How Planet 9 Would Make Ours a More Typical Solar System

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The six most distant known objects in the solar system with orbits exclusively beyond Neptune (magenta) all mysteriously line up in a single direction. The new report shows a planet with 10 times the mass of the earth in a distant eccentric orbit anti-aligned with the other six objects (orange) is required to maintain this configuration. Image: Caltech/R. Hurt (IPAC)"
The six most distant known objects in our solar system with orbits (magenta) exclusively beyond Neptune all mysteriously line up in a single direction. A new report identifies the potential presence of a distant solar system planet — with 10 times the mass of the Earth and in a distant and eccentric orbit (orange) — as the reason why.  (JPL/Caltech; R. Hurt)

There’s been a ton of justifiable excitement these days about the possible discovery of a ninth planet in our solar system — an object ten time the mass  of Earth and 200 times further from the sun.  Especially in the context of the recent demotion of Pluto from a planet to a dwarf planet, the announcement of a potential replacement seems almost karmic, stage managed, in its take-and-give.  This is especially so since the astronomer probably most responsible for the diminished position of Pluto is also the one who now asserts the very far away presence of a different Planet 9 — planetary astronomer Michael Brown of the California Institute of Technology.

The validity of the possible detection of a Planet 9 has set off hot debates — with NASA officials, for instance, making clear that the agency sees the “discovery” as an exciting but early step towards establishing the existence of possible new planet.  We are all drawn to discovery and controversy, so the presence, or non-presence, of the planet has been the focus of attention.

But another most intriguing aspect of the finding has been largely ignored — the way  that such a Planet 9 would make our solar system surprisingly more similar to the many more eccentric exoplanet solar systems now known to be out there.  Our solar system would also suddenly have a range of planets sized more like the galactic norm.

What’s more, there’s reason to consider that a Planet 9 could have been spun off another solar system rather than having been ejected from the inner solar system, as proposed by Brown and colleague Konstantin Batygin.

In other words, Planet 9 may be an “exoplanet” in origin.  And if not, a finding that it was ejected long ago from our inner solar system would answer some questions about why our system seems to be so different from many of the other exoplanetary systems discovered so far.

Mike Brown and Konstanytin Batyglin of Caltech
Astronomers Mike Brown and Konstantin Batygin of Caltech.  They took research by Scott Shepard of the Carnegie Institution for  Science and Chad Trujillo of the Gemini Observatory in Hawaii regarding the unusual paths of objects orbiting beyond Pluto and carried it further to conclude there is a Planet 9 in the distant solar neighborhood.  (Lance Hayshida/Caltech)

“Our Planet 9 has a very eccentric orbit like planets in many other solar systems, and it’s a size of planet not found in our solar system but is the most common in other solar systems,”  said Brown.  “Seems odd to say, but it would make our solar system more normal.”

More specifically, here are the reasons why:

  • The most commonly sized exoplanet detected so far is larger than Earth and smaller than the next largest planet in our solar system, Neptune.  Since the difference in size is substantial — Neptune’s diameter is 4 times greater than Earth’s and its mass is 17 times greater — that leaves a lot of exoplanets of a size category different from anything in our solar system.  This absence has been a puzzle and would be reduced if Planet 9, some 10 times more massive than Earth, was determined to be real.
  • Many, if not most, solar systems identified so far are home to planets with very eccentric orbits.  In our solar system, the eight planets orbit on a generally singular plane, and most orbits are more circular than not.  The proposed Planet 9 would orbit on a very different plane — thirty degrees off the rest of the solar system’s planetary plane — and it circles the Earth in a most peculiar 10,000 to 20,000-year orbit.
  • Astronomers theorize that planets are ejected from their solar systems all the time, and roam through space without an orbit.  But in theory, they can easily move into a solar system where a sun and other planets pull it into an orbit around them.  This kind of planet capture has been successfully modeled many times, and has even once been identified.

 

Artist rendering of possible Planet 9, described in a recent edition of the Astronomical Journal. The authors estimate that the planet comes as close to the sun as 100-200 astronomical units (the distance from the Earth to the Sun) and travels as far away as 1200 AUs. (Caltech/R. Hunt)
Artist rendering of possible Planet 9, described in a recent edition of the Astronomical Journal. The authors estimate that the planet comes as close to the sun as 100-200 astronomical units (the distance from the Earth to the Sun) and travels as far away as 1200 AUs. (Caltech/R. Hunt)

The question of whether the object identified came from the inner solar system (as deemed likely  by Brown) or from elsewhere is a complicated one with a special interest for exoplanet researchers.

As reported by Brown and Batygin, the best theory to explain the faraway presence of Planet 9 is that it was ejected long ago from the region around Jupiter to Neptune.  Such solar system ejections are understood to happen all the time, and it would be a logical explanation given the relative closeness of our solar system planets.  The planet could have gotten knocked off course by coming too close to Jupiter, with its strong gravitational pull.

As theorized by the two authors, the planet could have then come to an orbital rest after being slowed down by gases.  But that wouldn’t occur until it was well past the solar system we know:  each orbit around the sun would take an estimated 15,000 years.

But Hagai Perets, an astrophysicist formerly at the Harvard-Smithsonian Center for Astrophysics and now at the Israel Institute of Technology, says it is equally or perhaps more plausible that Planet 9 (if it exists) came from another solar system entirely.  Having studied “roaming planets” kicked out of their solar systems, he says he is convinced that it could happen.

