Exoplanets

The Planetary Society’s LightSail 2

Overview

On June 24th at 22.30 EST (4.30am BST on June 25th), The Planetary Society (TPS) launches it’s CubeSat space probe called LightSail 2 on board the mighty SpaceX Falcon Heavy rocket.  All going well, LightSail 2 will deploy a 32 square-metre Solar Sail, and commence it’s enigmatic mission to orbit the Earth propelled only by the momentum of sunlight, and steered by the Earth’s magnetic field.

If successful, LightSail 2 will use the momentum of the Sun’s light to move about the Earth while using orientation changes in relation to the Sun to tack into a higher Earth orbit, in a manner similar to how a sailing ship tacks through wind to traverse it.

The outcomes of LightSail 2 will herald a new era in innovative propulsion whose value has long been understood and which is now in our sights – that of using sailed spacecraft to beach the enormous distances to the planets, and even the nearest stars, potentially within our lifetime.

Crowd funded by The Planetary Society – the largest non-profit space interest organisation in the World – more than 40,000 people have made the LightSail 2 mission possible: a mission of our time, but one in the hearts and minds of those who founded The Planetary Society in 1980 when co-founders Carl Sagan and Louis Friedman then championed the idea, through to the present day where Planetary Society CEO Bill Nye and Director of Projects Bruce Betts now carry the baton toward a fully operational Light Sail space craft.

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Figure 1: An artists impression Light Sail 2 fully deployed in Earth orbit. Credit: The Planetary Society

Sunlight Propulsion – The Background

The Principle

Although light – a form of energy – has no mass, it does possess momentum. As a result, if a ray of light (or a photon of light – depending on how you regard it) impacts on any surface, it constitutes a momentum collision, where a momentum transfer occurs between the ray of light and the object it collides with, producing what is often termed radiation pressure that causes the object to move.

Although the momentum and resulting pressure from sunlight is tiny, it is not totally insignificant. The radiation pressure upon the palm of your hand facing the Sun is about equivalent to that of a grain of pepper landing gently upon it.

But such is the supply of sunlight, inexhaustible in the vacuum of space, that the affect of such a tiny pressure can build up over time. As just one example, each of the two 1970s Viking missions to Mars weighted in at a hefty 3.5 Tonnes each, yet had NASA not corrected for the radiation pressure from sunlight falling upon each spacecraft as it travelled to Mars, they would have missed the planet by 15,000 kilometres on arrival! Such a tiny pressure, experienced relentlessly in the vacuum of space, builds up to a significant factor in space flight.

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Figure 2: A diagrammatic representation of sunlight impacting upon a solar sail, causing it to move Credit: Physics dot org

History and Context

The idea that sunlight might bare a propulsive pressure is not new. Johannes Kepler, as far back as 1619, proposed a pressure associated with sunlight when attempting to explain why comet tails always point away from the Sun; and indeed Kepler is partly correct in this assessment, though he couldn’t have known that the Sun also emits high energy particles, and possesses a vast magnetic field that combine to produce a Solar Wind that also contributes significantly to comet tails being pushed outward from the Sun.

Jules Verne in his 1867 novel “From the Earth to the Moon” also hypothesised the use of  sunlight pressure as an alternative to the 900-foot canon used to fire his fictitious ship to the Moon. And indeed, at that same time the Scottish scientist James Maxwell actually produced the theoretical principles that revealed that light does indeed possess momentum; not long after experimentally verified at the turn of the 20th century by the scientists Ernst Fox Nichols and Gordon Ferrie Hull.

Light Sail Propulsion

As the new century unfolded and we began to understand more acutely the vastness of space, the realization of the necessity for new and innovative propulsion systems began to arise if we were to breach the distances to the planets and stars.

Of course since the dawn of the space era spaceflight has been dominated by chemical rockets: burn chemical propellants and they produce enormous quantities of gas at high pressure, released through a nozzle to create a thrust that propels the rocket upwards. From the earliest Chinese rockets over a thousand years ago to the great Saturn V that carried people to the Moon, all have been based on the principle of the chemical propellant rocket.

And while the extraordinary efficiency and reusability of SpaceX’s Falcon rockets will help revolutionize space enterprise in Earth orbit and beyond; and the gargantuan power of Boeing/NASA’s new Space Launch System (SLS) – one and a quarter times as powerful as Saturn V – will be capable of getting people to Mars and back, it has also been known from the outset that chemical rockets can only get us so far into space.

Even sending unmanned space probes to the outer planets of the Solar System using chemical rocket alone is an enormous challenge. The Voyagers to the Gas Giants, Cassini to Saturn and New Horizon to Pluto all needed extra propulsion from gravity assist manoeuvres as they passed the Gas Giant planets, to take years off their travel time.

