In 1995, astronomers finally 

found an extrasolar planet.

A gas giant planet like 51 Pegasi b 

isn’t habitable... but it proved that 

exoplanets did indeed exist.

Giordano Bruno, 

1548-1600. (3)

Edwin Hubble, 

1920s.

Haute-Provence 

Observatory, 1995.

...but their planets 

remain invisible to us 

because they are smaller 

and non-luminous.

We see only the suns 

because they are large 

and luminous...

A giant planet 

rapidly orbits the 

star 51 Pegasi! (4)

Wait, those 

clouds of light 

are galaxies!

Mon 

dieu...

4
As a planet 

orbits its star, 

it tugs ever so 

slightly on it.

The star isn’t 

stationary. It ‘dances’ 

around a central point 

in space.

How much the 

star moves in this 

dance can tell us things 

about the planet, like 

its mass. 

The more 

massive the planet, 

the more it pulls 

on its star.

And toward 

us... it’s blue.

When the 

star moves 

away from us, it 

looks red.

This movement 

affects the light 

spectrum of a star, or 

its color signature.

This discovery raised important 

questions for astrobiologists...

This is how 

we found 51 

Pegasi b!

Right now, we 

mainly use two techniques 

to find planets, the radial 

velocity method and the 

transit method.

And how 

can we find the 

planets that 

might? 

Michel Mayor, 

University of 

Geneva

Didier Queloz, 

Cambridge and 

University of 

Geneva

Giada Arney, 

NASA GSFC

The radial velocity method* 

measures the gravitational 

affect of a planet on its sun.

* aka Doppler 

spectroscopy

Could extrasolar planets 

have life?

5
The amount of light 

blocked, and how often it 

happens, can tell us about the 

planet’s size, its orbital period 

(or “year”), and its distance 

from the star.

If you know 

the radius of the 

star, that is.

Planets orbiting close 

to a star are easier to study, 

because close planets transit 

more often, giving us more 

observations to look at.

Natalie Batalha, NASA Ames 

Research Center (ARC)

The transit technique works 

when a planet passes in 

front of its star.

Bill Borucki, 

NASA ARC

With both 

techniques, it’s easier 

to spot big planets.

Gas giants 

similar to Jupiter have 

more gravity to affect 

their star. And they’re 

big, so they block out 

more light when they 

transit.

Radial velocity was 

the first method to find 

exoplanets in large numbers, but 

now it has been overtaken by 

the transit method, both in the 

number of planets discovered and 

in its ability to characterize 

them. (5)

When a planet 

moves in front of its 

star, it blocks part 

of the light that 

we see. 

6
7

But as techniques 

improved, smaller and 

smaller planets were 

discovered.

After the first exoplanet 

discovery, astronomers got 

to work hunting for more.

The first planets 

were the easiest to 

spot - huge gas giants 

similar to Jupiter.

Spitzer Space Telescope (NASA - 2003)

Finding small, rocky planets 

raised questions about whether 

or not any could be habitable.

And finding an inhabited planet 

could answer so many questions 

about Earth’s place in the Universe.

There were stars 

with multiple planets 

in orbit.

And bizarre 

worlds unlike 

anything we’d 

seen before.

Things started to 

get very interesting 

for astrobiologists.

Hubble Space Telescope 

(NASA - 1990)

W. M. Keck 

Observatory**

Convection, Rotataion and 

planetary Transits (CoRoT) 

(CNES/ESA - 2006)*

HARPS spectrograph at 

ESO, La Silla Observatory**

European Southern Observatory (ESO), Cerro Paranal

Anglo-Australian 

Planet Search (APS)**

Exaple of facility using: *transit 

method, or **radial velocity method

Gaia (ESA - 2013)

From 1995 onward, 

ground- and space-based 

telescopes were used to 

add to the list.

New discoveries 

flooded in, including 

planets unlike anything 

in the Solar System.

7
With so many 

planets, we wanted to 

figure out what their 

climates might 

be like...

In 2001, the Virtual Planetary Laboratory was 

founded with support from NASA Astrobiology.

