Our Galactic Environment

While I’m focused on the Earth these days, I can’t help looking up and thinking about outer space now and then.

So, let me tell you about the Kuiper Belt, the heliosphere, the Local Bubble—and what may happen when our Solar System hits the next big cloud! Could it affect the climate on Earth?

New Horizons

We’re going on a big adventure!

New Horizons has already taken great photos of volcanoes on Jupiter’s moon Io. It’s already closer to Pluto than we’ve ever been. And on 14 July 2016 it will fly by Pluto and its moons Charon, Hydra, and Nix!

But that’s just the start: then it will go to see some KBOs!

The Kuiper Belt stretches from the orbit of Neptune to almost twice as far from the Sun. It’s a bit like the asteroid belt, but much bigger: 20 times as wide and 20 – 200 times as massive. But while most asteroids are made of rock and metal, most Kuiper Belt Objects or ‘KBOs’ are composed largely of frozen methane, ammonia and water.

The Earth’s orbit has a radius of one astronomical unit, or AU. The Kuiper Belt goes from 30 AU to 50 AU out. For comparison, the heliosphere, the region dominated by the energetic fast-flowing solar wind, fizzles out around 120 AU. That’s where Voyager 1 is now.

New Horizons will fly through the Kuiper Belt from 2016 to 2020… and, according to plan, its mission will end in 2026. How far out will it be then? I don’t know! Of course it will keep going…

For more see:

• JPL, New Horizons: NASA’s Pluto-Kuiper Belt Mission.

The heliosphere

Here’s a young star zipping through the Orion Nebula. It’s called LL Orionis, and this picture was taken by the Hubble Telescope in February 1995:

The star is moving through the interstellar gas at supersonic speeds. So, when this gas hits the fast wind of particles shooting out from the star, it creates a bow shock half a light-year across. It’s a bit like when a boat moves through the water faster than the speed of water waves.

There’s also a bow shock where the solar wind hits the Earth’s magnetic field. It’s about 17 kilometers thick, and located about 90,000 kilometers from Earth:

For a long time scientists thought there was a bow shock where nearby interstellar gas hit the Sun’s solar wind. But this was called into question this year when a satellite called the Interstellar Boundary Explorer (IBEX) discovered the Solar System is moving slower relative to this gas than we thought!

IBEX isn’t actually going to the edge of the heliosphere—it’s in Earth orbit, looking out. But Voyager 1 seems close to hitting the heliopause, where the Earth’s solar wind comes to a stop. And it’s seeing strange things!

The Interstellar Boundary Explorer

The Sun shoots out a hot wind of ions moving at 300 to 800 kilometers per second. They form a kind of bubble in space: the heliosphere. These charged particles slow down and stop when they hit the hydrogen and helium atoms in interstellar space. But those atoms can penetrate the heliosphere, at least when they’re neutral—and a near-earth satellite called IBEX, the Interstellar Boundary Explorer, has been watching them! And here’s what IBEX has seen:

In December 2008, IBEX first started detecting energetic neutral atoms penetrating the heliosphere. By October 2009 it had collected enough data to see the ‘IBEX ribbon’: an unexpected arc-shaped region in the sky has many more energetic neutral atoms than expected. You can see it here!

The color shows how many hundreds of energetic neutral atoms are hitting the heliosphere per second per square centimeter per keV. A keV, or kilo-electron-volt, is a unit of energy. Different atoms are moving with different energies, so it makes sense to count them this way.

You can see how the Voyager spacecraft are close to leaving the heliosphere. You can also see how the interstellar magnetic field lines avoid this bubble. Ever since the IBEX ribbon was detected, the IBEX team has been trying to figure out what causes it. They think it’s related to the interstellar magnetic field. The ribbon has been moving and changing intensity quite a bit in the couple of years they’ve been watching it!

Recently, IBEX announced that our Solar System has no bow shock—a big surprise. Previously, scientists thought the heliosphere created a bow-shaped shock wave in the interstellar gas as it moved along, like that star in the Orion Nebula we just looked at.

The Local Bubble

Get to know the neighborhood!

I love the names of these nearby stars! Some I knew: Vega, Altair, Fomalhaut, Alpha Centauri, Sirius, Procyon, Denebola, Pollux, Castor, Mizar, Aldebaran, Algol. But many I didn’t: Rasalhague, Skat, Gaorux, Pherkad, Thuban, Phact, Alphard, Wazn, and Algieba! How come none of the science fiction I’ve read uses these great names? Or maybe I just forgot.

The Local Bubble is a bubble of hot interstellar gas 300 light years across, probably blasted out by the supernova called Geminga near the bottom of this picture.

Geminga

Here’s the sky viewed in gamma rays. A lot come from a blazar 7 billion light years away that erupted in 2005: a supermassive black hole at the center of a galaxy, firing particles in a jet that happens to be aimed straight at us. Some come from nearby pulsars: rapidly rotating neutron stars formed by the collapse of stars that went supernova. The one I want you to think about is Geminga.

Geminga is just 800 light years away from us, and it exploded only 300,000 years ago! That may seem far away and long ago to you, but not to me. The first Neanderthalers go back around 350,000 years… and they would have seen this supernova in the daytime, it was so close.

But here’s the reason I want you to think about Geminga. It seems to have blasted out the bubble of hot low-density gas our Solar System finds itself in: the Local Bubble. Astronomers have even detected micrometer-sized interstellar meteor particles coming from its direction!