“Solar systems,” he said, “throw around their planets like we toss balls.

“We know there are planets with very wide orbits, thousands of astronomical units {the distance from the sun to Earth} from their suns.  We need a mechanism to explain this phenomenon, since the planets could not be formed in that region.

Hagai Perets, an astrophysicist at Technion- Israel Institute of Technology. He has studied rogue planets kicked out of their solar systems, and argues that the possible Planet 9 could have arrived from somewhere other than our solar system.
Hagai Perets, an astrophysicist at Technion- Israel Institute of Technology. He has studied rogue planets kicked out of their solar systems, and argues that the possible Planet 9 could have arrived from somewhere other than our solar system.

“That’s where stellar clusters come in, because most stars are formed in these clusters.  With so much activity going on as stars and solar systems are formed, it makes sense that there would be a great scattering of planets in their early epochs, and some of those planets would be ejected completely.

“They become free-floating, rogue planet,” he said.  “We have observational evidence that they exist, as well as our theoretical models.”

Brown agrees that Planet 9 could have come from another solar system, but he believes that an ejection from our inner solar system is the most plausible explanation.

Proposed orbit for a Planet 9 -- eccentric and distant from the sun, like many exoplanets and their host stars. For more information about planets that orbit far, far from their host stars, check out this recent discovery: http://planetquest.jpl.nasa.gov/news/247
Proposed orbit for a Planet 9 — eccentric and distant from the sun, like many exoplanets and their host stars. For more information about planets that orbit far, far from their host stars, check out this recent discovery: http://planetquest.jpl.nasa.gov/news/247

The potential discovery of a Planet 9 was made the way that Neptune was first identified — by detecting its gravitational effects on other objects.  (In the case of Neptune, that meant the effects on Uranus.)  This indirect process of discovery is not dissimilar from the first, and still widely used, method of finding exoplanets — by detecting through radial velocity the gravitational “wobble” that exoplanets cause in their host stars.

Brown and Batygin found evidence for the planet’s existence in the peculiar orbits of objects well beyond Neptune detailed in a previously published study by Scott Shepard of the Carnegie Institution for Science.  The authors analyzed six of the objects and found that they moved in their elliptical orbits while pointing in the same direction and while tilted at similar 30 degrees angles.

“It’s almost like having six hands on a clock all moving at different rates, and when you happen to look up, they’re all in exactly the same place,” Brown said in a statement. “Basically it shouldn’t happen randomly. So we thought something else must be shaping these orbits.”

Brown has a long history of studying the vast Kuiper Belt well beyond Neptune and its untold objects large and small.  It was in the course of his research of these “trans-Neptune objects” that he came to the conclusion that Pluto didn’t meet the accepted standards for what defines a planet.  It was just too small and its presence has little or no effect on surrounding objects.  Having reached that conclusion, he became a leader in the effort to have the planet demoted.

So having been involved in the undoing of the original Planet 9, he is now convinced there is another — very different — Planet 9.  Brown specifically calls it “Planet 9” rather than the long-discussed “Planet X” because, he said, there have been so many false claims made about a possible “Planet X.”

As he explained it:  “We wanted to highlight the strong science behind the finding.”

Now that stronger evidence for the distant world has been discovered, Brown thinks that within five years the planet can be directly imaged by astronomers — or perhaps will be discounted as unable to be confirmed.

Ironically, the naming of a “Planet 9” has already hit some headwinds — well before its existence is confirmed or rejected.  As with the change of the name of Pluto from a “planet” to a “dwarf planet,” there is interesting science behind the objections.

Alan Stern, principal investigator for the New Horizons mission to Pluto, has dismissed the name “Planet 9” due to his firm belief that there are many objects orbiting out beyond Pluto that are potentially planet size.

“I think the number of planets in our own solar system is going to explode, and that this is going to be one of the important lessons of 21st century astronomy.  I think people will get over worrying about their names pretty quickly.”

The reported Planet 9 inhabits the icy realm of the Kuiper Belt. (NASA)
The reported Planet 9 inhabits the icy realm of the Kuiper Belt. (NASA)

Stern pointed to research suggesting the early presence in our solar system of large planets that were later ejected to places unknown.  Some of those planets likely stuck around in far-off orbits like the proposed Planet X (or Planet 9.)

If this turns out to be the case,  Stern said, their existence would confirm his (and others’) long-held belief “that the majority of the planets in our solar system orbit far beyond the classical ones we grew up with.”

In addition to being compelling science, such detections would also support the view that the primary difference between planets in our solar system and exoplanets beyond is simply where they orbit.  Consequently, just as the study of our solar system informs the exploration and characterizing of exoplanets and their systems, so too does the science of exoplanets help better understand our solar system.

Together, they also tell us that our understanding of the vast menagerie of planets out there remains quite limited, with far less known than unknown.

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Movement in The Search For ExoLife

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A notional version of an observatory for the 2030s that could provide revolutionary direct imaging of exoplanets. GSFC/JPL/STScI
A notional version of an observatory for the 2030s that could provide revolutionary direct imaging of exoplanets. GSFC/JPL/STScI

Assuming for a moment that life exists on some exoplanets, how might researchers detect it?

This is hardly a new question.  More than ten years ago, competing teams of exo-scientists and engineers came up with proposals for a NASA flagship space observatory capable of identifying possible biosignatures on distant planets. No consensus was reached, however, and no mission was developed.