VPL is based at the University of 

Washington, but includes researchers 

at 20 different institutions 

around the world. (6)

Geochemists Molecular 

evolutionary 

biologists

Spectroscopists

Planetary 

scientists

Atmospheric 

chemists

Climatologists Biologists

Just to 

name a few.

Astrobiology 

includes scientists 

from many fields, and 

so does the VPL.

Biophysicists

There are 

Geophysicists astronomers. Geomicrobiologists

Virologists

...and answer the 

big question: can 

they support life?

We’ve developed 

a ‘tool kit’ of computer 

models to simulate possible 

environments on distant 

worlds.

And we study how 

observations and modeling 

can help determine if a 

planet is (or was) able to 

support life. (6)

This work will help 

us determine what to 

look for with missions 

like JWST.

...and apply it to 

extrasolar planets.

Our goal is to take 

what we have learned about 

habitability from our own 

solar system....

Astrobiology 

was ready to join 

in the search for 

exoplanets.

8
**See Issues 2,3 & 5

* See Issue 5

Mercury

Venus

Earth

Mars

Analog environments 

on Earth help us study 

life’s potential in the 

Solar System*.

But Earth, the 

entire planet, is like 

one giant analog for 

habitable worlds.

Earth science 

helps us predict what 

our telescopes might 

see in other planetary 

systems.

We start 

our search with 

terrestrial planets.

Now, ‘terrestrial’ 

does not mean ‘Earthlike’ or habitable.

Our system has four 

terrestrial planets, but 

only one with life**.

Bodies like the 

dwarf planet Pluto aren’t 

terrestrial because they 

have so much ice and 

frozen material.

It means the 

planet is made of 

metals and rocks.

Understanding 

habitable planets 

starts with 

Earth.

We also need to 

understand the Earth 

through time... ...how it formed, 

evolved, and became a 

home for life.

9
Earth-sized 

means with a 

similar diameter 

to the Earth.

Leslie Rogers, 

University of 

Chicago

Earth is the 

biggest terrestrial 

planet in our system, but 

it’s hard to say how big 

a terrestrial planet in 

another system 

could be.

Earth-mass

means it has a 

similar mass.

That’s where 

you might hit the 

boundary between rocky 

‘super Earths’, and small 

gaseous planets we call 

‘mini-Neptunes’.

So far, we 

think it has to 

be less than 1.6 

Earth radii. (9)

We start 

with determining 

their orbits.

Once we find 

terrestrial planets, 

the next step is 

figuring out what 

they’re like.

Just because a 

planet has the same 

mass, that doesn’t mean 

that it is the same size 

as Earth... and vice 

versa. (7, 8)

Two planets 

with the same 

mass could have 

different 

diameters.

It just 

depends on 

the planet’s 

density...

...in other 

words, what 

it is made out 

of.

10
Or is 

it...

We want to look at small planets. 

The easiest planets to find with 

our techniques are ones that 

orbit close to their sun. Being near the sun can make it too 

hot for life... just look at Mercury.* 

We need to be able to 

spot terrestrial planets 

further from their sun...

...but not so far 

that they are too 

cold for life.**

Early on, scientists came up with the simple 

idea of the ‘habitable’ or ‘Goldilocks’ zone 

by looking at our own system. (10)

This is the zone where 

liquid water can exist on 

the surface of a planet.

The habitable zone is 

not too hot, and not 

too cold... ...it’s just right 

for water, which 

is essential for 

life as we know it.

*See Issue 3

**See Issues 4 and 5

That doesn’t 

sound too hard. We 

look for a planet 

just like Earth.

Same diameter, 

same mass, same 

orbit.

After all, 

Earth is the only 

planet we know 

with life.

11
The Earth, today. The Earth, 4 

billion years ago.

In some ways, 

Earth isn’t the only 

analog we have for an 

inhabited world.

Maybe Earth-like 

exoplanets don’t have 

to look like the Earth 

we know today. 

Remember, we must 

understand the Earth 

through time.

For so much of 

its history, Earth didn’t 

even have an ‘oxygen’ 

atmosphere.

The most ‘alien’ 

planet that we have 

data for is actually the 

ancient Earth...