We may think of interstellar space as all the same—empty and boring—but that’s far from true. The density of interstellar space varies immensely from place to place! The Local Bubble has just 0.05 atoms per cubic centimeter, but the average in our galaxy is about 20 times that, and we’re heading toward some giant clouds that are 2000 to 20,000 times as dense. The fun will start when we hit those…. but more on that later.

Nearby clouds

While we live in the Local Bubble, several thousand years ago we entered a small cloud of cooler, denser gas: the Local Fluff. We’ll leave this in at most 4 thousand years. But that’s just the beginning! As we pass the Scorpius-Centaurus Association, we’ll hit bigger, colder and denser clouds—and they’ll squash the heliosphere.

When will this happen? People seem very unsure. I’ve seen different sources saying we entered the Local Fluff sometime between 44,000 and 150,000 years ago, and that we’ll stay within it for between 4,000 and 20,000 years.

We’ll then return to the hotter, less dense gas of the Local Bubble until we hit the next cloud. That may take at least 50,000 years. Two candidates for the first cloud we’ll hit are the G Cloud and the Apex Cloud. The Apex Cloud is just 15 light years away:

• Priscilla C. Frisch, Local interstellar matter: the Apex Cloud.

When we hit a big cloud, it will squash the heliosphere. Right now, remember, this is roughly 120 AU in radius. But before we entered the Local Fluff, it was much bigger. And when we hit thicker clouds, it may shrink down to just 1 or 2 AU!

The heliosphere protects us from galactic cosmic rays. So, when we hit the next cloud, more of these cosmic rays will reach the Earth. Nobody knows for sure what the effects will be… but life on Earth has survived previous incidents like this, and other problems will hit us much sooner, so don’t stay awake at night worrying about it!

Indeed, ice core samples from the Antarctic show spikes in the concentration of the radioactive isotope beryllium-10 in two seperate events, one about 60,000 years ago and another about 33,000 years ago. These might have been caused by a sudden increase in cosmic rays. But nobody is really sure.

People have studied the possibility that cosmic rays could influence the Earth’s weather, for example by seeding clouds:

• K. Scherer, H. Fichtner et al, Interstellar-terrestrial relations: variable cosmic environments, the dynamic heliosphere, and their imprints on terrestrial archives and climate, Space Science Reviews 127 (2006), 327–465.

• Benjamin A. Laken, Enric Pallé, Jaša Čalogović and Eimear M. Dunne, A cosmic ray-climate link and cloud observations, J. Space Weather Space Clim. 2 (2012), A18.

Despite the title of the second paper, its conclusion is that “it is clear that there is no robust evidence of a widespread link between the cosmic ray flux and clouds.” That’s clouds on Earth, not clouds of interstellar gas! The first paper is much more optimistic about the existence of such a link, but it doesn’t provide a ‘smoking gun’.

And—in case you’re wondering—variations in cosmic rays this century don’t line up with global warming:

The top curves are the Earth’s temperature as estimated by GISTEMP (the brown curve), and the carbon dioxide concentration in the Earth’s atmosphere as measured by Charles David Keeling (in green). The bottom ones are galactic cosmic rays as measured by CLIMAX (the gray dots), the sunspot cycle as measured by the Solar Influences Data Analysis Center (in red), and total solar irradiance as estimated by Judith Lean (in blue).

But be careful: the galactic cosmic ray curve has been flipped upside down, since when solar activity is high, then fewer galactic cosmic rays make it to Earth! You can see that here:

I’m sorry these graphs aren’t neatly lined up, but you can see that peaks in the sunspot cycle happened near 1980, 1989 and 2002, which is when we had minima in the galactic cosmic rays.

For more on the neighborhood of the Solar System and what to expect as we pass through various interstellar clouds, try this great article:

• Priscilla Frisch, The galactic environment of the Sun, American Scientist 88 (January-February 2000).

I have lots of scientific heroes: whenever I study something, I find impressive people have already been there. This week my hero is Priscilla Frisch. She edited a book called Solar Journey: The Significance of Our Galactic Environment for the Heliosphere and Earth. The book isn’t free, but this chapter is:

• Priscilla C. Frisch and Jonathan D. Slavin, Short-term variations in the galactic environment of the Sun.

For more on how what the heliosphere might do when we hit the next big cloud, see:

• Hans-R. Mueller, Priscilla C. Frisch, Vladimir Florinski and Gary P. Zank, Heliospheric response to different possible interstellar environments.

The Aquila Rift

Just for fun, let’s conclude by leaving our immediate neighborhood and going a bit further out. Here’s a picture of the Aquila Rift, taken by Adam Block of the Mt. Lemmon SkyCenter at the University of Arizona:

The Aquila Rift is a region of molecular clouds about 600 light years away in the direction of the star Altair. Hundreds of stars are being formed in these clouds.

A molecular cloud is a region in space where the interstellar gas gets so dense that hydrogen forms molecules, instead of lone atoms. While the Local Fluff near us has about 0.3 atoms per cubic centimeter, and the Local Bubble is much less dense, a molecular cloud can easily have 100 or 1000 atoms per cubic centimeter. Molecular clouds often contain filaments, sheets, and clumps of submicrometer-sized dust particles, coated with frozen carbon monoxide and nitrogen. That’s the dark stuff here!

I don’t know what will happen to the Earth when our Solar System hits a really dense molecular cloud. It might have already happened once. But it probably won’t happen again for a long time.