But early this year, NASA Astrophysics Division Director Paul Hertz announced the formation of four formal Science and Technology Definition Teams to analyze proposals for a grand space observatory for the 2030s.  Two of them in particular would make possible the kind of super-high resolution viewing needed to understand the essential characteristics of exoplanets.  As now conceived, that would include a capability to detect molecules in distant atmospheres that are associated with living things.

These two exo-friendly missions are the Large Ultraviolet/Optical/Infrared (LUVOIR) Surveyor and the Habitable Exoplanet (HabEx) Imaging Mission.   Both would be on the scale of, and in the tradition of, scientifically and technically ground-breaking space observatories such as the Hubble and the James Webb Space Telescope, scheduled to launch in 2018.  These flagship missions provide once in a decade opportunities to move space science dramatically forward, and not-surprisingly at a generally steep cost.

A simulated spiral galaxy as viewed by Hubble, and the proposed High Definition Space Telescope (HDST) at a lookback time of approximately 10 billion years (z = 2) The renderings show a one-hour observation for each space observatory. Hubble detects the bulge and disk, but only the high image quality of HDST resolves the galaxy’s star-forming regions and its dwarf satellite. The zoom shows the inner disk region, where only HDST can resolve the star-forming regions and separate them from the redder, more distributed old stellar population. Image credit: D. Ceverino, C. Moody, G. Snyder, and Z. Levay (STScI)500 light years away, as imaged by Hubble and potential of the kind of telescope the exoplanet community is working towards.
A simulated spiral galaxy as viewed by Hubble, and as viewed by the kind of high definition space telescope now under study.   Hubble detects the bulge and disk, but only the high definition image resolves the galaxy’s star-forming regions and its dwarf satellite. The zoom shows the inner disk region, where only high definition can resolve the star-forming regions and separate them from the redder, more distributed old stellar population. (D. Ceverino, C. Moody, G. Snyder, and Z. Levay (STScI)

Because the stakes are so high, planning and development takes place over decades — twenty years is the typical time elapsed between the conception of a grand flagship mission and its launch.  So while what is happening now with the science and technology definition teams  is only a beginning — albeit one with quite a heritage already — it’s an essential, significant and broadly-supported start.  Over the next three years, the teams will undertake deep dives into the possibilities and pitfalls of LUVOIR and HabEx, as well as the two other proposals.  There’s a decent chance that a version of one of the four will become a reality.

Aki Roberge, an astrophysicist at the Goddard Space Flight Center and staff scientist of the LUVOIR study, said that the explicit charge to the teams is to cooperate rather than compete.  Any of the four observatories under consideration, she said, would enable transformative science. But from an exoplanet perspective, the possibilities she described are pretty remarkable.

“What we’re aiming for is the capability to really search for the true Earth analogues out there, the Earth-sized planets in the habitable zones of sun-like stars.  We need to understand their atmospheres, their climates, their compositions.  And ultimately, the goal is to search for life.”

The co-chair of the HabEx team, Bertrand Menneson of the Jet Propulsion Lab, said the goals are the same:  A major jump forward in our ability to understand exoplanets and a serious effort to find life.

actual image of venus crossing in front of the sun. Exoplanets will not be imaged like this in our lifetimes, but this is the goal.
Actual image of Venus crossing in front of the sun in 2012 taken by NASA’s Solar Dynamics Observatory. Exoplanets will not be imaged like this in our lifetimes, but this is the ultimate goal.

The field of exoplanet detection and research has exploded over the past two decades, with an essential boost from increasingly capable observatories on Earth and in space.  With at least three more major exoplanet-friendly space telescopes scheduled (or planned) for the next decade — as well as first light at several enormous ground-based mirrors — the brisk pace of discoveries is sure to continue.

So why are so many scientists in the field convinced that a grand, Flagship-class NASA space observatory is essential, and that it needs to be developed and built ground-up with exoplanet research in mind?  Can’t the instruments in use today, and planned for the next decade, provide the kind of observing power needed to continue making breakthroughs?

Well, no, they can’t and won’t.  That has been the conclusion of numerous studies over the years, and most recently an in-depth effort by the Association of Universities for Research in Astronomy (AURA,)   http://www.hdstvision.org/report which last summer called for development of a 12-meter (about 44 feet across) High Definition Space Telescope with the super high resolution needed to study exoplanets.  Generally speaking, a larger light-collecting mirror allows astronomers and astrophysicists to see further and better.

 

A direct, to-scale, comparison between the primary mirrors of the Hubble Space Telescope, James Webb Space Telescope, and the proposed High Definition Space Telescope (HDST). In this concept, the HDST primary is composed of 36 1.7 meter segments. Smaller segments could also be used. An 11 meter class aperture could be made from 54 1.3 meters segments. Image credit: C. Godfrey (STScI)
A direct, to-scale, comparison between the primary mirrors of the Hubble Space Telescope, James Webb Space Telescope, and the High Definition Space Telescope (HDST) proposed by the AURA group. In this concept, the HDST primary is composed of 36 1.7 meter segments.  The LUVOIR mirror under consideration is in the eight to twelve meters range. C. Godfrey (STScI)

The group, headed by Julianne Dalcanton of the University of Washington and Sara Seager of MIT, began with this overview of the state of play when it comes to exoplanets, instruments, and what is possible now and might be in the future:

While we now have a small sample of potentially habitable planets around other stars, our current telescopes lack the power to confirm that these alien worlds are truly able to nurture life. This small crop of worlds may have temperate, hospitable surface conditions, like Earth. But they could instead be so aridly cold that all water is frozen, like on Mars, or so hot that all potential life would be suffocated under a massive blanket of clouds, like on Venus. Our current instruments cannot tell the difference for the few rocky planets known today, nor in general, for the larger samples to be collected in the future. Without better tools, we simply cannot see their atmospheres and surfaces, so our knowledge is limited to only the most basic information about the planet’s mass and/ or size, and an estimate of the energy reaching the top of the planet’s atmosphere. But if we could directly observe exoplanet atmospheres, we could search for habitability indicators (such as water vapor from oceans) or for signs of an atmosphere that has been altered by the presence of life (by searching for oxygen, methane, and/or ozone).