Life has been on 

Earth for billions of 

years and, long ago, the 

planet looked a lot 

different.

...which we study 

through geochemistry 

and geology.

Maybe there’s an 

inhabited planet that looks 

more like the Archean 

Earth (4 billion to 2.5 billion years ago)...

...when life first 

originated! (11)

12
Ignas Snellen, Leiden 

University (12)

Dimitar Sasselov, 

Harvard University Sara Seager, MIT 

Drake Deming, University 

of Maryland at College Park

We just need to 

find the right planet to 

study, and put these 

ideas to the test!

That’s a big 

challenge. Exoplanets are so 

far away.

But we can still 

study the planet, 

even though we 

can’t see it.

What do 

you mean by 

that?

When a planet 

passes in front of a 

star it blocks some of 

the star’s light.

If we are 

careful, we can see 

some of that light 

filtered by a planet’s 

atmosphere.

We separate 

the tiny amount of light 

that makes it through 

the atmosphere from the 

huge amount of light you 

see from the star. It’s tough, but 

our techniques are 

getting better 

and better.

As light passes 

through the planet’s 

atmosphere, the gas 

changes what it looks 

like to us.

Different gas 

molecules absorb 

light at different 

wavelengths.

First, we made 

models to help determine 

how we could study the 

atmospheres of transiting 

planets. (13)

We see those 

changes in the star’s 

transmission spectrum. 

The trick is figuring out 

what they mean.

13
Then we began 

to characterize the 

composition of gas giant 

atmospheres. 

(16, 17, 18, 19)

Gas giants are a good 

test for our techniques, but now 

we need to refine our methods and 

develop technology that will let us study 

the atmospheres of smaller and 

smaller planets.

But what can an 

atmosphere tell us 

about life?

Remember, 

we’re not looking 

for intelligent, 

sci-fi ‘aliens.’

Turns out, it 

can tell us all kinds 

of things.

We’re looking 

for chemical signs 

of life.

The effect of 

primitive organisms like 

microbes might be the 

most likely thing 

we’ll see.

Although they’re 

microscopic, we know 

that microbes impact a 

planet globally...

Eddie Schwieterman, 

University of California, 

Riverside

... from atmospheric 

composition to ocean 

chemistry.

Gases created by 

biology, from microbes 

to plants, can alter the 

atmosphere.

Strange as it 

sounds, it’s microbes

that could speak to us 

across the parsecs.

The technique 

was validated when we 

observed the planet HD 

209458. (14, 15)

14
On a planet without life, the 

atmosphere comes to a stable 

balance of chemicals. 

It was James Lovelock* who first suggested that biology could be detected 

by its effect on the atmosphere.

Biology can shift this balance in a way 

that can’t happen on a lifeless planet.

Nancy Kiang, 

NASA GISS

* See Issue 5

We have lots 

of oxygen because 

microbes in the 

oceans...

...and plants on the 

land are performing 

photosynthesis.

Photosynthesis 

makes the oxygen, 

and it also makes cool 

colors that can be seen 

from space!

It’s especially 

easy to see things like 

forests and grasslands 

covering the land.

*See Issue 2

Some kinds of 

chemicals don’t co-exist 

easily in the atmosphere 

because of chemical 

reactions.

For example, oxygen 

and methane don’t normally 

co-exist easily. But on Earth 

today, they do... 

...because 

of biology!

If Earth 

had no life, the 

atmosphere would 

look different. 

(20)

15
*See Issue 4

Ancient Earth’s 

atmosphere was different 

than today, but there 

was also life.

We’re not totally 

sure what it looked like, 

but there might have 

been a lot more methane 

than today.

This means the 

Earth would’ve looked 

completely different 

from space.

Carl Sagan 

described Earth 

today as a pale blue dot*. 

Compare that to Saturn’s 

moon Titan... 

... Titan has a 

methane-rich haze 

that looks kind of 

orange.

But Titan 

doesn’t have life 

that we know of.

Plants have their color, 

because photosynthetic 

pigments select light to use 

from the Sun.

Our sun is a yellow 

star, but there are 

lots of other stars out 

there with different 

colors of light.