We have known from the outset, that if we wish for long-term and sustainable engagement with the Solar System; and if we hope to reach the stars, then better propulsion systems are needed, a realisation perhaps more poignantly today as our ambitions towards deep space become ever clearer.

There is no shortage of contenders however: Ramjets, Nuclear Thermal Rockets, Fusion Propulsion and indeed Light Sails among others – all contend for future space propulsion.

While such contenders have remained on the drawing board, the idea of spacecraft using sails to harness the radiation pressure of light has garnered particular interest. Firstly, it does not require an extreme or currently unrealized energy source. And while the idea of propelling a sailed spacecraft by the minute pressure from light may seem at first impractical, means have been realized by which it might become a viable alternative to chemical rockets for long time-scale space missions to the planets and even potentially the nearest stars.

For example, using only sunlight, a solar sail measuring hundreds of metres along each side can generate enough thrust to navigate across the Solar System – endlessly. A solar sail spacecraft with an albeit enormous sail of dimensions 100km square and made of an extremely thin material could achieve extraordinary velocities – in the order of 4 million kilometres per hour – enabling such a craft to reach the our nearest stellar neighbour Alpha Centauri at 4 light years distance in approximately 1000 years. While sounding like a long time, when compared to the Voyagers propelled by chemical rockets and gravity assist and achieving velocities of 20,000 km/h and requiring 70,000 years to reach Alpha Centauri, then the benefit becomes clear.

An alternative to constructing such an enormous sail propelled by sunlight is to construct tiny space craft attached to sails of dimensions of perhaps 5 metre square, then propelled by powerful lasers on Earth or the Moon rather than by Sunlight. Such a system could in principle propel the spacecraft at a staggering 200 million kilometres per hour – one fifth of the speed of light – enabling it to reach the nearest stars in only 20 years or so. The Breakthrough Starshot program, currently underway, is investigating such a possibility, with an aspiration to realising a real mission to Proxima Centauri using light sails in only a few decades from now.

While using light sailed space craft would be impractical for short journeys to the Moon and near by planets, as Bill Nye says: “…[for] interstellar travel, really the only way to do it that anybody can think of right now is with solar sail[s]…”

Light Sail

And so it has been on this basis that alternative propulsion systems, and light sails in particular, have remained of keen interest over the years.

Carl Sagan himself championed the idea in the 1970s, famously showing a model of a light sail on the Johnny Carson Show. Subsequently he, along with Ann Druyan (author, co-writer of the original Cosmos series, principal writer and Executive Producer of Neil deGrasse Tyson’s Cosmos and spouse to Carl Sagan) and NASA Engineer and TPS Co-founder Louis Friedman all progressed the idea from the outset of the formation of The Planetary Society.

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Figure 3: Carl Sagan on The Johnny Carson Show, demonstrating a solar sail spacecraft.

And while it is currently unfeasible to construct 100km square sails, or propel sails in space via powerful lasers, it is completely possible to do an enormous amount of preliminary work with a light sail mission: construct light sails with innovative materials capable of being pushed by sunlight; learn how to launch and deploy such sails in space safely and securely; learn to control and steer lights sails deployed in space and develop the mechanisms to do so; and examine the theory of light sail propulsion through experimentation: in short, devise missions that take light sail propulsion off the drawing board and into space. Such preliminary but ground-breaking work is what has underpinned TPS Light Sail.

Cosmos-1 and Light Sail 1

TPS’s first light sail attempt in 2005 was called Cosmos 1: a light sail mission launched in collaboration with Ann Druyan’s company Cosmos Studios. The light sail was launched upon a Russian converted military rocket from a submarine in the Barents Sea. Unfortunately the rocket did not reach its intended orbit and Cosmos 1 could not be deployed.

A planned replacement – Cosmos 2 was subsequently replaced by a new approach using lower mass sail technology, announced in 2009 as LightSail 1. Using a NASA designed Nano-Sail D and deployed into Earth Orbit as a CubeSat in 2015, LightSail 1 successfully deployed it’s sail; albeit in an orbit too low to allow for the Sun’s photons to propel it.

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Figure 4: Artists impression of Cosmos 1 deployed in Earth orbit. Credit: The Planetary Society / Cosmos Studios

JAXA Ikaros

Indeed, LightSail 1 was not the first successful solar sail mission – that honour goes to the 2010 Japanese Aerospace Exploration Agency (JAXA) Ikaros mission – a light sail larger than LightSail 1 (14m along each side) that deployed successfully in interplanetary space close to the planet Venus where it established a 10 month orbit about the Sun, sending data back to Earth and providing valuable insights into attitude control of a light sail craft that has helped the development of TPS’s LightSail 2.

 
JAXA are fully committed to solar sail propulsion, and intend to use an enormous 250m x 250m sail to send a research probe to the Trojan Asteroids out near Jupiter in the early 2020’s, cementing solar sail propulsion as a viable means of travelling to the planets. Both JAXA and TPS maintain active communications and a sharing of expertise and knowhow to strengthen the future for light sail technology.