A central goal for both LUVOIR and HabEx is to provide that “seeing” through much more sophisticated direct imaging — that is, capturing the actual reflected light from exoplanets rather than relying on indirect techniques and measurements.  The many indirect methods of finding and studying exoplanets have played and will continue to play an essential role.  But there is now a community consensus that next generation direct imaging from space is the gold standard.

 

Kepler exoplanets candidates, both confirmed and unconfirmed, orbiting G, K, and M type main sequence stars, by radii and fraction of the total. (Natalie Batalha and Wendy Stenzel, NASA Ames)
There are more than 4,000 Kepler exoplanets candidates, both confirmed and unconfirmed, orbiting G, K, and M type main sequence stars.  This graphic shows their distribution by radii and fraction of the total. (Natalie Batalha and Wendy Stenzel, NASA Ames)

That a major space observatory for the 2030s just might be exoplanet-focused reflects a definite maturing of the field.  From a science perspective, the discoveries of the Kepler mission in particular made clear that exoplanets are everywhere, and not infrequently orbiting in habitable zones.  The work of the Curiosity rover on Mars, and especially the conclusion that the planet once was wet and “habitable,” added to the general interest and excitement about possible life beyond Earth.

And then there are the lessons learned from the earlier bruising battles among exoplanet scientists, who had developed a reputation for serious in-fighting.  THEIA, the Telescope for Habitable Exoplanets and Interstellar/Intergalactic Astronomy, was put forward as a flagship direct imaging mission in 2010, when the Astronomy and Astrophysics Decadal Survey that sets priorities for the field was being put together by the National Academy of Sciences.  But THEIA was not adopted.

A cartoon from Chas Beichman’s ExoPAG presentation illustrates the infighting within the exoplanet science community during the 2010 decadal survey, with cosmologists, represented by “dark energy” to the side, ready to reap the benefits of that debate.
A cartoon from a exoplanet science presentation illustrates the infighting within the exoplanet science community during the 2010 decadal survey, with cosmologists, represented by “dark energy” to the side, ready to reap the benefits of that debate. ( Chas Beichman)

With the 2020 Decadal Survey on the horizon, exoplanet scientists have tried to limit conflicts and to work with the larger astronomy community.  The formal NASA/community study group, the Exoplanet Exploration Program Analysis Group (ExoPAG), brought two related groups together and ultimately recommended the intensified study for LUVOIR, HabEx and the two other proposals —  which focus on black holes, ancient galaxy formation, and other aspects of the early cosmos.  https://exep.jpl.nasa.gov/files/exep/ExoPAG_Large_Missions.pdf

When completed, the studies will go to the National Academy of Sciences for further review, discussion, and ultimately a recommendation to NASA regarding which project should go forward.

The leader of the ExoPAG  group was astronomer Scott Gaudi of Ohio State University, who specializes in characterizing exoplanets but played no favorites in the ExoPAG report and recommendations.

“What we want is to set up a fair process of intense review so the most compelling science can be chosen to go forward.  At this point, we don’t know if the necessary technologies will be available in time, and we don’t know what the costs will be.  There’s only so much money that comes from NASA for our (astrophysics) community, and maybe a top choice will cost more than the community is willing to spend.  So there are so many factors to consider.”

(The LUVOIR mission is generally considered to be somewhat more ambitious than HabEx, and would require a larger telescope mirror — greater than 8 meters across –and more funding.  Flagship missions are expensive, as NASA learned once again with the James Webb telescope, which will have cost $8.8 billion by the time of its scheduled launch.)

I asked Gaudi if the seemingly substantial public interest in exoplanets could play any role in subsequent decision-making, and he replied that it possibly would.  “In the past five or ten years, exoplanets have become a prominent topic for sure.  And the public is clearly very, very interested in that topic.”  But that public interest, he said, won’t mean much if the science and technical feasibility isn’t there.

Scott Gaudi, chairman of ExoPAG in 2015.
Scott Gaudi, chairman of ExoPAG in 2015.

We won’t know for some years if the stars will align in a way that will lead to a major observatory with direct imaging and exoplanets at its center.  But for those active in the field, the opportunity to take part in a major effort to formally determine its scientific merit and feasibility is irresistible.

Shawn Domagal-Goldman, a research space scientist at Goddard, was selected to be a deputy on the LUVOIR science and technology team, which he sees as a much-anticipated “proof of concept” effort for the exoplanet research of the future.

Between 12 and 18 scientists and engineers will be selected by NASA headquarters for each team, and Domagal-Goldman said it’s essential that they make up a broad and inter-disciplinary group, including people from industry.  Scientists from abroad not associated with an American institution can’t be formal members, but they can observe and may become more involved if their national space agencies decide to join in the effort. He encourages researchers — from newly minted PhDs to career scientists — to nominate themselves to join.

