Nearby Worlds, and a Cosmic Horizon

We're learning more about planets circling other stars: and about light that's been traveling for billions of years.

1. "Earth-Like," Not Habitable
   • GJ 436b and GJ 1214b: "Super-Earths," But Not Particularly Earth-Like
   • Lots of Clouds, or None at All
2. "Super-Earth," Yes; Earth-Like, Not So Much
3. The Universe: As Far as We Can See
   • More a Horizon, Than an Edge
   • Across the Horizon, Toward Infinity
4. Oldest Light in the Universe
   • When the Fog Cleared

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Ancient Mesopotamia and the Universe

A few thousand years ago, quite a few folks said that we live on a sort of plate, under a dome that held back an overhead ocean.


(From "The Three-Story Universe," © N. F. Gier, God, Reason, and the Evangelicals (1987), via Nick Gier, University of Idaho, used w/o permission.)

That wasn't the only set of ancient ideas about the universe, but it's one the Hebrews knew about. I'm somewhat familiar with this Mesopotamian cosmology, partly because it's used as powerful imagery in the Bible.

I'm a Catholic, though, so although I need to take Sacred scripture very seriously: I am not required to believe that it is a science textbook. (January 2, 2013)

Plato, Aristotle, and Getting a Grip

Folks kept being curious about the world we live in. About two dozen centuries back, folks like Plato and Aristotle made sense of their observations by assuming that the universe was a set of nested spheres.

Around eight centuries ago, European academics were getting access to ancient texts. That was an exciting era, particularly since they were gathering new data and were developing new ways of looking at what had been learned earlier.

Some folks suggested that we might not be standing on the only world. Others, predictably, didn't like the idea: and said it was impossible.

That's when the Church stepped in, as I've said before. Ever since 1277, Catholics haven't been allowed to say that there can't be other worlds.

I can live with that.

I prefer taking the universe as it really is, and enjoy trying to keep up with what we're learning about this vast and ancient cosmos. Besides, I think expressing disapproval of the way creation is designed makes no sense.

Not everyone feels that way, particularly when it comes to humanity's early days:

• "Personal Preference, Reality, and Dealing with Knowledge"
  (January 2, 2014)

Peering Into Vastness

Just a few years after we started collecting data from orbiting observatories like Hubble and Kepler, scientists have discovered more than a thousand planets orbiting other stars. None of them are quite like Earth, and many aren't like anything in the Solar system.

Other scientists mapped the oldest light in the universe, arriving from the time when our universe became transparent. We've learned that this universe is roughly 13,800,000,000 years old, and quite probably infinite in extent.

This creation is a big place. Again, I can live with that.

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1. "Earth-Like," Not Habitable

(From NASA, ESA, and STScl; via HubbleSite; used w/o permission.)

"NASA's Hubble Sees Cloudy Super-Worlds With Chance for More Clouds"
NASA news release (December 31, 2013)

"Scientists using NASA's Hubble Space Telescope have characterized the atmospheres of two of the most common type of planets in the Milky Way galaxy and found both may be blanketed with clouds.

"The planets are GJ 436b, located 36 light-years from Earth in the constellation Leo, and GJ 1214b, 40 light-years away in the constellation Ophiuchus. Despite numerous efforts, the nature of the atmospheres surrounding these planets had eluded definitive characterization until now. The researchers described their work as an important milestone on the road to characterizing potentially habitable, Earth-like worlds beyond the solar system. Their findings appear in separate papers in the Jan. 2 issue of the journal Nature...."

As I've said before, "Earth-like" doesn't necessarily mean "habitable."

For example, Venus is almost the same diameter and density as Earth, made of very nearly the same materials. It's so much like Earth that some folks imagined that the planet was covered by swamps and jungles.

Venus is very Earth-like, nearly a twin to our home, except for a few details.

The atmosphere of Venus is much thicker than ours, mostly carbon dioxide, with clouds of sulfuric acid and a surface pressure about 92 times Earth's. It's hot, too: about 467° Centigrade at the surface. That's 872° Fahrenheit, just a little less than what you'd find in a self-cleaning oven.

The good news is that 50 to 65 kilometers above the surface of Venus, air pressure and temperature is about the same as on Earth's. That's about 35 miles. We have the technology to build habitable structures that would float at that altitude, and that's another topic.

• "Cloud Cities of Venus"
  Apathetic Lemming of the North (July 25, 2008)

GJ 436b and GJ 1214b: "Super-Earths," But Not Particularly Earth-Like

"...The two planets fall in the middle range in mass, between smaller, rockier planets such as Earth and larger gas giants such as Jupiter. GJ 436b is categorized as a 'warm Neptune' because it is much closer to its star than frigid Neptune is to the sun. GJ 1214b is known as a 'super-Earth' because of its size. Both GJ 436b and GJ 1214b can be observed transiting, or passing in front of, their parent stars. This provides an opportunity to study these planets in more detail as starlight filters through their atmospheres.