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Figure 5: JAXA Ikaros deployed in space near the planet Venus. Credit: JAXA

Lights Sail 2

And so for TPS the culmination of more than 40 years thought and effort in light sail technology is the Light Sail 2 mission, to launch at 22.30 EST on June 24th 2019.

Mission Objectives

So what are the objectives of LightSail 2? In short, to fully deploy a 32 square metre (5.6m x 5.6m) solar sail spacecraft at a high enough altitude orbit (720km above the Earth), free of Earth’s atmospheric drag so as to enable the Sun’s photons to move the spacecraft.

Once secure in orbit, LightSail 2 will continually reorienting it’s solar sails as it orbits the Earth so that it only captures sunlight when moving away from it, receiving a momentum push to raise its orbit and demonstrate solar sail propulsion at work.

And so the primary objective of LightSail 2 is to attempt to elevate it’s orbit from an initial 720km by about half a kilometre each day as it orbits the Earth. If this can be achieved, it will successfully demonstrate both the capability of sunlight to propel the spacecraft, and the ability to adjust its attitude with precision in order to enable its orbit to grow.

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Figure 6: LightSail 2 reorientation its sail so as to gain a push from the Sun’s light when pointing away from it, delivering energy to the spacecraft to raise it’s orbit. Credit: The Planetary Society

The People

While LightSail 2 is a TPS driven project, no fewer than 40,000 people world wide crowd funded a sizeable part of the $7 million total budget, making it a true citizen-science mission. True to TPS’s ethos of democratizing the exploration of space, LightSail 2 is a mission made possible by citizens across the planet passionate about space exploration. It is a truly international mission.

Under the TPS stewardship of CEO Bill Nye, Director of Projects Bruce Betts and Purdue University’s David Spencer, the funding raised have enabled TPS to employ necessary expert space systems design and testing companies Stellar Explorations Inc. and Ecliptic Enterprises Corp. Meanwhile, many LightSail 2 and associated ground station and software systems have also been designed and tested by students in Georgia Tech and Cal Poly.

With such talent on board from so many quarters, the systems of LightSail 2 have been designed and tested to extraordinary levels. As just one example, even if LightSail 2 looses contact with Earth, it has the ability to reboot its systems and initiate contact with Earth of its own accord – a very sophisticated capability for a mission costing about 1/20th as much as an equivalent NASA mission!

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Figure 7: TPS CEO Bill Nye demonstrates the Mylar Sail material

Spacecraft Characteristics

Perhaps the most intriguing aspect to LightSail 2 is it’s tiny CubeSat foot print. On launch, it measures no more than the size of a loaf of bread! Once deployed in space, 4 mini solar panels will open from its sides to power the on-board computers, communications systems, detectors and sail actuators.

The sails themselves are made of Mylar, and at just 4.5 microns thick (one tenth the width of a human hair) are light and reflective enough for sunlight alone to push them into a higher orbit.

Before deployment the entire spacecraft measures just 30 x 10 x 10 cm, while when fully open the sails span an area of 5.6 x 5.6 m or about 32 square metres; and an entire spacecraft weights just 5kg!

 

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Figures 8 and 9: Images showing LightSail 2 Solar Panel Deployed and Solar Sail Deployed. Credit: The Planetary Society

Mission timeline

The Falcon Heavy mission upon which LightSail 2 is attached is due for launch on June 24th at 22.30 EST (4.30am BST on June 25th). With a three hour launch window on that day, all being well LightSail 2 will be deployed into an orbit at 720km altitude (by comparison the ISS is at 408km).

LightSail 2 is a secondary payload attached to the Falcon Rocket in a small washing machine-sized spacecraft called a Prox-1. Once detached from the Falcon rocket and at a safe distance, Prox-1 will eject LightSail 2 for orbital insertion.

The orbit LightSail 2 initially settles into will be circular, and of low inclination meaning it will remain within about 24 degrees north and south of the Earth’s equator and unlikely therefore to be visible in skies from latitudes beyond 42 degrees north and south.

After about 5 days of testing, the sails will be deployed. Four Cobalt-alloy tape-like booms unroll from within the tiny spacecraft, and attached are the four wedge shapes Mylar sails which when fully deployed cover an area of 32 square metres.

As the spacecraft orbits the Earth, momentum wheels will enable LightSail 2 to change orientation with respect to the Sun: on the part of it’s orbit facing the Sun it will orientate the thin edge of its sails toward the Sun so as not to be affected by the Sun’s radiation pressure, while on the part of its orbit moving away from the Sun it will re-orientate itself by 90 degrees face on to the Sun, experiencing a slight radiation pressure push that will accelerate the spacecraft by a tiny amount of just 0.058 mm/s². While this sounds like a tiny acceleration, it is continuous, unlike with a chemical rocket whose acceleration ceases once it’s fuel is used up, and so in just one one month of constant sunlight, LightSail 2’s speed will increase by 549 kilometers per hour!