“Nobody gets paid for this, it’s a labor of love,” he said.  “But what would be more satisfying than having some of your intellectual contribution go into the formulation of missions like these.

“Direct imaging of exoplanets is clearly a direction where the community is headed. These are the missions of the future in one form or another, and if you’re a PhD or postdoc who’s qualified, this could be your career.”

Of course, it just might make the greatest discovery of modern science — finding life beyond Earth.

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The IAU on ExoNames

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The IAU, in the person of Executive Committee member and former General Secretary Thierry Montmerle, wrote the following response to an earlier column, “(Mostly) Thumbs Down on ExoNames.”  The response to the article was first posted as a comment on the Many Worlds site, but to ensure that it is seen by readers I am posting the full email now:

 

We found it quite interesting, since, for once, it concerns the feedback from the astronomers’ community, which is certainly as important for us as the reactions from the public.

We do have a few comments to offer, that may supplement your already rich article.

They are listed below.

Many thanks for you interest in the IAU and in the “NameExoWorlds” contest !

1) The exoplanet community has been involved from the start. IAU Commission C53 set up a Working Group including, among others, Didier Queloz and Geoff Marcy. This Working Group made the recommendation for defining the initial list of 305 confirmed exoplanets for public naming. They also agreed that the names could be given by the public (not by the discoverers or scientists), with the aim of having various names representing different cultures all over the world. Given the results of the contest, it is obvious this goal has been successfully reached.

The discoverers of the named planets have also been contacted at the beginning of the process and most of them expressed that they were enthusiastic about it; they provided comments on their discovery, which were published from the start on the NameExoWorlds web page.

The Working Group also agreed that the public names would never supersede the scientific designations. This has been recalled many times at each step.

2) Astronomy clubs and associations were invited to engage into the contest by voting for the top “most popular” systems (i.e., most interesting in their opinion). As a result, we decided to select the top 20 for naming proposals by the public. The resulting list is remarkably diverse and reflects rather closely the wide range of the present-day “exoplanet science” (restricted to confirmed objects having been studied for many years, not to recent “frontier” objects like the Kepler exoplanet candidates)

3) All the names (including star names) were the subject of 275 argued proposals sent by over 600 registered clubs, then submitted to a public, worldwide vote, and over 500,000 were cast by voters from 180 countries. The contest also generated over 800 articles in 54 countries. Thus the large impact of the contest on the public across the world is undeniable. Also, we have seen in many occasions that the contest gave the opportunity to the astronomical associations to address locally the non-specialist public at large about stars and planets. In this regard, the contest in itself is also a success in arousing a wider interest for astronomy.

4) Whether or not the scientific community uses these names is actually irrelevant, this has never been the purpose of the contest. Professional astronomers still use concurrently common “nicknames” and scientific designations for astronomical objects: alpha CMa and Sirius, M42 and Orion, NGC7293 and the Helix Nebula, M31 and the Andromeda galaxy, etc., but the public certainly prefers to use the nicknames when they exist. The fact is the names approved by the IAU are already quoted in Wikipedia, so the public will very likely use them whenever an opportunity arises.

5) On the other hand, the public names are now officially sanctioned by the IAU and included in the SIMBAD database (and are in the process of being included in other professional databases).

6) We’ll see in the long term whether the names are caught up by the public in general, but in our opinion it will be more a matter of the future scientific interest of the objects themselves (exoplanets and/or stars) than of their public name. There is little doubt that any future press release on HD149026b, for instance, even if written by scientists, will speak about the planet “Smertrios” rather than use the scientific “license plate” designation.

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Enceladus and Water Worlds

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Glittering geysers of water ice erupt from Saturn's enigmatic moon Enceladus as seen during a previous flyby. The plumes are backlit by the sun, which is almost directly behind the moon. The moon's dark side that we see here is illuminated by reflected Saturn-shine. Today, the Cassini spacecraft flew right through the plumes in order to let its instruments 'taste' them. Credit: NASA/JPL/SSI/Ugarkovich
Glittering geysers of water ice erupt from Saturn’s enigmatic moon Enceladus as seen during a previous flyby. The plumes are backlit by the sun, which is almost directly behind the moon. The moon’s dark side that we see here is illuminated by reflected Saturn-shine.  Credit: NASA/JPL/SSI/Ugarkovich

As if the prospect of billions of potentially habitable exoplanets wasn’t enough to get people excited, what about all those watery exo-moons too?

The question arises as the Cassini mission makes its final pass near the now famous geysers at the south pole of the moon Enceladus ,scheduled for Saturday.  The plumes are currently in darkness and so it’s a perfect time to tease out a particularly compelling aspect of the Enceladus story:  how hot is the inside of the mini-moon.  Earlier measurements of the water ice spray took place when the sun was on that southern pole, so this will be the first time Cassini can measure precisely how much of the already detected heat comes from the moon’s interior.

The expectation is that much of the heat does indeed come from inside, warmed substantially by tidal forces and perhaps hydrothermal vents that together serve to keep liquid a subsurface ocean all around the moon.  As a result, the evolving scientific view is that tiny Enceladus, one of 63 moons of Saturn, just may have the ingredients and characteristics that put it into an improbable habitable zone.

“Step by step, we’re learning about an environment that seemed impossible not long ago,” said Cassini Mission Scientist Linda Spilker.  “We know that Enceladus has some rocky core, and that it touches the liquid water.  We also know that some of the compounds identified in the geysers can only be formed when rock is in contact with hot water, and that must be happening at the bottom of the moon’s ocean.  All the pieces are coming together to tell us that the moon has an ocean that might be able to support life.”