Hmm... if there 

was a break in the 

clouds and we could 

see the surface.

And what if 

the atmosphere was 

completely different 

than either modern or 

ancient Earth? (22)

Josh KrissansenTotton, UW

Rodrigo Luger, UW

Right... if the Archean 

Earth had life and Titan 

doesn’t, how do you tell 

the difference?

Maybe the 

Archean Earth 

was a pale orange

dot. (21)

Maybe colors at 

the surface could 

be signs of life!

Plants on worlds 

orbiting those stars 

could be something 

other than green. 

(23, 24)

16
Tim Lyons, 

UC Riverside

Take salt 

ponds on Earth 

today. Here, microbes 

called archaea can 

be purple, orange, 

red...

On ancient Earth 

archaea could have 

dominated. (25) 

And it’s not just Earth 

(or ancient Earth) that 

can teach us what a 

planet’s atmosphere has 

to say about a planet.

Building on what they 

know about Mars, Earth, and 

Venus, astrobiologists are developing 

new planetary atmosphere and climate 

models to try to understand what 

planets might be like in other 

solar systems.

The differences 

among the planets in our 

solar system can tell us a 

lot about what is possible 

on exoplanets.

The balance of Earth’s atmosphere 

is explained by biology.

Jupiter’s atmosphere 

balances out chemically.

Mars’ atmosphere can be 

explained by photochemistry*.

Caleb Scharf, 

Columbia University

Tony Del Genio, 

NASA GISS

Gavin Schmidt, 

NASA GISS

One thing 

to note...

There’s a lot of 

untapped energy in 

Mars’ atmosphere in 

the balance between 

carbon dioxide and 

oxygen.

Life uses 

pigments for 

lots of things.

*See Issue 2

It might be these 

pigments that we’re 

more likely to spot on 

an exoplanet. (26)

On Earth, there are 

lots of biological pigments 

that aren’t green, and many 

that aren’t used by 

photosynthesis.

17
Once again, 

it seemed so easy 

at first...

But, of course, 

we realized there 

was more to it 

than that. ...look for 

something 

small...

...and an 

oxygenated

atmosphere!

Life makes the 

composition of Earth’s 

atmosphere possible.

Free energy is an 

anti-biosignature.

We know that from 

watching life on Earth, and 

how it maintains the balance 

of things like oxygen, nitrogen, 

and water on our planet. 

“The low oxygen on 

Mars is what made me think 

the planet was lifeless back 

in the days of Viking.”

(20)*

...in the 

habitable 

zone...

Ravi Kumar Kopparapu, 

NASA GSFC

*See Issue 2

If life had 

access to Mars’ 

atmosphere, it would 

use that energy to 

survive.

18
Now we know multiple 

ways for making 

oxygen abiotically. 

(27, 28, 29, 30, 31)

At a coffee shop 

in Seattle, 2010...

Uh-oh!...

What, is 

something 

wrong?

Uh... yeah.

Turns out, 

you can get lots 

of oxygen without 

biology. (27)

More 

coffee?

No, not 

that...

Oxygen 

doesn’t have 

to come from 

life...

Or, carbon dioxide 

can be broken apart 

by solar energy 

through photolysis*.

Now scientists had to look at 

what other features could act as 

a signal for life in an atmosphere.

For instance, energy from a 

young star can strip hydrogen 

away from water molecules...

...leaving only the 

oxygen behind.

*Chemical decomposition 

under the influence of light.

Jen Domagal-Goldman, 

American Democracy Project

19
...maybe 

something else 

happens alongside 

that process that 

we can detect?

Then we could 

tell the difference 

between oxygen 

from biology, and 

oxygen from something else. 

It was dubbed the 

Nexus for Exoplanet 

System Science 

(NExSS). (32)

NExSS encourages collaboration among 

the four science communities supported 

by NASA’s Science Mission Directorate.

Planetary Science.

Heliophysics

Astrophysics

Earth Science

If the 

oxygen comes from 

photolysis, or some 

other process...

We realized that 

we can’t only think about 

how to identify life.

We also need to 

think about how we 

might be fooled.

Exactly.