In this way it will gain energy and rise up to a higher orbit. Small mirrors on the edges of the sails will hopefully be tracked from the ground using laser-ranging, and if successful, will enable us to track and determine LightSail 2’s orbit with exquisite precision, gaining new insight into the workings of solar sails.

Alas LightSail 2 does not have full attitude control, and so as one side of its orbit increases (apogee) the other side of its orbit (perigee) will decrease, getting ever closer to the Earth on each successive orbit. And so it is expected that within about a year of operation LightSail 2’s perigee will be close enough to the Earth’s atmosphere to succumb to drag forces that will cause it to finally re-enter the atmosphere where it will burn up, ending the mission.

As of June 24th 2019 you can follow the mission daily from the online feeds presented below, and if you are a radio expert you can track it live via radio receiver – again details provided in list of resources below.

The Future – NASA, JAXA and Breakthrough Starshot

While the science and know-how gleaned from LightSail 2 will be invaluable to all interested in light sail spacecraft, The Planetary Society will likely not pursue further light sail missions. Upon completion, LightSail 2 will have fulfilled the dreams of Carl Sagan, Ann Druyan and Louis Friedman of a successful light sail mission.

As a non-profit organisation whose central remit is to enthuse and inspire citizens across the planet in space exploration, TPS will move onto other projects. Nevertheless, to ensure that the results of LightSail 2 help future light sail missions, TPS have been working through the entirety of LightSail 2 with a team from NASA toward conducting a follow on light sail mission called Near-Earth Asteroid Scout – a fully fledged solar sail mission to conduct reconnaissance of an asteroid, to be launched as one of 13 CubeSats on the first SLS mission. Meanwhile the planned solar sail mission of JAXA to the Trojan asteroids and Breakthrough Starshot’s aspiration so using powerful lasers to send no less than one thousand 4 metre squared space craft to Proxima Centauri all mean that the future for light sail propulsion is very bright, and here to stay.

Perhaps most poignant and gratifying of all is that, at a time of increased awareness of the necessity of environmental sustainability on out planet, it so happens that among the most innovative space missions being planned is for a sustainable model of propulsion, based purely on sunlight.

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Figure 10: NASA’s Near-Earth Asteroid Scout, based on a similar configuration to LightSail 2. Credit: NASA

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Figure 11: Breakthrough Starshot proposes sending no fewer than 1000 light sails, each almost identical in size to LightSail 2 but with their attached spacecraft only grams in weight, and to fire a 1 Giga Watt Laser array situated on the Earth’s surface to accelerate them by 100km/s/s for 10 minutes in order to achieve a velocity of 0.2 the speed of light. With 1000 light sails sent on the journey, the hope is that at least a few would reach Proxima Centauri after (more…)

Detection of Terrestrial Planet Candidate in a Temperate Orbit about Proxima Centauri

Summary
Proxima Centauri is the closest star to our Solar System. At 4.24 light years distant, it is the closest of a triple star system called the Alpha Centauri system comprising of two Sun-like stars at a distance of 4.38 light years, and Proxima Centauri, a red dwarf star residing about two trillion kilometres or a little under 0.2 light years closer to us, and in orbit of its two companions. In our night sky they are located in the southern hemisphere constellation of Centaurus.

Given the detection in recent years of thousands of planets around other stars, called exoplanets, there has been a major aspiration to detect a planet around any of the three stars of the Alpha Centauri system. To this end, the European Southern Observatory (ESO), the largest astronomical institution in the world, set a science (and science-outreach) project in motion in January 2016 called PaleRedDot – a specific search for the existence of a planet exhibiting potential Earth-like characteristics around Proxima Centauri; and on August 24th 2016 the PaleRedDot consortium of scientists from the UK, Spain, Chile, the US and other countries published findings in the journal Nature that strongly indicates the presence of an Earth-sized planet orbiting Proxima Centauri. PaleRedDot has achieved success – within just months of its initiation!

What is intriguing about this discovery is that the planet, currently designated Proxima B, is similar in size to the Earth at around 1.3 Earth masses, and also resides at a distance of 7.4 million kilometres from its cool red parent star within what is called its habitable zone – the region about the star that could allow any water on an orbiting planet to exist in liquid state. These characteristics make Proxima B a candidate terrestrial or candidate Earth-like planet.

While details of the planet’s make up and surface conditions are currently unknown and none of the new evidence suggests any actual terrestrial characteristics, such is the rapid development of telescopes and analysis techniques that it is likely only a matter of a years before we gain sound insight into many of Proxima B’s characteristics, including whether any of its potential for terrestrial-like behaviour has been realised.