"One of the papers presents an atmospheric study of GJ 436b based on such transit observations with Hubble, led by Heather Knutson of the California Institute of Technology in Pasadena, Calif. The Hubble spectra were featureless and revealed no chemical fingerprints whatsoever in GJ 436b's atmosphere...."
(NASA)

What's fascinating about the spectra of GJ 436b and GJ 1214b, planets about 36 and 40 light-years away from us, is that they lack 'chemical fingerprints.'

That's because they both have a lot of very high-altitude clouds. At least, that's the most likely explanation.

Clouds on Earth are made of water, apart from the occasional effluvia from volcanoes and fires. Clouds on these planets are probably potassium chloride or zinc sulfide - at several hundred degrees Fahrenheit.

On the other hand, it's possible that GJ 436b doesn't have any clouds at all.

Lots of Clouds, or None at All

"...'Either this planet has a high cloud layer obscuring the view, or it has a cloud-free atmosphere that is deficient in hydrogen, which would make it very unlike Neptune,' said Knutson. 'Instead of hydrogen, it could have relatively large amounts of heavier molecules such as water vapor, carbon monoxide, and carbon dioxide, which would compress the atmosphere and make it hard for us to detect any chemical signatures.'

"Observations similar to those obtained for GJ 436b had been obtained previously for GJ 1214b. The first spectra of this planet also were featureless, but indicated GJ 1214b's atmosphere was dominated by water vapor or hydrogen, with high-altitude clouds.

"Using Hubble, astronomers led by Laura Kreidberg and Jacob Bean of the University of Chicago took a closer look at GJ 1214b. They found what they consider definitive evidence of high clouds blanketing the planet and hiding information about the composition and behavior of the lower atmosphere and surface. The new Hubble spectra also revealed no chemical fingerprints in GJ 1214b's atmosphere, but the data were so precise they could rule out cloud-free compositions of water vapor, methane, nitrogen, carbon monoxide, or carbon dioxide for the first time...."
(NASA)

Planets in the Solar system come in two varieties: smaller ones made mostly of rock and metal, close to the sun; bigger ones made mostly of hydrogen, methane, and other substances that are gasses at Earth-like temperatures. Between the inner and outer planets we found a lot of much smaller solid objects: the asteroids.

Scientists figured that we'd find very nearly the same arrangement around other stars: if we ever found planets out there.

My guess is that we'll find planetary systems like ours: maybe even some with a planet like Earth in the 'Goldilocks zone;' with an atmosphere that's about three quarters nitrogen, one quarter oxygen; a nice mix of oceans, mountains, and plains; and life.

We might even find folks who recently learned how to use fire, or nuclear fission, or who stopped traveling between stars a million years ago. And that's another topic. Topics.

So far, what we're learning is that many planetary systems aren't like ours, and that objects orbiting other stars come in many varieties.

Someone's probably already called the diversity of planetary systems we're discovering "chaotic." It's a cool word these days, and well on its way to becoming a cliche. My opinion.

I'll grant that scientists are still working on explanations for how all these different planets formed. What's surprising, in a way, is how little existing models for stellar formation and evolution have had to be revised as we started testing theoretical models with data from real planetary systems.

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2. "Super-Earth," Yes; Earth-Like, Not So Much

(From G. Bacon, NASA, ESA, and Space Telescope Science Institute; via HubbleSite; used w/o permission.)
"Artist's interpretation of a cloudy exoplanet."

"Hubble Sees Cloudy Super-Worlds with Chance for More Clouds"
HubbleSite news release (December 31, 2013)

"Weather forecasters on exoplanet GJ 1214b would have an easy job. Today's forecast: cloudy. Tomorrow: overcast. Extended outlook: more clouds.

"Two teams of scientists using NASA's Hubble Space Telescope report they have characterized the atmospheres of a pair of planets with masses intermediate between gas giants, like Jupiter, and smaller, rockier planets, like Earth. A survey by NASA's Kepler space telescope mission showed that objects in this size range are among the most common type of planets in our Milky Way galaxy. The researchers described their work as an important milestone on the road to characterizing potentially habitable, Earth-like worlds beyond the solar system....

"The findings appear in separate papers in the January 2 issue of the journal Nature.

"...The two planets studied are known as GJ 436b and GJ 1214b. GJ 436b is categorized as a 'warm Neptune' because it is much closer to its star than frigid Neptune is to our Sun. The planet is located 36 light-years away in the constellation Leo.

"GJ 1214b is known as a 'super-Earth' type planet. Super-Earths are planets with masses between that of Earth and Neptune. Because no such planet exists in our solar system, the physical nature of super-Earths is largely unknown. GJ1214b is located just 40 light-years from Earth, in the constellation Ophiuchus...."