NASA's Cassini spacecraft captured this view as it neared icy Enceladus for its closest-ever dive past the moon's active south polar region. The view shows heavily cratered northern latitudes at top, transitioning to fractured, wrinkled terrain in the middle and southern latitudes. The wavy boundary of the moon's active south polar region -- Cassini's destination for this flyby -- is visible at bottom, where it disappears into wintry darkness. This view looks towards the Saturn-facing side of Enceladus. North on Enceladus is up and rotated 23 degrees to the right. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Oct. 28, 2015. The view was acquired at a distance of approximately 60,000 miles (96,000 kilometers) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 45 degrees. Image scale is 1,896 feet (578 meters) per pixel.
The Cassini spacecraft, sponsored by NASA, the European Space Agency and the Italian space Agency,  captured this view on Oct. 28 as it neared Enceladus. The wavy boundary of the moon’s active south polar region — Cassini’s destination for this flyby — is visible at bottom. The image was taken in visible light with the Cassini spacecraft narrow-angle camera from approximately 60,000 miles away. (Cassini Imaging Team, SSI, JPL, ESA, NASA)

That a moon might have habitable conditions is not a new idea:  science fiction great Arthur C. Clarke (as well as many scientists and now members of Congress) have pressed for a mission to Jupiter’s moon Europa because its internal ocean has been identified as similarly promising.

But what is so compelling about Enceladus is that its potential habitability pretty much came out of nowhere.  While Europa is the sixth largest moon in the solar system, Enceladus is but 370 miles in diameter.  It is covered in ice, but in 2004 its four parallel “tiger stripe” fractures were discovered, leading to the conclusion that some kind of volcanic action was taken place beneath them.  Spilker was a scientist with the Voyager mission that passed by Saturn in 1980-81 and said that Enceladus was then in relative darkness and made little impression on the team.  “We definitely missed the tiger stripes,” she said.

The plumes emerge from the south pole region, not far from the tiger stripe fractures, and appear to come from  near-surface pockets of liquid water. (The oceans of Europa are not nearly as accessible, lying 6 to 20 miles below the icy surface.)  Making Enceladus even more interesting, the 70 geysers spit out organic chemicals known as the building blocks of life along with its water ice.

The chemical composition of the plumes of Enceladus's includes hydrocarbons such as ammonia, methane and formaldehyde in trace amounts similar to the makeup of many comets. (NASA)
The chemical composition of the plumes of Enceladus’s includes hydrocarbons such as ammonia, methane and formaldehyde in trace amounts similar to the makeup of many comets.  The presence of the organic compounds suggests that very interesting chemistry is taking place where the moon’s oceans touch its core. (NASA, ESA)

So Enceladus (and perhaps Europa, too) provide a kind of emerging “proof of concept” that ice-covered water worlds can and do exist elsewhere.  (Jupiter’s giant moons Ganymede and Callisto also have massive ocean, but far below their surfaces and sandwiched between layers of ice.)  In fact, subterranean oceans may be common because, in recent years, scientists have come to understand that water — especially in its vapor and ice stages — is ubiquitous in the solar system, the galaxy and the universe.  Comets, which are generally half water ice and half rock, are one of numerous delivery systems.

And we already have, of course, one good example of what would generally be considered a waterworld without the ice covering — Earth.  But that’s not all, even without leaving our solar system.   We look at planets such as Mars and Venus and now see desiccated landscapes.  Yet it is broadly accepted that Mars once was quite wet on the surface, based on findings from the Curiosity mission and years of satellites imaging, and some have speculated that Venus might once have been wet as well.

So are there many waterworlds or aquaworlds with surface liquid water out there?

“It would be fair to say there is a consensus view that exoplanets and moons with lots of water are all over the place,” said Joel Green, an exoplanet scientist with the Space Telescope Science Institute. “The known presence of so much H2O makes that non-controversial.”

Kepler-62e has been described as being a possible waterworld, with large oceans. UPR Arecibo
Kepler-62e has been described as being a possible waterworld, with large oceans. UPR Arecibo

 

What is indeed controversial is whether any waterworlds, or even potential waterworlds, have been detected. There actually has been wide coverage of waterworld discoveries far, far away,  and even some declared confirmations.  But so far at least, those confirmations have not lasted.  As early as 2004, the Hubble Space Telescope identified the signature of water vapor in the atmosphere of an exoplanet, and similar detections have followed.  But that information and more that scientists have collected and modeled about H2O on exoplanets has never been sufficient to make a confirmation stick.

“It’s a very challenging detection to make, and many don’t think we haven’t gotten there yet,” Green said.  Especially challenging is the detection of liquid water, since it does not show up using optical, infrared, ultraviolet or any other kind of light, and so can’t be identified with a spectroscope. It’s presence can only be inferred based on other conditions.  Water vapor and water ice are, however, detectable via spectroscope.

But waterworld theories abound.  For instance, scientists know that Neptune and Uranus in the outer part of our solar system consist of vast amounts of water ice, and they also know that planets tend to change orbits and sometimes migrate closer to their suns.  Marc Kurchner of NASA’s Goddard Space Flight Center has proposed that if similar planets were to migrate inward in different solar systems, the result could be a very wet planet, with oceans hundreds of miles deep.