The more 

complicated our questions 

got, the more scientists 

we needed to try and 

answer them.

It was time to 

get insights from an 

even wider scientific 

community.

In 2014, NASA Astrobiology led an 

effort to create a new, coordinated 

network of exoplanet explorers.

Cecilia Bitz, UW

Rika Anderson,

Carleton College

20
** See Issue 3

We have to look 

to the future and get ready 

for new technology that will 

let us study small planets 

in detail.

And we need 

better models to 

understand what the 

telescopes see.

As we build more 

powerful telescopes, 

we need a place to 

point them.

Tools developed 

in the 1960s to study 

Venus changed our 

view of Earth**.

Now we’re applying 

climate simulations developed 

in the 1980s to known exoplanets 

in order to accelerate new 

discoveries.

NExSS includes several research 

teams, each dedicated to solving 

a piece of the exoplanet puzzle. With a growing 

community of astronomers, 

astrophysicists, Earth scientists 

and heliophysicists at NASA working 

together, we’re developing an even 

deeper understanding of 

...how they might how planets evolve...

be like or un-like 

Earth...

...how they 

interact with 

their stars...

NExSS research 

covers every aspect of 

habitability, from planet 

formation to the end of 

a planet’s lifecycle.

Steve Desch, 

Arizona State 

University

Eric Ford, 

Penn State

...and their 

past or present 

habitability.

21
These findings 

indicate that we might 

find a terrestrial planet 

that theoretically sits 

in the habitable zone... 

yet isn’t actually 

habitable.

With new ideas 

from even more scientists, 

let’s look again at the 

‘habitable zone.’

The habitable zone 

concept lets us start our 

search where life is most likely 

to be, but it doesn’t guarantee 

habitability. (33)

Antígona Segura Peralta, 

Universidad Nacional 

Autónoma de México

The ‘habitable 

zone’ makes things 

sound easy. We just need 

a small planet with an 

orbit similar to 

the Earth.

Mars is at 

the outer edge 

of our system’s 

habitable zone.

Earth sits in 

the middle.

Venus is 

outside the habitable 

zone, but it’s close to 

the inner limit.

If only it 

were that 

simple... As soon as a 

planet deviates from 

that composition, it can 

get into trouble.

The habitable zone 

is based on an Earth-like 

planet, with an Earthlike atmosphere.

So far, 

we haven’t found 

evidence of past or 

present life on either 

Mars or Venus.

22
* See Issue 3

Mark Kuchner, 

NASA GSFC

Even a planet with 

exactly the same size 

and orbit as Earth might 

look very different.

One thing the 

habitable zone concept 

doesn’t take into account is 

clouds, which can have an 

effect on surface 

temperature.

Venus might have been 

habitable long ago, if it had clouds 

like Earth’s that kept a lot of sunlight 

out. But today, carbon dioxide in Venus’ 

atmosphere causes a greenhouse 

effect that makes the planet 

way too hot for life*.

Whew!

Maybe ancient 

Venus was habitable... 

but now it might be 

an example of what 

the future holds 

for Earth*.

Venus could be an example of 

the end state of habitable 

planet evolution.

At some point in its history, 

maybe life could have survived... 

but not anymore.

New code is being developed to show 

if clouds have a cooling or a warming 

effect on different types of planets.

Tony Del Genio, 

NASA GISS

23
With 

today’s instruments, we just can’t get a 

spectra that tell us what the 

atmosphere is made of.

* See Issue 4

Astronomers 

have found one such 

planet, named GJ 

1214 b. (34)

*See page 16

Hazy planet 

spectra might not be 

flat at all wavelengths, 

and we can still characterize basic properties 

like planet colors.*

Things like a planet’s 

orbit, tidal locking, 

or tidal heating* can also 

mess up observations. 

(35, 36, 37)

...but be heated so 

much that it’s covered in 

magma and all liquid water 

evaporates away to be 

blown into space! (38)

A planet could 

appear to be in 

the coziest spot 

of the habitable 

zone... 

Hazy or cloudy 

planets with flat spectra 

are difficult to characterize. 

A hazy mini-Neptune might 

look the same as a hazy 

terrestrial planet.