We'll probably find "super-Earths" that are Earth-like, scaled-up versions of our home. These two planets, apart from not having quite enough mass to qualify as 'giant' planets, are very different from ours.

Earth is mostly rock and metal, with an average density of about 5.5 grams per cubic centimeter.

GJ 436b is about four and a third times Earth's diameter, with an average density of about 1.51 grams per cubic centimeter. It may have a solid core, or solid layers, but it can't be made from the same mix of stuff as Earth.

GJ 1214b is about two and two thirds bigger than Earth, in terms of diameter. It's about six and a half times as massive as our world, and if I did the conversion from kilograms and cubic meters right, its average density is 1.87 grams per cubic centimeter. Again, it can't be mostly rock and metal. It's nowhere near dense enough.

Neither of these exoplanets are particularly Earth-like in the sense of closely resembling Earth: but studying them will tell us more about planets in general.

More:

• "Planet GJ 436 b"
  The Extrasolar Planet Encyclopaedia (updated December 31, 2013)
• "Planet GJ 1214 b"
  The Extrasolar Planet Encyclopaedia (updated December 18, 2013)
• "Interior structure models of GJ 436b"
  N. Nettelmann, U. Kramm, R. Redmer, R. Neuhaeuser; Astrophysics; Cornell University Library (Submitted on 24 February 24, 2010 (v1), last revised 1 October 1, 2010 (this version is v2)
• "A super-Earth transiting a nearby low-mass star"
  (GJ 1214b)
  David Charbonneau, Zachory K. Berta, Jonathan Irwin, Christopher J. Burke, Philip Nutzman, Lars A. Buchhave, Christophe Lovis, Xavier Bonfils, David W. Latham, Stephane Udry, Ruth A. Murray-Clay, Matthew J. Holman, Emilio E. Falco, Joshua N. Winn, Didier Queloz, Francesco Pepe, Michel Mayor, Xavier Delfosse, Thierry Forveille; Astrophysics; Cornell University Library (Submitted December 16, 2009)
• "Spitzer Transit and Secondary Eclipse Photometry of GJ 436b"
  Drake Deming, Joseph Harrington, Gregory Laughlin, Sara Seager, Sarah B. Navarro, William C. Bowman, Karen Horning; Astrophysics; Cornell University Library (Submitted July 18, 2007)

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3. The Universe: As Far as We Can See

(From NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team; via Space.com; used w/o permission.)
"Called the eXtreme Deep Field, or XDF, the photo was assembled by combining 10 years of NASA Hubble Space Telescope photographs taken of a patch of sky at the center of the original Hubble Ultra Deep Field. The XDF is a small fraction of the angular diameter of the full Moon. Image released September 25, 2012."

"How Big is the Universe?"
Nola Taylor Redd, Space.com (December 24, 2013)

" As technology has evolved, astronomers are able to look back in time to the moments just after the Big Bang. This might seem to imply that the entire universe lies within our view. But the size of the universe depends on a number of things, including its shape and expansion. Just how big is the universe? The truth is, scientists can't put a number on it. The observable universe

"Astronomers have measured the age of the universe to be approximately 13.8 billion years old. Because of the connection between distance and the speed of light, this means they can look at a region of space that lies 13.8 billion light-years away. Like a ship in the empty ocean, astronomers on Earth can turn their telescopes to peer 13.8 billion light-years in every direction, which puts Earth inside of an observable sphere with a radius of 13.8 billion light-years. The word 'observable' is key; the sphere limits what scientists can see but not what is there.

"But though the sphere appears almost 28 billion light-years in diameter, it is far larger. Scientists know that the universe is expanding. Thus, while scientists might see a spot that lay 13.8 billion light-years from Earth at the time of the Big Bang, the universe has continued to expand over its lifetime. Today, that same spot is 46 billion light-years away, making the diameter of the observable universe a sphere around 92 billion light-years...."

There's big, and there's really big. A sphere that's about 92,000,000,000 light-years across is somewhere beyond 'really big.'

Recapping what Nola Taylor Redd said, the universe is about 13,800,000,000 years old. All we can see is light and other radiation that started traveling when the universe became transparent, shortly after it began.

Since space has been expanding, we can detect objects which are now 46,000,000,000 light-years away.

It gets better: or worse, depending on your point of view. That enormous sphere has Earth in its center. That doesn't mean that we're in the center of the universe. Nola Taylor Redd gave one of the better explanations I've seen.

More a Horizon, Than an Edge

"Centering a sphere on Earth's location in space might seem to put mankind in the center of the universe. However, like that same ship in the ocean, we cannot tell where we lie in the enormous span of the universe. Just because we cannot see land does not mean we are in the center of the ocean; just because we cannot see the edge of the universe does not mean we lie in the center of the universe."
(Nola Taylor Redd, Space.com)

The "edge" of the observable universe is no more or less real than the horizon. It's the limit of what we can observe, but almost certainly not the edge of what exists. A world at what we see as the universe's horizon would be in the center of its own horizon.

