The Pale Red Dot Campaign

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Alpha and Beta Centauri are the bright stars; Proxima Centauri is the small, faint one circles in red.
Alpha Centauri A and B are the bright stars; Proxima Centauri, a red dwarf star, is the small, faint one circled in red. (NASA, Julia Figliotti)

Astronomers have been trying for decades to find a planet orbiting Proxima Centauri, the star closest to our sun and so a natural and tempting target.  Claims of an exoplanet discovery have been made before, but so far none have held up.

Now, in a novel and very public way, a group of European astronomers have initiated a focused effort to change all that with their Pale Red Dot Campaign.  Based at the La Silla Observatory in Chile, and supported by  networks of smaller telescopes around the world, they will over the next three months observe Proxima and its environs and then will spend many more months analayzing all that they find.  And in an effort to raise both knowledge and excitement, the team will tell the world what they’re doing and finding over Twitter, Facebook, blogs and other social and traditional media of all kind.

“We have reason to be hopeful about finding a planet, but we really don’t know what will happen,” said Guillem Anglada-Escudé  of Queen Mary University, London, one of the campaign organizers.  “People will have an opportunity to learn how astronomers do their work finding exoplanets, and they’ll be able to follow our progress.  If we succeed, that would be wonderful and important.  And if no planet is detected, that’s very important too.”

The Pale Blue Dot, as photographed by Voyager 1 (NASA)
The Pale Blue Dot, as photographed by Voyager 1 (NASA)

The name of the campaign is, of course, a reference to the iconic “Pale Blue Dot” image of Earth taken by the Voyager 1 spacecraft in 1990, when it was well beyond Pluto.  The image came to symbolize our tiny but precious place in the galaxy and universe.

But rather than potentially finding a pale blue dot, any planet orbiting the red dwarf star Proxima Centauri would reflect the reddish light of the the star, which lies some 4.2 light years away from our solar system.  Proxima — as well as 20 of the 30 stars in our closest  neighborhood — is reddish because it is considerably smaller and less luminous than a star like our sun.

Anglada-Escudé said he is cautiously optimistic about finding a planet because of earlier Proxima observations that he and colleagues made at the same observatory.  That data, he said, suggested the presence of a planet 1.2 to 1.5 times the size of Earth, within the habitable zone of the star.

“We did not and are not making claims in terms of having discovered a planet,”  he said.  “We’re saying that we detected signals that could mean there is a planet.  This is why we’ve planned this campaign — to see if the signal is telling us something real.”  He described the campaign as a “partnership between scientists involved in the observations and European Southern Observatory.”

Even without a previous signal, it’s a reasonable bet that Proxima does have at least one planet orbiting it.  Based on the results of the Kepler Space Telescope survey in particular, there is a consensus of sorts in the astronomy community that on average, every star has at least one planet circling it.

Alpha Centauri A and Alpha Centauri B are a binary pair, while Proxima Centauri is far away but is xxx
Alpha Centauri A, Alpha Centauri B and Proxima Centauri make up a three-star system, although Proxima Centauri is a distant .2 lightyears away rom the other two.  (Ian Morrison)

Paul Butler, a pioneer in planet hunting at the Carnegie Institution of Washington who has done extensive observing of Proxima with his team at Las Campanas Observatory in Chile, will be providing data to the Pale Red Dot campaign.  Proxima search results from the ESO’s Very Large Telescope at Paranal, Chile, will also be provided to campaign.

Butler said that in some ways Proxima “is the most exciting star in the sky.  It’s the very nearest star and so the discovery of a planet there would be huge – front page of the paper around the world.”

What’s more, he said, such a discovery could be enormously helpful in motivating Congress and taxpayers to spend the money needed for what is considered the holy grail of planet hunting — building a space-based exoplanet observatory that could directly image exoplanets.  “We have to give people a clear reasons to spend all that money and finding a potentially habitable planet around Proxima, that would be it.”