Rory Barnes, 

UW

Russell 

Deitrick, UW

It’s difficult to 

get information 

from a hazy planet, 

but spotting one 

isn’t a total loss.

There are other 

things to consider 

about the habitable 

zone.

Yeah, there 

is a lot more to 

habitability than 

a planet’s distance 

from its star.

If a planet has 

clouds, or haze, light 

only penetrates the 

top part of the 

atmosphere.

For the 

wavelengths of light 

we look at, the planet’s 

spectrum ends up flat 

and featureless.

24
Planets in our 

solar system orbit 

in a similar plane, 

like a plate around 

the Sun.

Eric Agol, UW

A planet on a 

crazy orbit might 

transit its star...

... so we can 

spot it...

...but we might not 

realize that it swings both so 

far away from and so close to 

its star during its orbit that it 

is uninhabitable for life as

we know it. (39)

Planets in other 

systems might not, 

making their habitable 

zones completely 

different.

Whoa!

The gravity of 

a planet pulls on its 

star (and vice versa), 

and planets also pull 

on each other.

This is where 

orbital dynamics can 

affect the climate 

and habitability 

of a planet. If the orbit isn’t 

circular, a planet could 

cook part of the time... 

and freeze for the 

other part! (40, 41)

25
It could be 

important to 

find out.

And there are 

so many different 

types of planets, with 

so many unanswered 

questions.

Or a star 

with a lot of 

flares?

And what 

happens if a planet 

like Earth orbits a 

different type of 

star? (42)

Like... can a 

planet without an 

atmosphere still 

have an ocean?

We need to 

know everything we 

can about a planet... its 

atmosphere... interior... 

orbit... its 

host star...

How do we 

know what goes on inside 

a planet when all we see is 

a tiny pixel of light from 

billions of miles away?

Right. Can 

we tell if it has 

things like plate 

tectonics? 

Or a magnetic 

field to protect 

life from things 

like radiation? 

Are interior 

properties somehow 

reflected in the exterior 

of the planet? 

If we’re going to 

believe that we found 

a biosignature, we need to 

understand the entire 

context of a planet.

But how? 

Like a red 

dwarf? (43) 

And what 

about the planet’s 

rotation? (44, 45)

Aomawa Shields, 

University of 

California, Irvine

26
Like in sub-ice 

oceans of liquid 

water on worlds 

like Europa*

Life might also 

inhabit areas outside 

of the traditional 

habitable zone.

Or deep 

underground on 

rocky planets.** 

But if life is hidden 

somewhere, like the 

subsurface, we can’t 

detect it from trillions 

of miles away.

At least not with the 

technology we have right 

now... or anytime soon.

So, a planet with life 

at the surface is our best 

chance for spotting a 

habitable planet.

It’s not the only place 

we could find one... but with 

limited resources, it’s the 

best place to start our 

hunting.

And remember, we’ll 

only get a few chances 

with JWST.

Life at the surface 

brings us back to looking 

for planets that could 

have liquid water at the 

surface.

Hints about what a planet with surface liquid 

might look like have come from missions that 

turned their cameras back to Earth.

We also have 

images of Titan and 

its hydrocarbon 

lakes.*

And one picture 

from the EPOXI 

mission was really 

interesting.

Tyler Robinson, University 

of California, Santa Cruz

John Baross, UW

Viking

EPOXI

Cassini

LCROSS *See Issue 4

*See Issue 4

**See Issue 5

27
Ralf Jaumann, Deutsches 

Zentrum für Luft- und 

Raumfahrt (DLR)

...reflecting off of the surface 

of a hydrocarbon lake on Titan.

While orbiting Saturn, 

the Cassini spacecraft 

caught a glint of light... EPOXI caught a bright 

glint of sunlight reflecting 

off of bodies of water on 

Earth’s surface.

* Extrasolar Planet Observation and 

Deep Impact Extended Investigation

Could we detect 

a similar glint 

of light with our 

telescopes? (48)

Maybe... ... such a tiny 

amount of light 

added to the light 

of the star...

It wouldn’t 

prove there’s 

life, but liquid 

is a big step. 