There is a general tendency to associate the presence of water with the presence of life, and the ubiquity of water in the galaxies with the likelihood of finding life.   But while life is found everywhere that water exists on Earth, that does not at all mean that the discovery of water elsewhere means life will be present, too.

All it means is that one of many prerequisites for life (as we know it) will have been met. But as Enceladus shows, when water is present, all kinds of interesting things start of happen.

A view of Enceladus’ southern hemisphere in enhanced color (IR-green-UV). The “tiger stripe” fractures, the source of plumes venting gas and dust into space, are prominently visible in the center. {NASA/JPL-Caltech/SSI/Lunar and Planetary Institute, Paul Schenk (LPI, Houston)
A view of Enceladus’ southern hemisphere in enhanced color (IR-green-UV). The “tiger stripe” fractures, the source of plumes venting gas and dust into space, are prominently visible in the center. {NASA/JPL-Caltech/SSI/Lunar and Planetary Institute, Paul Schenk (LPI, Houston)
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Faint Worlds On the Far Horizon

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Faintest distant galaxy ever detected, formed only 400 million years after the Big Bang. NASA, ESA, and L. Infante (Pontificia Universidad Catolica de Chile)
Faintest distant galaxy ever detected, formed only 400 million years after the Big Bang. NASA, ESA, and L. Infante (Pontificia Universidad Catolica de Chile)

For thinking about the enormity of the canvas of potential suns and exoplanets, I find images like this and what they tell us to be an awkward combination of fascinating and daunting.

This is an image that, using the combined capabilities of NASA’s Hubble and Spitzer space telescopes, shows what is being described as the faintest object, and one of very oldest, ever seen in the early universe.  It is a small, low mass, low luminosity and low size protogalaxy as it existed some 13.4 billion years ago, about 4oo million years after the big bang.

The team has nicknamed the object Tayna, which means “first-born” in Aymara, a language spoken in the Andes and Altiplano regions of South America.

Though Hubble and Spitzer have detected other galaxies that appear to be slightly further away, and thus older, Tayna represents a smaller, fainter class of newly forming galaxies that until now have largely evaded detection. These very dim bodies may offer new insight into the formation and evolution of the first galaxies — the “lighting of the universe” that occurred after several hundred million years of darkness following the big bang and its subsequent explosion of energy.

This is an illustration by Adolf Schaller from the Hubble Gallery (NASA). It is public domain. It shows colliding protogalaxies less than 1 billion years afer the big bang.
This is an illustration by Adolf Schaller from the Hubble Gallery and shows
colliding protogalaxies less than 1 billion years after the big bang. (NASA)

Detecting and trying to understand these earliest galaxies is somewhat like the drive of paleo-anthropologists to find older and older fossil examples of early man. Each older specimen provides insight into the evolutionary process that created us, just as each discovery of an older, or less developed, early galaxy helps tease out some of the hows and whys of the formation of the universe.

Leopoldo Infante, an astronomer at Pontifical Catholic University of Chile, is the lead author of last week’s Astrophysical Journal article on the faintest early galaxy.  He said there is good reason to conclude there were many more of these earliest protogalaxies than the larger ones at the time, and that they were key in the “reionization” of the universe — the process through which the universe’s early “dark ages” were gradually ended by the formation of more and more luminous stars and galaxies..

But the process of detecting these very early protogalaxies is only beginning, he said, and will pick up real speed only when the NASA’s James Webb Space Telescope (scheduled to be launched in 2018) is up and operating.  The Webb will be able to see considerably further back in time than the Hubble or Spitzer.

Estimates of how many galaxies might exist in the universe are in flux, with recent studies producing results ranging from 100 to 225 billion.  On average a galaxy will have some 100 billion stars, giving the universe a low-end estimate of 10,000,000,000,000,000,000,000 stars.

When it comes to planets, a consensus of sorts has formed around the conclusion that in the Milky Way, and perhaps elsewhere, there is on average at least one planet per star.  So assuming that the planetary dynamics of our galaxy are similar to those of others, that’s an awful lot of potential exoplanets.

PSR B1620-26 b is an extrasolar planet located approximately 12,400 light-years away from Earth in the constellation of Scorpius. It bears the unofficial nicknames "Methuselah" and "the Genesis planet" due to its extreme age
PSR B1620-26 b is an extrasolar planet located approximately 12,400 light-years away from Earth in the constellation of Scorpius. It bears the unofficial nicknames “Methuselah” and “the Genesis planet” due to its extreme age. (NASA and G. Bacon, STScI)

All this has significant implications for the field of exoplanet research.

“We know that basically, planets form at about the same time as their stars from all the leftover dust and gas kicked up,” said Joel Green, Project Scientist at Space Telescope Science Institute’s Office of Public Outreach (STScI.)  The Institute operates the science for the Hubble Space Telescope as an international observatory.

“The earliest planets may have been very different kinds of planets because there was not as much metallicity (heavier elements) in those stars.  But as soon as you have stars, you have planets.”

He said that in theory, that means that when the very earliest stars formed — during a time when the universe was essentially dark — planets were formed too. “They don’t need a universe of light to form; they need one star.”

The most ancient exoplanet detected so far (PSR B1620-26 b) has had a rather unusual history, first born 12.7 billion years ago outside of a “globular cluster”  of stars (a comparatively older, compact group of up to a million old stars, held together by mutual gravitation), it then migrated closer to the cluster and into a rough astrophysical neighborhood. As viewed today, it orbits a pair of burned-out stars in the crowded core of a globular star cluster. It was first identified as a possible planet in 1992 — before the detection of 51 Pegasi b — but it took more than a decade to confirm that it is.

