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As scientists and astronomers
peer more closely and with

ever more fidelity at nearby
stars, they are discovering

exoplanets, worlds outside
our own solar system. Stars with

planets were once thought
to be a rarity. They're turning

out to be the norm and not
the exception. With some

confidence, scientists now
calculate there could be as many

as 40 billion Earth-like planets
in our Milky Way galaxy alone.

The first exoplanets were
detected in 1992, orbiting a

pulsar, the remnants of a
massive exploded star that was now

a fast-spinning neutron
star. With data collected from

the Arecibo antennae in
Puerto Rico, Aleksandr Volshan

discovered three terrestrial
planets orbiting the pulsar PSR

B1257 plus 12. Other pulsars
have been detected with dust

clouds and disks orbiting
them. This suggests that these

three planets are second
generation. The original planets

were destroyed by the stellar
explosion, and the subsequent

debris disk enabled these
new planets to form. It was

another three years before
the first planet orbiting a G

-class star similar to our
sun was identified, a gas giant

like Jupiter orbiting 51 Pegasi
every four days. Five years

later, another main sequence
star was found to have multiple

planets. Exoplanets were not
easy to detect. They are tiny,

their light smothered by
the light of their star, and they

are so far away.

There are three main methods
for planetary detection. The

hardest is direct visual
observation via telescope, both on

the ground and in orbit. Some 59
planets have been discovered this way.

More indirect methods are
employed, one of which is

the radial velocity method.
As a planet orbits, its

gravitational pull causes the
parent star to move back and

forth. This tiny radial motion
shifts the observed spectrum

of the star by a correspondingly
small amount because of the

Doppler shift. With supersensitive
spectrographs, the shifts

can be measured and used to infer
details of a planet's mass and orbit.

The 3.6-meter telescope
at La Silla, Chile has such a

spectrographic instrument
called HART and is the leading

exoplanet hunter.

This is Talbotis, one of
the first planets discovered

utilizing this method. 51
light-years from Earth, this

planet is massive, some four
times the mass of Jupiter. So

far, over 600 planets have
been detected by this method.

Another observational tool,
and the most successful to date,

is the transit method. A
planet that passes in front of its

parent star relative to us
produces a slight dimming

of the star's light, which
can be detected by sensitive

instruments. Some 1,200
have been located this way.

Until now, it was expected
that exoplanets would orbit in

more or less the same plane
and in the same direction as the

star's rotation. However, new
results unexpectedly show that

many exoplanets actually
orbit at a large angle to their

star's spin axis. In the case
shown here, Wasp 8b, the orbit

is completely reversed. There
are other tools and techniques

in the planet-hunting
toolbox, including microlensing,

oculation and TTV or transit
timing variation. Often, more

than one technique is
used to verify findings.

In fact, revisiting a
planet after some time for

verification can lead to
surprising results, as in HD

189733b, a hot Jupiter-type
planet. On revisiting it, Hubble

discovered the atmosphere
was being stripped from it by a

violent stellar flare.

With the Hubble Space
Telescope dividing its valuable time

between many varied tasks
and objectives, a dedicated planet

hunter called Koro was launched.

The mission was led by the
French space agency CNES, with

contributions from ESA, Austria,
Belgium, Germany, Spain and

Brazil. Launched in 2006,
the mission lasted seven years.

It located two planets around
the star Koro 7, one of which

was the first found to display
a density similar to Earth's.

In all, it located 32 planets and a
hundred others are awaiting confirmation.

The NASA Spitzer Infrared
Space Telescope was launched to

study protoplanetary and
debris disks around stars and the

curious brown dwarfs, often
referred to as failed stars.

Spitzer's infrared capability
quickly led to numerous

planetary discoveries and
infrared mapping of other known

planets, like hot Jupiter HD
149026b. Some 256 light-years

away in the constellation
Hercules, a planet dubbed a hot

Jupiter, it is a sweltering
2,040 degrees Celsius, the

hottest planet yet detected,
and also the darkest,

reflecting no sunlight back
into space. It speeds around its

star every 2.9 days.

A world just two-thirds
the size of Earth, one of the

smallest on record, and only
33 light-years away around the

star GJ 436, planet UCF-1.01
might be the nearest world to

our solar system that is
smaller than Earth. The planet, a

rocky world, orbits so close
to the star that the surface is

probably molten. As with HD
189733b in the constellation of

Alpacula, it appears to
be tidally locked to its star,

showing only one face as
it orbits. Spitzer was able to

distinguish the various
temperatures of its clouds from 650

to 1700 degrees Celsius.
Thermal imaging of these hot giants

has provided more details
of these distant worlds. In

2009, the game changed with
the launch of the Kepler Space

Observatory.