Interestingly, our horizon is getting bigger, as light keeps pouring in from the the early moments of this universe. It's not getting bigger by much, though. Each year the observable universe is about two light-year wider: but 92,000,000,002 isn't all that much more than 92,000,000,000.

There's almost certainly more of the universe that's 'over the horizon.'

Across the Horizon, Toward Infinity

"...The shape of the universe

"The size of the universe depends a great deal on its shape. Scientists have predicted the possibility that the universe might be closed like a sphere, infinite and negatively curved like a saddle, or flat and infinite.

"A finite universe has a finite size that can be measured; this would be the case in a closed spherical universe. But an infinite universe has no size by definition.

"According to NASA, scientists know that the universe is flat with only about a 0.4 percent margin of error (as of 2013). A flat universe is an infinite universe; thus the size of the universe is infinite."
(Nola Taylor Redd, Space.com)

Astronomers and cosmologists have been collecting and analyzing data about the space-time continuum for decades, getting increasingly precise results. My guess is that our knowledge of the universe during 2013 isn't the final word: but I'm not surprised that the current best estimate is that the universe is flat.

Maybe new measurements or analysis a year from now show that the universe has positive or negative curvature: or not. Either way, I think we have a very great deal more to learn about this astounding creation.

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4. Oldest Light in the Universe

(From ESA and the Planck Collaboration, via Space.com, used w/o permission.)
"A 2013 map of the background radiation left over from the Big Bang, taken by the ESA's Planck spacecraft, captured the oldest light in the universe. This information helps astronomers determine the age of the universe."

"How Old is the Universe?"
Nola Taylor Redd, Space.com (December 20, 2013)

"Age may only be a number, but when it comes to the age of the universe, it's a pretty important one. According to research, the universe is approximately 13.8 billion years old. How did scientists determine how many candles to put on the universe's birthday cake? They can determine the age of the universe using two different methods: by studying the oldest objects within the universe and measuring how fast it is expanding....

"...The oldest known globular clusters have stars with ages between 11 and 18 billion years old. The wide range comes from problems in pinpointing the distances to the clusters, which affects estimates of brightness and thus mass....

"...The uncertainty still creates a limit to the age of the universe; it must be at least 11 billion years old. It can be older, but not younger....

"...In 2012, WMAP estimated the age of the universe to be 13.772 billion years, with an uncertainty of 59 million years. In 2013, Planck measured the age of the universe at 13.82 billion years...."

We've learned quite a bit about stars and how they work in the last half-century, including how fast they run through their supply of hydrogen.

A star that's ten times as massive as our sun will use fuse hydrogen into helium for something like 20,000,000 to 32,000,000 years before swelling into a red giant. Heavier stars explode as supernovas, producing heavier elements, and that's another topic.

We know more now than we did in 1963: but we still don't have all the answers. That's one reason for uncertainty about the age of stars in the oldest globular clusters.

WMAP is NASA's Wilkinson Microwave Anisotropy Probe, Plank is an ESA probe on a similar mission. They're mapping the CMB, or Cosmic Microwave Background: a sort of background glow that we're pretty sure is from the Big Bang.

When the Fog Cleared

This is one of the better descriptions of what happened about 380,000 years after this universe began. That's when the fog cleared, so to speak:

"...When the universe was young, before the formation of stars and planets, it was denser, much hotter, and filled with a uniform glow from a white-hot fog of hydrogen plasma. As the universe expanded, both the plasma and the radiation filling it grew cooler. When the universe cooled enough, protons and electrons combined to form neutral atoms. These atoms could no longer absorb the thermal radiation, and so the universe became transparent instead of being an opaque fog...."
("Cosmic microwave background," Wikipedia)

Photons existed before that moment, photons but they didn't have much elbow room. Not that they had elbows, of course. Back to Wikipedia.

"...the event shortly afterwards when photons started to travel freely through space rather than constantly being scattered by electrons and protons in plasma is referred to as photon decoupling. The photons that existed at the time of photon decoupling have been propagating ever since..."
("Cosmic microwave background," Wikipedia)

Photons lost energy as they traveled, since the universe has been expanding. These 'stretched' photons from the big decoupling have a longer wavelength now, which is why it's called the cosmic microwave background.

Scientists expected the CMB to be pretty much the same temperature, whichever direction they looked. Instead, they found unexpectedly large irregularities. An intriguing explanation for these cool spots is that they're where our universe collided with others. And that's another topic.

More:

• "Cosmic Microwave Background: Big Bang Relic Explained (Infographic)"
  Karl Tate, Space.com (April 3, 2013)

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