 Hubble Space Telescope image is our closest stellar neighbour: Proxima Centauri, just over four light-years from Earth. Although it looks bright through the eye of Hubble, Proxima Centauri -- with only about one eight the mass of our sun -- is not visible to the naked eye.Shining brightly in this Hubble image is our closest stellar neighbour: Proxima Centauri. Proxima Centauri lies in the constellation of Centaurus (The Centaur), just over four light-years from Earth. Although it looks bright through the eye of Hubble, as you might expect from the nearest star to the Solar System, Proxima Centauri is not visible to the naked eye. Its average luminosity is very low, and it is quite small compared to other stars, at only about an eighth of the mass of the Sun. However, on occasion, its brightness increases. Proxima is what is known as a “flare star”, meaning that convection processes within the star’s body make it prone to random and dramatic changes in brightness. The convection processes not only trigger brilliant bursts of starlight but, combined with other factors, mean that Proxima Centauri is in for a very long life. Astronomers predict that this star will remain middle-aged — or a “main sequence” star in astronomical terms — for another four trillion years, some 300 times the age of the current Universe. These observations were taken using Hubble’s Wide Field and Planetary Camera 2 (WFPC2). Proxima Centauri is actually part of a triple star system — its two companions, Alpha Centauri A and B, lie out of frame. Although by cosmic standards it is a close neighbour, Proxima Centauri remains a point-like object even using Hubble’s eagle-eyed vision, hinting at the vast scale of the Universe around us.
A Hubble Space Telescope image of Proxima Centauri, just over four light-years from Earth. Proxima Centauri — with only about one eight the mass of our sun — is not visible to the naked eye. Its average luminosity is very low but, on occasion, its brightness increases. Proxima is what is known as a “flare star” — where convection processes within the star’s make it prone to random and dramatic changes in brightness. (NASA)

Proxima and the other Alpha Centauri stars are also an especially appealing target because they have loomed so large in science fiction.  From Robert Heinlein’s “Ophans of the Sky” stories of crews traveling to Proxima to Isaac Asimov’s “Foundation and Earth ” set around Alpha Centauri and more recently to the James Cameron’s movie “Avatar,” also set in the Centauri neighborhood, these closer-by have been a frequent and logical destination.

While Alpha Centauri B has gotten much scientific attention in recent years with a reported but still unconfirmed and now often dismissed planet candidate, Proxima Centauri has been the object of much observation, too, and that has begun to define what kinds of planets might and might not be present.

So far, the work of Butler’s team has not found any particularly promising signs of a planetary-caused Proxima wobble.  But he said nothing established so far about Proxima rules out the presence of a small planet relatively close to the sun — the very time-consuming observations needed to potentially detect that size planet just haven’t been done.

Similarly, the Very Large Telescope results ruled out the presence of Saturn-size planets with many-year orbits and Neptune-size planets with orbits less than about 40 day, and no planets more than 6 to 10 Earths in the habitable zone.  This is actually promising news, since the absence of larger planets in the habitable zone leaves the field open for smaller ones.

Two other teams are now focused on Proxima as well.  One is led by David Kipping of Columbia University  using the Canadian Microvariability & Oscillations of STars space telescope (MOST) to search for transits.  The other is led by Kailash Sahu of the Space Science Telescope Institute in Baltimore, using the Hubble Space Telescope for microlensing of the star. The stars are aligned for the microlensing event this month.

A ring of telescopes at ESO's La Silla observatory. La Silla, in  the  southern part of the Atacama desert, 600 km north of  Santiago de  Chile,  was ESO's first observation site. The telescopes are 2400 metres  above  sea level, providing excellent observing conditions. ESO  operates the 3.6-m telescope, the  New Technology Telescope (NTT), and   the 2.2-m Max-Planck-ESO telescope  at La Silla. La Silla also hosts  national telescopes, such as the 1.2-m  Swiss  Telescope and the 1.5-m  Danish Telescope.
A ring of telescopes at ESO’s La Silla observatory. La Silla, in the southern part of the Atacama desert, 600 km north of Santiago de Chile, was ESO’s first observation site. The telescopes are 2400 metres above sea level, providing excellent observing conditions. ESO operates the 3.6-m telescope, the New Technology Telescope (NTT), and the 2.2-m Max-Planck-ESO telescope at La Silla. La Silla also hosts national telescopes, such as the 1.2-m Swiss Telescope and the 1.5-m Danish Telescope. (ESO)