It would 

certainly help 

us narrow our 

search. 

Particularly 

if the liquid is 

water.

Surface water 

could mean life 

at the surface. 

Where we might 

stand a chance of 

detecting it! 

...and focus on 

looking for life that 

uses the dominant 

energy source at 

the surface... 

... light from 

its star!

That’s a scenario 

that could produce a 

biosignature in a planet’s 

atmosphere that we could 

detect with upcoming 

missions. 

46

47

Our best hope right 

now is to find a planet 

with surface water...

28
The diversity of worlds 

in our solar system is 

puny compared to what 

we’ve seen beyond it.

But Kepler’s planets are so far away, 

and it’s extremely hard to get any 

information about their atmospheres.

What we know so far about exoplanets 

is built on the work of some amazing 

missions and technology.

One such mission is Kepler, which 

has made a huge contribution to 

exoplanet science. (49)

For its primary mission, Kepler 

was fixed at observing a single 

spot in the sky. One tiny percent 

of the entire Universe. In this one spot, Kepler found 

a HUGE number of planets. For 

years to come, scientists will 

be combing through data to 

find even more.

Kepler has shown us 

that rocky planets 

are not rare, and that 

there are a lot of 

potentially habitable 

planets out there.

We need spectroscopy measurements 

to study the atmosphere of planets, 

and we don’t have any instruments 

that can see things as far as Kepler.

Determining their mass is difficult as 

well... limiting what else we can learn 

about these numerous worlds for now.

Kepler taught us what we 

need to look for next.

We need to find Kepler-like planets 

closer to Earth. Within 10 parsecs 

(more like 32 light years). And because 

Kepler found so many planets, we now 

know that we have a good chance of 

finding similar planets closer to home.

29
Unlike Kepler, 

a mission like TESS would 

be able to move around and 

observe the entire sky.

And TESS could 

search for nearby 

planets.

Most nearby stars are 

dim M-dwarfs, which 

could be a good thing.

Because these stars are dim, 

habitable zones for M-dwarfs 

are close to them.

That means potentially habitable 

planets would transit more often, 

and might be easier to spot.

New telescopes could do just 

that, like TESS*. (50)

But JWST has limitations in 

terms of studying planets.

If JWST is able to get a 

transmission spectrum from an 

exoplanet, we’ll still only be able 

to learn about the uppermost 

part of the atmosphere.

Then comes the 

James Webb Space 

Telescope (JWST).

JWST will give 

us a really good 

look at nearby 

M-dwarfs.

Catarina Alves de 

Oliveira, European 

Space Agency (ESA)

* Transiting Exoplanet 

Survey Satellite

30
Starlight won’t penetrate deep 

enough to reveal any information 

about lower levels of the atmosphere, 

where biosignature gases might be.

For JWST, all 

of our eggs are in the 

‘transit basket.’ It’s high 

stakes... but with potentially 

high rewards.

Looking at a single 

planet with JWST is going 

to take a HUGE amount of 

observation time.

This is why, in its 

lifetime, JWST will let us 

look at a few planets at 

the MOST.

A handful out 

of thousands of 

possibilities. Right now, we’re 

building a habitability 

index to rank our 

targets.

How do we 

chose?

All we’ve learned from 

planets in our solar system 

will help us determine 

targets for JWST.

JWST is a 

major stepping 

stone toward 

even better 

technology.

In the long term, 

we want a mission that 

can directly image and 

characterize habitable 

planets!

...but each new 

mission gets 

us closer.

We’re still a 

long way from 

that goal...

Doug Hudgins, Program 

Scientist, NASA Exoplanet 

Exploration Program

Karl Stapelfeldt, Exoplanet 

Exploration Program Office 

Chief Scientist at JPL

31
WFIRST could make observations 

in the near-infrared (NIR) part 

of the spectrum. Its main camera 

would take images that are 100 

times larger than those of the 

Hubble Space Telecscope. (51)

To take a picture 

of a planet, we have to 

block the huge amount 

of light coming from 

the star it orbits. We get 

10,000,000,000 

photons from a 

star for every 

ONE photon from 

an Earth-like 

planet!