And so this discovery marks the beginning of an intensive period of endeavour and discovery, heralds a new era in exoplanetary science given the proximity of the planet, that will surely lead to significant new insights into Proxima B in particular and planets and solar systems in general.

With highly innovative technical proposals emerging for long range robotic exploration of the outer Solar System and beyond, the discovery of Proxima B also offers a new and unprecedented planetary target at our closest stellar neighbour, and will act as a major sign post toward the stars in the coming decades. We have our first confirmed planetary exploration destination beyond our own Solar System!

This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image between the planet and Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface.

This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image between the planet and Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface. Credit: ESO/M. Kornmesser

Background

The Sun and Stars
Our Sun provides virtually all energy in our Solar System and Earth could not have given rise to or sustained life as we know it without its presence. At just one hundred and fifty million kilometres distance, the Sun is a very close neighbour indeed.

All of the stars in the night sky are also suns, some much larger and hotter than ours such as the brilliant blue-white star Rigel in the constellation of Orion; with many others smaller and redder in colour and shining more dimly than our Sun. Such dim stars are called red dwarfs.

By contrast our Sun is an in-between star – yellow in colour – and with a diameter of one million kilometres is unremarkable in size, mass and brightness when compared to the most massive stars like Rigel.

Of the 400 billion of stars in our Milk Way galaxy, the vast majority (75%) are red dwarfs, while just 4% are like our Sun. Still, that’s approximately 16 billion Sun-like stars in just our galaxy.

The distances between the stars are truly vast. While we measure the distances between the planets of our Solar System in tens or hundreds of million of kilometres, stellar distances are typically many trillions of kilometres. Such distances are truly gargantuan and so the light year has been adopted to indicate the distances between the stars, where one light year is the distance travelled by a ray of light in one year – 10 trillion kilometres (well, 9.6 trillion km).

Our Milky Way galaxy, containing upwards of 400 billion stars, is a flattened disk extending more than one hundred thousand light years, and with a central bulge more than twenty thousand light years in thickness.

Exoplanets
In the quest to fully understand our Milky Way galaxy and the Universe in general – and our place in the greater scheme – we have always wondered whether the far off stars are accompanied by planetary systems. But with such vast distances to contend with, evidence of planets around other stars, called exoplanets, has been beyond our detection capabilities until quite recently.

Over the past twenty years however, telescopes, detectors, computers and analytical techniques have improved so radically that we have been able to detect planets around many stars. And so today the detection of exoplanets is one of the most active fields of science, involving the World’s great telescopes such as the European Southern Observatory located in Chile, and space probes such as NASA’s Kepler Exoplanet Detector and the Hubble Space Telescope. As of the time of writing, approximately 3500 exoplanets have been confirmed, with new planets being discovered on a near daily basis.

Such has been the transformation of the detection and study of exoplanets that by now we have even achieved the first tentative census of planets in the entire Milky Way galaxy; and the numbers are staggering.

For example, we are confident that almost every star in the galaxy has at least one planet, so that makes for a minimum of four hundred billion planets, and the full count is likely to run into the trillions. It is estimated that there may be upwards of 30 billion Earth-sized planets in the Milky Way (note – ‘sized’ not ‘like’) and intriguingly we suspect that of the 16 billion Sun-like stars in the Milky Way, one in five of those has an Earth-sized planet residing in its habitable zone. That makes for about 3 billion stars like our Sun each with a planet the size of the Earth residing in its habitable zone, in just our Milky Way galaxy alone.

Exoplanet Detection
Detecting exoplanets is an extraordinarily challenging business even today. In most cases, our largest telescopes are still not powerful enough to actually see planets around other stars directly; and so we have had to develop sophisticated techniques to indirectly detect the existence of exoplanets.

Of the many techniques available, two stand out for now. The first is called the Transit Method. Here, we monitor a star’s brightness over a prolonged period of time, and if a planet happens to be orbiting the star along our line of sight, we should see the star temporarily dim in brightness as the planet passes in front of the star. The Kepler space probe has detected all of its exoplanets using this method, but as powerful as this techniques is, it only works if the alignment is right – and it’s only right for a few percent of all stars visible form Earth.

The second method is called the Doppler or Radial Velocity method. Here, we monitor changes in the spectrum or detailed colour of the star as it wobbles, where the wobble is caused by a nearby orbiting object such as planet. As the star wobbles toward us we see it more blue, and as it moves away from us we see it more red. The changes in colour are minute, but they are detectable, especially stellar wobbles caused by large Jupiter-sized planets orbiting their parent star.

Earth-like Planets and the search for Life in the Universe
Of course one of the fundamental quests for humanity is the two-pronged question of the origin of life on Earth and the cosmic abundance of life. With so many astounding advances in science, answers to those questions have remained frustratingly elusive.

But with the development of exoplanet detection we can begin to open new avenues of investigation, most especially in the search for planets like our own that might harbour life.