As part of NASA's Discovery
Program, the Kepler Space

Telescope was launched to
survey and monitor a fixed field

of stars of the nearby Milky
Way trailing behind Earth. It

observed around 165,000
stars, watching for any changes in

their brightness. It has located
over 1,100 planetary candidates.

Now these are candidates,
but most of them, I'm convinced,

will be confirmed in the
coming months and years. That's

more than all the people
have found so far in history. A

veritable menagerie of planet
types is emerging like Kepler

-16b orbiting two stars,

ice worlds and water
worlds like Gliese 1214b.

The Kepler team announced
today 1,094 new planet candidates,

bringing the total roster up
to 2,326. Of those, 207 are

Earth-size, and we now have
48 that are in the habitable

zone, 10 of which are smaller
than two Earth radii. So these

are planets that could
potentially be rocky. So it's an

exciting milestone because
we are really honing in on truly

Earth-sized habitable planets.

The combined surface and
space-based system observations

have led scientists to believe
that planets around stars are

the rule rather than the
exception, and the average

number of planets per
star is greater than one.

Kepler has discovered at
least 86 stars with multiple

planetary systems. Kepler-11,
for example, has six confirmed

planets orbiting a sun-like
star. The Kepler-11 planetary

system is amazing. It's
amazingly compact. It's amazingly

flat. There's an amazingly
large number of big planets

orbiting close to their star.
We didn't know such systems

could even exist. There are
certainly far fewer than one

percent of stars have systems
like Kepler-11. But whether

it's one in a thousand, one
in ten thousand, or one in a

million, that we don't know
because we only know one of

them.

The growing number
of confirmed planets is

opening up new insights
into planet formation.

We're learning so much more
about the orbits of planets,

the masses of planets, the
sizes of planets, and we're just

beginning. Kepler is still
returning data, and we're going

to learn a fantastic amount
about the diversity of planets

out there around stars
within our galaxy. Around each

star is a circumstellar region
called the habitable zone.

Sometimes referred to as
the Goldilocks zone, this is a

region neither too cold nor
too hot, where a planet could,

under the right conditions,
support liquid water

at its surface and in
turn could support life.

The first Earth-sized planet
in a habitable zone was

discovered around a red dwarf.
Named Kepler-186f, it is just

ten percent larger than
Earth. Kepler-186f is the first

validated Earth-sized planet
in the habitable zone of its

star. It's the outermost of
five planets to orbit a star

that is smaller and cooler
than the sun. This planet orbits

its star every 130 days,
and so this places it in the

habitable zone, where it's in
a region where it could have

liquid water on its surface.
The star Kepler-186 is 500

light years from Earth in
the constellation Cygnus. This

planet Kepler-186f orbits a
star that's cooler and dimmer

than the sun. So while we may
have found a planet that's the

same size as Earth and receives
a similar amount of energy

to what Earth receives, it
orbits a very different star. So

perhaps instead of an Earth
twin, we've discovered an Earth

cousin. Believed to be a rocky
world, its mass and density

are yet to be determined.
This is one of the big milestones

that we've been looking for
in our attempts to find out if

there are places just like
home and if there's life out

there. One of the big steps
is to say, is there somewhere

that looks, to all intents and
purposes, like Earth? Well,

we don't know just yet, but
we know that there are places

that at least look similar.

To date, over 48 Earth-like planets
have been located within habitable zones.

Gliese 581 has four known
planets. The outer D planet is

thought to be an icy planet
that has migrated closer to the

star and would thus be covered
by a large and deep ocean.

Kepler 62f is likely to have a
rocky composition and is only

40% larger than Earth, making
it the exoplanet closest to

the size of our planet
known in the habitable zone of

another star. Kepler 62e
orbits on the inner edge of the

habitable zone and is roughly
60% larger than Earth. Other

recent discoveries include
Kepler 438b, 442b and 440b, a

super-Earth.

The super-Earth exoplanet
GJ 1214b orbits its faint red

parent star. This is the first
super-Earth exoplanet to have

had its atmosphere analyzed.
It has a mass about six

times that of the Earth and
appears to be surrounded by an

atmosphere of steam
or thick clouds or haze.

Based on observations,
scientists believe that of the Sun

-like stars, some 22% have
an Earth-sized planet orbiting in

the habitable zone. Assuming
200 billion stars in the Milky

Way, that would be 11 billion
potentially habitable Earths,

rising to 40 billion worlds if
brown dwarfs are included.