The oldest known exoplanet solar system is Kepler -444, formed 11.2 billion years ago in the Milky Way, itself 13.2 billion years old. Located in the constellation Lyra  116 light-years away, it hosts five rocky planets, all orbiting close to their sun.

Kepler-444 hosts five Earth-sized planets in very compact orbits. The planets were detected from the dimming that occurs when they transit the disc of their parent star, as shown in this artist's conception. Credit: Tiago Campante/Peter DevineKepler-444 is a metal-poor Sun-like star located in the constellation Lyra, 116.4 light-years away. Also known as HIP 94931, KIC 6278762, KOI-3158, and LHS 3450, this pale yellow-orange star is very bright and can be easily seen with binoculars. It was formed 11.2 billion years ago, when the Universe was less than 20 percent its current age. It is approximately 25 percent smaller than the Sun and substantially cooler.
Kepler-444 hosts five Earth-sized planets in very compact orbits. A metal poor sun (composed largely of hydrogen and helium), it is very bright and easily seen with binoculars. (Tiago Campante/Peter Devine)

The discovery of a solar system with rocky planets of this age (more than twice the age of our solar system’s rocky planet quartet), opens the door to the prospect of an early universe with many more rocky planets than once thought.  That means there could be vast numbers of very ancient Earth-like planets out there.

Returning to the faintest protogalaxy, it is described as being comparable in size to the Large Magellanic Cloud (LMC), a very small satellite galaxy of our Milky Way seen in the southern hemisphere. Tayna is rapidly making stars at a rate ten times faster than the LMC, and is likely the growing core of what will evolve into a full-sized galaxy.

This faintest ancient galactic find is part of a discovery of 22 young galaxies at ancient times located nearly at the observable horizon of the universe, research that substantially increases in the number of known very distant galaxies.

“The big unanswered question is how and when did the stars and galaxies turn on to end those Dark Ages,” said Green.  “There was a point when they started popping like popcorn.  With Hubble we can go back only so far and can’t see anymore, but the James Webb can go significantly further and see back to the Dark Ages.”

Massive cosmic objects, from single stars to galaxy clusters, bend and focus the light that flows around them with their gravity, acting like giant magnifying glasses. This effect is called gravitational lensing or, when it is detected on tiny patches on the sky, microlensing. Credit: ESA/ATG medialab Read more at: http://phys.org/news/2015-07-astronomers-cosmic-gravity-black-hole-scope.html#jCp
Massive cosmic objects, from single stars to galaxy clusters, bend and focus the light that flows around them with their gravity, acting like giant magnifying glasses. This effect is called gravitational lensing or, when detected on distant plants and faint galaxies, microlensing. (ESA/ATG medialab)

Ironically, Infante and his team were able to find the faintest distant galaxy so far without having it be the hardest to see.  That’s because they were able to use a technique of observing first proposed by Albert Einstein.  As described on the HubbleSite:

The small and faint galaxy was only seen thanks to a natural “magnifying glass” in space. As part of its Frontier Fields program, Hubble observed a massive cluster of galaxies, MACS J0416.1-2403, located roughly 4 billion light-years away and weighing as much as a million billion suns. This giant cluster acts as a powerful natural lens by bending and magnifying the light of far-more-distant objects behind it. Like a zoom lens on a camera, the cluster’s gravity boosts the light of the distant protogalaxy to make it look 20 times brighter than normal. The phenomenon is called gravitational lensing and was proposed by Einstein as part of his General Theory of Relativity.

While gravitational lensing uses a galaxy cluster as its magnifying glass, “microlensing” takes advantage of the same physics but uses a single star in our galaxy as the lens.  That technique is the only known method capable of discovering planets at truly great distances from the Earth. Radial velocity searches look for planets in our immediate galactic neighborhood, up to 100 light years from Earth, transit photometry can potentially detect planets at a distance of hundreds of light-years, but only microlensing can find planets orbiting stars near the center of the galaxy, thousands of light-years away.

And in the spirit of the wonder that microlensing tends to engender, let me leave you with another of those defining astronomical images that are impossible to ignore or forget.

This is the third version of the Hubble Ultra Deep Field, first assembled from 2003-2004 images, upgraded to the Hubble eXtreme Deep Field (XDF) image in 2012 and then enhanced further in 2014 and returned to the original Hubble Ultra Deep Field name.  Both the XDF and the 2014 version capture a patch of sky at the center of the original Hubble Ultra Deep Field.  That initial effort, which looked back in time approximately 13 billion years, picked up many unintentionally microlensed galaxies.

The newer images feature about 5,500 galaxies even within its smaller field of view. The faintest galaxies are one ten-billionth the brightness of what the human eye can see; just imagine that ratio for a single star or a planet.

So while there undoubtedly are an untold numbers of planets in the field, they will remain hidden for a very long time to come.

Hubble Ultra Deep from 2014. using full range of ultraviolet to near infrared, includes some of the most distant galaxies imaged by an optical telescope.
Hubble Ultra Deep Field from 2014. using full range of ultraviolet to near infrared, includes some of the most distant galaxies imaged by an optical telescope.  It is the third iteration of the Hubble Ultra Deep Field image, and combines more than 10 years of Hubble photographs taken of a patch of sky at the center of the original creation. (NASA)
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