And so the search for both Earth-sized and Earth-like planets has become a hot pursuits in science today. While planets of Earth size may reside anywhere in a given solar system from the scorching inner regions to the frozen outskirts; Earth-like or potentially Earth-like suggests a planet of similar mass and make-up to Earth residing in a particular region of its solar system called the habitable zone – a relatively narrow zone around the star where liquid water may exist on the planet’s surface. Such a constraint renders the detection of such planets incredibly challenging, and as already indicated, so new are even our best detection techniques that the best we can hope for today is the detection of potential or candidate Earth-like planets, with actual confirmations of any present Earth-like characteristics being some years off. Nevertheless, significant progress is been made of late, and currently, of the 3,500 known exoplanets, about 600 are known to be Earth-sized, with approximately 40 of those being candidate Earth-like planets.

So while the search for candidate Earth-like planets is making ground-breaking progress, detailed examinations of the currently known potential Earth-liked planets remains firmly beyond our capabilities for now.

Red Dwarfs
But lets not forget about those 75%, or upwards of 300 billion red dwarf stars in our galaxy. While they are smaller, cooler and dimmer than the Sun, they are enormous in number, and each can of course possess a habitable zone, however close to the parent star that might be. And so of late there has been renewed interest in investigating red dwarfs too in the search for Earth-like planets. And while as indicated we have already detected about 40 potentially Earth-like planets, including red dwarfs in our search opens up one significant avenue of investigation – our closest stellar neighbour Proxima Centauri.

Alpha Centauri System and Proxima Centauri
The three closest stars to us – the gravitationally bound Alpha Centauri system – resides at a modest distance (on stellar scales) of just over 4 light years. While that’s still 40 trillion kilometres and currently too far to reach by space probe (the New Horizons space probe which took 10 years to travel to Pluto would take over 60,000 years to reach Alpha Centauri); nevertheless the three stars of the Alpha Centauri system are close enough that, if they contain exoplanets, we could determine the characteristics of those planets using our best current and near-future planned Earth-based observation techniques; and that would be a major break-through in exoplanet studies.

Of the three stars in the system, Alpha Centauri A and B are both very similar in age, size, mass and colour to our Sun. Nevertheless, given that they orbit each other at a distance equal to the distance of our Sun to the planet Saturn, Alpha Centauri A and B constitute a wholly different kind of system to our own. We can already tell there are no Jupiter-sized planets orbiting those stars for example, and while tentative evidence exists for an Earth-sized planet orbiting Alpha Centauri B, there is much debate on the interpretation of the available evidence.

Proxima Centauri on the other hand orbits both Alpha Centauri A and B at a distance of about 2 trillion kilometres, and indeed currently resides about two trillion kilometres closer to us than its stellar companions. It is a lot smaller than its far off stellar companions – a little larger than the planet Jupiter, but having accumulated more than 125 times the mass of Jupiter during its formation it was too massive to remain as a planet and became a red dwarf star, albeit with a luminosity far fainter than its companions or our Sun (about 0.0017 the Sun’s luminosity).

This picture combines a view of the southern skies over the ESO 3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lower-right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope. Proxima Centauri is the closest star to the Solar System and is orbited by the planet Proxima b, which was discovered using the HARPS instrument on the ESO 3.6-metre telescope.

This picture combines a view of the southern skies over the ESO 3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lower-right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope. Proxima Centauri is the closest star to the Solar System and is orbited by the planet Proxima b, which was discovered using the HARPS instrument on the ESO 3.6-metre telescope. Credit: Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani

PaleRedDot
With a pressing desire to detect any planet close by and a potential Earth-like planet in particular, the European Southern Observatory decided in January 2016 to initiate a project to see if we could detect any Earth-sized planets orbiting Proxima Centauri. The project was given the apt title PaleRedDot, a play on the famous “Pale Blue Dot” phrase coined by Carl Sagan upon seeing an image of the Earth as a tiny pale blue dot in a Voyager 1 photograph taken in 1990.

PaleRedDot set itself the challenging task of identifying whether there are any small planets orbiting Proxima Centauri using the Doppler method. This is challenging because the Doppler method is best suited to detecting Jupiter sized planets rather than Earth-sized planets which only causes a minute wobble of their parent star. But given the closeness of Proxima Centauri, it was deemed worth the attempt.

And so the ESO consortium, lead by astronomer Guillem Andlada-Escude of Queen Mary University UK, initiated their search in January 2016, accompanied with a high profile science-outreach campaign allowing anyone to look in online at the project and see its progress. And to the amazement of all, the data gathered in just ninety days from January to March 2016 was sufficient to confidently identify the existence of at least one planet orbiting Proxima Centauri.