Are any of these worlds
close to Earth? In fact, yes. The

closest star to our own is
the well-known Alpha Centauri

group, with the bright stars
Alpha and Beta Centauri, plus

the faint red star Proxima
Centauri, the closest star to

Earth.

Alpha Centauri b is known to
be orbited by an Earth-mass

planet, making it the closest
exoplanet to our solar system,

a meteor. It is located near 4.37
light-years away, almost within our reach.

Now in its fourth observing
campaign, the Kepler spacecraft

continues targeting 16,000
stars for exoplanets. It's

estimated that the onboard
fuel supply should last until at

least December 2017. So far,
Kepler has found an astounding

1,013 confirmed exoplanets
around 440 star systems.

The terrestrial telescopes
continue to do the heavy lifting

when it comes to verifying
possible planet candidates. The

Harpsk spectrographic
instrument Atlassia Chile is being

joined by the Next Generation
Transit Survey, or NGTS. It

will search for transiting
exoplanets with a focus on

discovering Neptune-sized
and smaller planets. NGTS is

designed to operate in a
robotic mode. It will continuously

monitor the brightness of
hundreds of thousands of

comparatively bright
stars in the southern skies.

ESO's Very Large Telescope
at Cerro Paranal in Chile,

composed of four individual
telescopes, will be improved

with the new Next Generation
of Adaptive Optic System called

Sphere. Other improved
technologies include the Gemini

Planet Imager at nearby Cerro
Pacion, now in operation, and

the SUBARU coronagraphic
extreme technology currently being

installed and tested.

New, bigger and more powerful
telescopes are in the pipeline as well.

The 30-meter telescope,
planned for Manuakea, Hawaii, will

have 492 small hexagon mirrors
arranged together to form the

primary mirror 30 meters across.

The GMT, or Giant Magellan
Telescope, will be built at the

Las Campañas Observatory
in Chile. It will consist of seven

8.4-meter mirrors arranged
together to make up the primary mirror.

Work on the mirrors is well underway.

The European Extremely Large
Telescope in the Atacama Desert

is due for completion in
2024. It will have a 39-meter

diameter mirror made up of
798 hexagonal mirrors and will be

the largest ever built. It
will enable scientists to study

the atmosphere of exoplanets more closely.

Space-based telescopes are
advancing as well. The Next

Generation Infrared Telescope,
James Webb, is nearing

completion as it goes
through rigorous testing.

A scheduled launch in October
2018 will put the 6.5-meter

telescope in orbit as a
replacement for the Hubble and

Spitzer telescopes.

Like unto the California Gold
Rush, there are planets out

there to be found, and the
race is on. TESS, the Transiting

Exoplanet Survey Satellite,
is scheduled to launch in 2017.

TESS will scan the entire sky
searching for exoplanets using

four state-of-the-art cameras.
It will be able to determine

the chemical compositions
of exoplanet atmospheres.

Also planned for a 2017
launch is KEOPS. Its function is

to characterize transiting
exoplanets orbiting bright host

stars. The satellite is a
small package, roughly 1.5

meters square, with a life
expectancy of five years.

The European Space Agency has
commenced a new program called

Cosmic Vision, set to run from
2015 to 2025, with PLATO, an

exoplanet hunter, expected
to be launched in 2024. Other

projects underway include
PEGAS, under development in

France, EXCEED, the Exoplanetary
Circumstaller Environments

and Disk Explorer, NASA,
and FINESSE, the Fast Infrared

Exoplanet Spectroscopy Survey
Explorer, due to launch 2019.

The Wide Field Infrared Survey
Telescope, WFIRST, is another

NASA observatory designed
to perform wide-field imaging for

the planet-hunting
community. It will be fitted with a

coronagraph instrument for
direct imaging of exoplanets and

debris disks.

Another NASA New Worlds
mission is the STARSHADE project,

scheduled for a 2019 launch.
It will physically block the

star's light with a parasol to allow
direct observation of exoplanets.

NASA is already thinking
about a future James Webb

replacement. Called the Advanced
Technology Large Aperture

Space Telescope, or ATLAST,
it will be 2,000 times more

light-sensitive than Hubble.
If all goes according to plan,

ATLAST could be launched
between 2025 and 2035.

With these new tools and
technologies, it is only a matter

of time before we are able
to detect Earth-like worlds

capable of supporting life.

Perhaps one day, even a
planet emitting radio or other

signals, indicative of a
sufficiently advanced intelligent

civilization. Finally answering
that great question, are we alone?