The Pale Red Dot observing began last week and will run for two and a half month using the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph at the European Southern Observatory (ESO) telescope at La Silla, Chile. The observations — like those made at the Magellan and at Paranal — look for tiny wobbles in the star’s motion created by the gravitational pull of an orbiting planet. (More on how the radial velocity method works, as well as other connections to and details about the campaign can be found at:  https://palereddot.org/introduction/)

The campaign is the beneficiary of a substantial amount of HARPS observing time — 25 minutes of observing for 60 nights in a row — which is essential to confidently detect the presence of a small, Earth-sized planet.

Other robotic telescopes — including the Burst Optical Observer and Transient Exploring System,  the Las Cumbres Observatory Global Telescope Network and the Astrograph for the Southern Hemisphere II — will participate.  The role of these automated telescopes is to measure the brightness of Proxima each night, a backup that will help astronomers determine whether the wobbles of the star detected via radial velocity are the tug of an orbiting planet or activity on the surface of the star. Anglada-Escudé said that after a full analysis, the findings will offered to a peer-reviewed journal and published.

While the goal of the campaign is definitely to detect a planet orbiting our closest stellar neighbor, it is also very consciously a public outreach effort for astronomy and exoplanets.  Everything about the campaign will be made public, and often immediately via Twitter and other social media.  It will provide a window, said Anglada-Escudé, into how planet-hunting astronomy works.

Guillem Anglada-Escude
Guillem Anglada-Escudé is leading the Pale Red Dot campaign.

“We think this to be a good way to explain things that are not obvious to the public, to show them that looking for planets is not always excitement and ‘eurekas.’   We’ll show life at the observatory, how our observations are made, what happens as we analyze the data.  And if in the end we don’t find evidence of a planet, we will have shown how we search for such tiny objects so far away, and do it with a pretty amazing precision.”

Involving the public so early and often definitely brings risks, since the campaign could certainly come up empty-handed.  But in terms of real-life planet hunting, that result is hardly unusual.  An awful lot of planet-hunting campaigns end without a detection.

When red dwarf stars, also called M dwarfs, are found with orbiting planets, they tend to be much closer in than with more massive stars, and their habitable zones are also much more narrow.  Initially, red dwarfs were not considered good candidates for habitable planets because they are so relatively small — between 50 to 5 percent the mass of our sun.  Any planets orbiting close to a red dwarf would likely be tidally locked as well, with only one side ever facing the sun.  The pull of the host star causes the locking.

These issues and more earlier led scientists to dismiss red dwarf exoplanets as unlikely to be habitable. That unpromising view has changed with the creation of models for tidally locked planets that could be habitable, and with the discovery of many exoplanets orbiting around the red dwarfs.  These small suns actually  constitute more than 70 percent of the stars in the sky, although very few of the ones you can see without a telescope.

So the time seems ripe for a substantial exoplanet campaign at Proxima — one that just might find a planet and that certainly has a lot to teach the public.

Sites where you can follow the campaign:
Twitter: @Pale_red_dot #palereddot
Facebook page:  ‘Pale Red Dot’

 Artist rendering of a cold desert on a planet orbiting Proxima Centauri. (Vladimir Romanyuk, Space Engine)
Artist rendering of a cold desert on a planet orbiting Proxima Centauri. (Vladimir Romanyuk, Space Engine)
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Counting Our Countless Worlds

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The Milky Way has several hundred billion stars, and many scientists are now convinced it has even more planets and moons. (NASA)
The Milky Way is home to several hundred billion stars, and many scientists are now convinced it has even more planets and moons. (NASA)

Imagine counting all the people who have ever lived on Earth, well over 100 billion of them.