Technology 

like starshades 

and coronagraphs 

could help.

Starshades are 

separate structures 

that can block 

light...

...and coronagraphs 

are built into the 

telescope. 

A coronagraph on 

WFIRST wouldn’t be used with 

the main camera, but it could still be 

powerful enough for us to image gas 

giant planets in visible light.

As always, viewing 

gas giants is where we’ll 

start, because they’re 

easier to see.

With its main 

camera, a mission like 

WFIRST could also use new 

planet-hunting techniques, 

like gravitational 

microlensing. (51)

Early versions 

of this technology could 

be included on upcoming 

missions, like the planned 

WFIRST*.

Over time, we’ll 

perfect the technique 

and study smaller and 

smaller planets.

* Wide Field Infrared 

Survey Telescope

32
Gravitational 

microlensing is a 

phenomenon caused 

by the bending of 

light in space.

It’s explained by my 

General Theory of 

Relativity!

Scott Gaudi, Ohio 

State University

Isaac Newton

Albert Einstein

Johann Georg 

von Soldner

It’s a bit like 

having a magnifying 

glass in the sky.

When we observe light from 

a distant star, and another 

star passes in front...

...gravity from the foreground 

star can actually bend the light 

from the star behind it.

If the foreground star has 

planets, they can 

also act as smaller 

lenses. (52)

Microlensing has 

been used to study black 

holes, and it could be good 

for studying planets that 

orbit far from their star.

Maybe with a 

mission like WFIRST, we 

can use this technique to 

learn even more about 

exoplanets.

33
While we’re working on 

new missions and getting close 

to choosing targets for JWST, 

exoplanet scientists keep 

making new discoveries....

In 2017, we made a 

huge discovery using data 

from Spitzer and groundbased telescopes...

TRAPPIST-1 is an ultra-cool 

star that is only slightly larger 

than the planet Jupiter.

Because TRAPPIST-1 is so small 

and cool, the system’s habitable 

zone is very close to the star.

It’s the first time 

that we’ve been able to 

infer masses for planets 

this small.

Three of the seven Earthsized planets orbit well 

within this habitable zone.

By studying the interactions of 

the planets, scientists were able 

to determine their masses.

So we have 

three planets in the 

habitable zone, with sizes 

AND masses similar to 

Earth’s...

...all orbiting close to 

their star, where it is 

easier for us to see 

them.

...a group of rocky 

planets orbiting the 

star TRAPPIST-1. 

(53)

But that’s 

not all...

34
Another amazing 

thing is that they are 

close, only 40 light 

years away! (54)

Right! We might 

actually be able to study 

their atmospheres with 

telescopes we have 

TODAY.

And, of 

course, they could be 

great candidates for the 

technology we’re building 

right now.

We could be so 

close to characterizing 

some truly habitable 

planets...

... or maybe 

even finding an 

inhabited one.

The good news is 

that we’re starting to 

figure out how we could spot 

any pale blue (or orange) 

dots like Earth.

Michaël Gillon, 

Université de Liège

Aki Roberge, 

NASA GSFC

The bad news is 

that we’ll just be looking 

at tiny dots of light in 

the sky.

35
The Earth through 

time provides many examples 

of different biospheres and 

habitable planets that we 

can look for.

Although, life could 

exist in forms we don’t 

know or understand.

Biospheres, much 

like planets, might be 

more diverse than we 

ever imagined.

But right now, 

we have only a handful 

of chances to spot an 

exoplanet with life.

We should start our 

search by looking for life 

as we know it.

To do that, 

we have to trade our 

telescopes for powerful 

microscopes, and turn our 

eyes back to life’s very 

beginnings.

If we want to think 

about life as we don’t know it... 

we need to understand life at 

its most basic level...

... and at a scale 

much smaller than 

entire planets.

Next issue:

Prebiotic Chemistry 

and the 

Origins of Life

36
Astrobiology

A History of Exobiology and Astrobiology at NASA

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46. This image shows the first flash of sunlight reflected off a lake on Saturn’s 

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47. A sun glint on Earth is captured (center of the black circle) in the middle frame 

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NP-2017-03-2376-HQ

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