Pale Red Dot was an international search for an Earth-like exoplanet around the closest star to us, Proxima Centauri. It used HARPS, attached to ESO’s 3.6-metre telescope at La Silla Observatory, as well as other telescopes around the world.  It was one of the few outreach campaigns allowing the general public to witness the scientific process of data acquisition in modern observatories. The public could see how teams of astronomers with different specialities work together to collect, analyse and interpret data, which ultimately confirmed the presence of an Earth-like planet orbiting our nearest neighbour. The outreach campaign consisted of blog posts and social media updates on the Pale Red Dot Twitter account and using the hashtag #PaleRedDot. For more information visit the Pale Red Dot website: http://www.palereddot.org

Pale Red Dot was an international search for an Earth-like exoplanet around the closest star to us, Proxima Centauri. It used HARPS, attached to ESO’s 3.6-metre telescope at La Silla Observatory, as well as other telescopes around the world. It was one of the few outreach campaigns allowing the general public to witness the scientific process of data acquisition in modern observatories. The public could see how teams of astronomers with different specialities work together to collect, analyse and interpret data, which ultimately confirmed the presence of an Earth-like planet orbiting our nearest neighbour. The outreach campaign consisted of blog posts and social media updates on the Pale Red Dot Twitter account and using the hashtag #PaleRedDot. For more information visit the Pale Red Dot website: http://www.palereddot.org Credit: ESO/Pale Red Dot

Proxima B
Most extraordinary of all is that the planet, designated Proxima B, has been found to have a mass at least 1.3 Earth masses, making it likely Earth-sized; while its orbit about Proxima Centauri is a close orbit of just 7.4 million kilometres distance. Given the star’s cool atmospheric temperature of 3000K, this sets Proxima B within the star’s habitable zone, making it a potential or candidate Earth-like planet.

While such a close orbit may sound somewhat bizarre, it is worth remembering that Proxima Centauri is just 1.3 times the diameter of the planet Jupiter; where Jupiter’s outermost Galilean moon Callisto (itself about the size of the planet Mercury) orbits Jupiter at a distance of just two million kilometres. So Proxima B’s orbit of 7.4 million kilometres distance may be small by Earth-Sun standards, but for the Proxima Centauri system, it is just fine.

This PaleRedDot project has been an overwhelming success, and its discovery of a candidate Earth-like planet orbiting our closest stellar is truly significant.

This infographic compares the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. Proxima Centauri is smaller and cooler than the Sun and the planet orbits much closer to its star than Mercury. As a result it lies well within the habitable zone, where liquid water can exist on the planet’s surface.

This infographic compares the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. Proxima Centauri is smaller and cooler than the Sun and the planet orbits much closer to its star than Mercury. As a result it lies well within the habitable zone, where liquid water can exist on the planet’s surface. Credit:ESO/M. Kornmesser/G. Coleman

This plot shows how the motion of Proxima Centauri towards and away from Earth is changing with time over the first half of 2016. Sometimes Proxima Centauri is approaching Earth at about 5 kilometres per hour — normal human walking pace — and at times receding at the same speed. This regular pattern of changing radial velocities repeats with a period of 11.2 days. Careful analysis of the resulting tiny Doppler shifts showed that they indicated the presence of a planet with a mass at least 1.3 times that of the Earth, orbiting about 7 million kilometres from Proxima Centauri — only 5% of the Earth-Sun distance.

This plot shows how the motion of Proxima Centauri towards and away from Earth is changing with time over the first half of 2016. Sometimes Proxima Centauri is approaching Earth at about 5 kilometres per hour — normal human walking pace — and at times receding at the same speed. This regular pattern of changing radial velocities repeats with a period of 11.2 days. Careful analysis of the resulting tiny Doppler shifts showed that they indicated the presence of a planet with a mass at least 1.3 times that of the Earth, orbiting about 7 million kilometres from Proxima Centauri — only 5% of the Earth-Sun distance. Credit: ESO/G. Anglada-Escudé

Proxima B and Habitability
While Proxima B resides in the habitable zone of Proxima Centauri, it must be made clear that the evidence accumulated to date provide absolutely no insight into the characteristics of Proxima B, other than that it’s a rocky planet at least 1.3 time the mass of the Earth residing in its parent star’s habitable zone. No information is currently forthcoming on the material make-up of its surface, whether is has an atmosphere, volatile systems including water, or on its actual habitability.

Indeed, there are some issues mitigating against clemency and stability that we are already aware of, such as that Proxima B’s rotation is likely tidally locked to its orbit (like the Moon which only shows one face to Earth at all times), and hence the dark side of Proxima B never receiving any direct sun light at all; as well as other issues associated with Proxima Centauri itself such as its very powerful magnetic field, the fact that it’s a flare star and associated disproportionately strong X-Ray emissions.

None of these can be claimed to be fatal to life supporting systems on Proxima B however – indeed the fact that it resides close to such an energetic star makes for a potentially very interesting environment because it will likely be a dynamic world in non-extreme ways (on a cosmic scale), and likely not dormant as with many of our Solar System’s large moons, for example.