Then imagine counting all the planets now orbiting stars in our Milky Way galaxy , and in particular the ones that are roughly speaking Earth-sized. Not so big that the planet turns into a gas giant, and not so small that it has trouble holding onto an atmosphere.

In the wake of the explosion of discoveries about distant planets and their suns in the last two decades, we can fairly conclude that one number is substantially larger than the other.

Yes, there are many, many billions more planets in our one galaxy than people who have set foot on Earth in all human history. And yes, there are expected to be more planets in distant habitable zones as there are people alive today, a number upwards of 7 billion.

This is for sure a comparison of apples and oranges. But it not only gives a sense of just how commonplace planets are in our galaxy (and no doubt beyond), but also that the population of potentially habitable planets is enormous, too.   “Many Worlds,” indeed.

The populations of exoplanets identified so far, plotted according to the radius of the planet and how many days it takes to orbit. The circles in yellow represent planets found by Kepler, light blue by using ground-based radial velocity, and pink for transiting planets not found by Kepler, and green, purple and red other ground-based methods. (NASA Ames Research Center)
The populations of exoplanets identified so far, plotted according to the radius of the planet and how many days it takes to orbit. The circles in yellow represent planets found by Kepler, light blue by using ground-based radial velocity, and pink for transiting planets not found by Kepler, and green, purple and red other ground-based methods. (NASA Ames Research Center)

It was Ruslan Belikov, an astrophysicist at NASA’s Ames Research Center in Silicon Valley who provided this sense of scale.  The numbers are of great importance to him because he (and others) will be making recommendations about future NASA exoplanet-finding and characterization missions based on the most precise population numbers that NASA and the exoplanet community can provide.

Natalie Batalha, Mission Scientist for the Kepler Space Telescope mission and the person responsible for assessing the planet population out there, sliced it another way. When I asked her if her team and others now expect each star to have a planet orbiting it, she replied: “At least one.”

Kepler-186f was the first rocky planet to be found within the habitable zone -- the region around the host star where the temperature is right for liquid water. This planet is also very close in size to Earth. (NASA Ames/SETI Institute/JPL-Caltech)
Kepler-186f was the first rocky planet to be found within the habitable zone — the region around the host star where the temperature is right for liquid water. This planet is also very close in size to Earth. (NASA Ames/SETI Institute/JPL-Caltech)

I caught up with Belikov, Batalha and several dozen others intimately involved in cataloguing the vast menagerie of exoplanets at a “Hack Event” earlier this month at Ames. The goal of the three-day gathering was to find ways to improve the already high level of reliability and completeness regarding planets identified by Kepler.

It also provided an opportunity to learn more about how, exactly, these scientists can be so confident about the very large numbers of exoplanets and habitable zone exoplanets they describe. After all, the total number of confirmed exoplanets is a bit under 2,000 – a majority found by Kepler but hundreds of others by pioneering astronomers using ground-based telescopes and very different techniques. Kepler has another 3,000 planet candidates that scientists are in the process of analyzing and most likely confirming, but still. Four thousand is minuscule compared with two hundred billion.

Not everyone completely agrees that we’re ready to estimate such large numbers of exoplanets—suggesting that we need more data before making such important estimates — but the community consensus is that their extrapolations from current data are solid and scientific. And here is why:

The Kepler telescope looks out at a very small portion of the sky with a limited number of stars – about 190,000 of them during its four year survey. And it identifies planets based on the tiny dimming of stars when an object (almost always a planet) crosses between the star and the telescope.

The Kepler telescope looked constantly for four years at almost 200,000 stars in the Cygnus constellation. (Carter Roberts)
The Kepler telescope looked constantly for four years at almost 200,000 stars in the Cygnus & Lyra constellations.  Its lens is always open, by design. (Carter Roberts)

By identifying those 4,000-plus confirmed and candidate planets over four years, Kepler infers the existence of many, many more. As Batalha explained, a transit of the planet is only observable when the orbit is aligned with the telescope, and the probability of that alignment is very small. Kepler scientists refer to this as a “bias” in their observations, and it is one that can be quantified. For example, the probability that an Earth-Sun twin will be aligned in a transiting geometry is just 0.5%. For every one that Kepler detects, there are 200 others that didn’t transit simply because of the orientation of their orbits.