Scientific Relevance
And so the discovery of a planet orbiting out next door neighbour will provide for opportunities to investigate a planet orbiting a different kind of star to our sun in unprecedented detail. When techniques improve, likely within just years, we will determine it’s material make up, atmospheric and other volatile material systems, interaction with parent star, presence of other planets and moons and other dynamisms on the planet and within its system.

Already the available data tentatively suggests a second planet in the system with an orbit somewhere between 80 and 600 days, though it must be restated that the evidence is tentative.

At a minimum we can be optimistic that Proxima B will be a very interesting and dynamic world, and if it happens to involve liquid water and other planetary volatile material systems, it may turn out to be of fundamental importance to our quest for origins and the cosmic abundance of life.

Sociological Relevance
By all measures, the importance of the discovery of a candidate Earth-like planet orbiting our closest stellar neighbour cannot be overstated.

As just described, if it turns out that Proxima B is Earth-like, it will provide significant new insights into the age old questions of origins and the cosmic abundance of life.

And in the context of our push to the far outer reaches of the Solar System with New Horizons to Pluto and into the Kuiper Belt, and the development of NASA’s Space Launch System (SLS) in 2018 which could deliver spacecraft to Pluto in just 3 years and will likely enable deep space missions to the Kuiper belt beyond Pluto; the discovery of Proxima B offers the coming generations of space explorers a major new destination to aim for.

It may seem far fetched to us today to contemplate sending a space probe there, but it is likely that in the next few decades new mechanisms to send small space craft to the nearest stars will begin to emerge. The discovery of Proxima B will contribute fundamentally to that push, because it offers a destination to aim for. What has been until now a notion of travelling to unknown destinations among the stars becomes a tangible mission to reach a specific planet in the Proxima Centauri system. There cannot be a closer target beyond our Solar system, and such tangibility will likely precipitate into a specific and targeted mission over the coming decades. We can be confident that all involved today in spawning projects to reach the nearest stars in the coming decades will be transfixed by this discovery.

As just two examples, currently in the US both NASA and DARPA are considering interstellar travel within the next 100 years; while Russian Entrepreneur Yuri Milner has donated no less than 100 million dollars to develop tiny space probe technology through an initiative called Breakthrough Stardust that could enable tiny robotic probes to travel to the Alpha Centauri or Proxima Centauri in just 20 years.

In a nutshell, we now have a destination planet orbiting our next door neighbour; and that will surely spur on innovation for the coming decades in space exploration.

Future Work
As ground-breaking as the discovery of Proxima B is, it represents only the very beginning in a new era of both exoplanetary studies and of interstellar space exploration. We could not ask for a better scenario in the study of candidate Earth-like planets; while the presence of this planet sets space exploration onto a whole new path beyond the Solar System.

And so many new astronomical studies of Proxima Centauri, and indeed of all our nearest stellar neighbours, are already ramping up. PaleRedDot will now aim at improving our understanding of the Proxima Centauri system and Proxima B, and will also continue into the future to study many near by stars.

More broadly, exciting new missions about to get under way such as the next generation space telescope (called the James Webb Space Telescope JWST), the new TESS Exoplanet Finder both due to launch in 2018, as well as the development of ESO EELT – a gigantic telescope to see first light in 2024, among many other projects; all mean that Proxima B will come under intense scrutiny in the next few years, and will surely give up its secrets as to its characteristics and whether it is, or is not, Earth-like.

The discovery of Proxima B heralds a new era in exoplanet studies and space exploration, and is likely to become a very important, and very familiar world to us in the coming decades.

This artist’s impression shows a view of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image to the upper-right of Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface.

This artist’s impression shows a view of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image to the upper-right of Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface. Credit: ESO/José Francisco ( http://josefrancisco.org )

Acknowledgment: thanks to Will Goodbody, Science and Technology Correspondent, RTE, for laying the groundwork for this blog.

Follow Proxima B, PaleRedDot and Exoplanet developments:

Pale Red Dot:
Web: https://palereddot.org
Twitter: @Pale_red_dot #PaleRedDot
Facebook http://www.facebook.com/PaleRedDot

European Southern Observatory
News https://www.eso.org/public/news/eso1629/
ESO Press Conference:
https://eso.adobeconnect.com/_a848728127/p3l3qqhq6un/?launcher=false&fcsContent=true&pbMode=normal

Research Paper in Nature:

Click to access eso1629a.pdf

Exoplanets:
http://exoplanets.org

Breakthrough Initiatives
http://www.breakthroughinitiatives.org

NASA / DARPA One Hundred Year Starship
http://100yss.org

JWST
http://www.jwst.nasa.gov

TESS
https://tess.gsfc.nasa.gov/index.html

EELT
https://www.eso.org/sci/facilities/eelt/

GAIA
http://sci.esa.int/gaia/