Then there’s the question of faintness and reliability. Kepler is looking out at stars hundreds, sometimes thousands of light years away.  The more distant a star, the fainter it is and the more difficult it is to gather measurements of –and especially dips in — brightness. When it comes to potentially habitable, Earth-sized planets, Batalha said that only 10,000 to 15,000 of the stars observed are bright enough for planets to be detectable even if they do transit the disk of their host star.

Here’s why: Detecting an Earth-sized planet would be roughly equivalent to capturing the image of a gnat as it crosses a car headlight shining one mile away. For a Jupiter-size planet, the bug would grow to only the size of a large beetle.

Add this bias to the earlier one, and you can see how the numbers swell so quickly. And since Kepler’s mission has been to provide a survey of planets in one small region – and not a census – this kind of statistical extrapolation is precisely what the mission is supposed to do.

There are numerous other detecting challenges posed by the dynamics of exoplanets, stars and the great distances. But then there are also innumerable challenges associated with the workings of the 95 megapixel CCD array that is collecting light for Kepler.   “Sensitivity dropouts” caused by those cosmic rays, horizontal “rolling bands” on the CCDs caused by temperature changes in the electronics, “optical ghosts” from binary stars that create false signals of transits on nearby stars — they are some of the many instrument artifacts that can be mistaken as a drop in light coming from a planet. Kepler’s data processing pipeline, much of which has been transferred over to the NASA Ames supercomputer, has the job of sorting all this out.

 

After the CCDs on the Kepler telescope record the light from stars in its viewing field, the data is sent back to Earth and goes through numerous steps before possibly delivering a “Kepler object of interest,” and possibly a planet candidate. Pleiades is the Ames supercomputer. (NASA Ames)
After the CCDs on the Kepler telescope record the light from stars in its viewing field, the data is sent back to Earth and goes through numerous steps before possibly delivering a “Kepler object of interest,” and possibly a planet candidate. Pleiades is the Ames supercomputer. (NASA Ames)

Adding to the challenge, said Jon Jenkins, a Kepler co-investigator at Ames and the science lead for the pipeline development, is that the stars viewed by Kepler turned out to be themselves “noisier” than expected. Stars naturally vary in their overall brightness, and the data processing pipeline had to be upgraded to account for that changeability.  But that stellar noise has played a key role in keeping Kepler from seeing some of the small planet transits that the team hoped to detect.

What the Hack event and other parallel efforts are doing is finding ways to, as Jenkins put it, “dig into the noise…to move towards the hairy edge of what our data can show.” The final goal: “To come up with the newest, best washer we can to clean the data and come out with an improved catalog of sparkling planets.”

All the data that will come from the primary Kepler mission, which came to a halt in the summer of 2013, has been collected and analyzed already on a first round. But now the entire pipeline of data is going to be reprocessed with its many improvements so the researchers can dig deeper into data trove. Batalha said they hope to find planets – especially Earth-sized planets – this way.

One of the key techniques to measure the performance of Kepler’s analysis pipeline is to inject fake transit signals into the data and see if it picks up their presence. As Batalha explained, this provides another way to gauge the biases in the system, its efficiency at detecting the planets that it could and should see. “If we inject 100 fake things into the pipeline and find 90 of them, that’s means we’re 90 percent complete.” She said the number would then be worked into the calculations of how many planets are out there, and how many of certain sizes will be caught and missed.

Natalie Batalha is the Chief Scientist for the Kepler mission, while announcing the discovery of Kepler’s first rocky planet, Kepler-10b, in January 2011.

So the Hack Event, which brought together astrophysicists, planetary scientists and computer hakers, was designed to come up with ways to improve Kepler’s completeness (seeing everything there to be seen) and reliability (the likelihood that the signal comes from a planet and not an instrument artifact or non-planetary phenomena in space). By computing both the completeness and reliability, scientists are confident that they can eliminate the observation biases and transform the discovery catalog into a directory of actual planets.

This is one of the key accomplishments of the Kepler mission – making it scientifically possible to say that there are billions and billions of planets out there. What’s more, the increased power of Kepler allowed for the discovery of smaller planets, which are now known to make up the bulk of the exoplanets. And while the number of Earth-sized planets detected in that habitable zone is small – around thirty – that’s still quite a remarkable feat. And remember, Kepler is looking at but one small sliver of the sky.

The twelve exoplanets detected so far closest to Earth in size, lined up with the type of stars they orbit. (NASA Ames)
The twelve exoplanets detected and confirmed so far closest to Earth in size, lined up with the type of stars they orbit. (NASA Ames)

Why does it matter how many exoplanets are out there, how many are rocky and Earth-sized, and how many within habitable zones? The last twenty years of exoplanet hunting, after all, has made clear that there are an essentially infinite number of them in the universe, and untold billions in our galaxy.

The answer lies in the insatiable human desire to know more about the world writ large, and how and why different stars have very different solar systems. But more immediately, there’s the need to know how to best design and operate future planet-finding missions. If the goal is to learn how to characterize exoplanets – identify components of their atmospheres, learn about their weather, their surfaces and maybe their cores – then scientists and engineers need to know a lot more about where planets generally, and some specifically, can be found. And those planet demographics just might open some surprising possibilities.

For instance, Belikov and his Ames colleague Eduardo Bendek have proposed a NASA “small explorer” (under $175 million) mission to launch a 30-to-45 centimeter mirror designed to look for Earth-sized planets only at our nearest stellar neighbor, Alpha Centauri. That’s as small a telescope as you can buy off-the-shelf.

Alpha Centauri is the closest star system to our Solar System at about 4.37 lightyears away. (NASA/Hubble Space Telescope)
Alpha Centauri is the closest star system to our Solar System at about 4.37 lightyears away. (NASA/Hubble Space Telescope)

Alpha Centauri is a two-star system, and until recently researchers doubted that binaries like it would have orbiting planets. But Kepler and other planet hunters have found that planets are relatively common around binaries, making Alpha Centauri a better target than earlier imagined.

To make it a truly viable project, ACESat – the Alpha Centauri Exoplanet Satellite – requires something else: a scientifically sound estimate of the likelihood that any star in our galaxy would have an Earth-sized planet in its system. Estimates so far have ranged from 10 percent to 50 percent, but Belikov said newer data is encouraging.

“If that number becomes more firm and approaches 50 percent, then an Alpha Centauri-only mission makes a great deal of sense,” he said. “For a small investment, we could have a real possibility of detecting a planet very close by.”

Intriguing, and an insight into how new space missions are designed based on the science already completed. Both NASA and the European Space Agency have plans to launch three significant exoplanet missions within the decade, and the powerful James Webb Space Telescope will launch in 2018 with some known and undoubtedly some not yet understood capabilities for exoplanet discovery. And perhaps most important, NASA is about to study how a potential mission in the 2030s could be designed with the specific purpose of directly imaging exoplanets – the gold standard for the field. All are being designed based on current exoplanet understandings, including the abundance calculations enabled by the Kepler mission’s observations.

Almost 2,000 exoplanets have now been identified, more than half by Kepler. Another 3,000 exoplanet candidates await confirmation. (NASA Ames)
Almost 2,000 exoplanets have now been detected and confirmed, more than half by Kepler. Another 3,000 exoplanet candidates await confirmation. (NASA Ames)

Future posts will dig deeper into a fair number of the subjects raised here, but for now this much is clear: Our galaxy has many billions of planets, and the process of detecting them is robust and on-going, the process of characterizing them has begun, and all the signs point towards the presence of enormous numbers of planets in habitable zones that, in the biggest picture at least, could possibly support life.

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