Did Planets from the Inner Disk of the Milky Way Become Rogue Planets?

 We think of a planet as a non-luminous body that orbits a star, like the planets of our solar system. But that’s not always the case.

All stars form from clouds of gas and dust that collapse inward due to the pull of gravity. The pressure caused by the gravitational crunch squeezes gas in the center of each cloud so tightly that it heated the gas to extreme temperatures, generating thermonuclear reactions, and a new star is born. Our sun flared into existence four and a half billion years ago, far younger than the oldest stars, which are born in other parts of the galaxy.

But there was still quite a bit of leftover gas and dust surrounding the young sun which formed a disk around the new star. This leftover bit eventually becomes all of the planets, moons, comets, and asteroids that orbit our sun.

Our solar system lives in the spiral arms of the Milky Way galaxy. Most of the galaxy’s younger stars like our sun are in the spiral arms. Astronomers estimate that virtually all of these stars have planets, an average of two and a half planets per star.

The older stars of our galaxy mostly reside in a bulge surrounding the center of the Milky Way. Stars there have on average barely one planet per star.  MIT astrophysicist Tim Hallatt thinks he knows why. “The puzzle is, these planets (in the spiral arms) are very common,” Hallatt says. “And yet when we look at this other dominant population of stars in the Milky Way, they’re less common. So what’s going on?”

As is the case with all large galaxies like ours, when the Milky Way first formed some 12 billion years ago, star formation was fast and furious, a time Hallatt describes as galactic chaos, what astronomers generally refer to as “cosmic noon.” Also, the stars there were more closely bunched together than the stars in our neighborhood. The greater levels of energetic radiation from the process of rapid star formation plus the relative proximity of stars meant that the stars during cosmic noon experienced ten million times greater levels of radiation. This intense radiation would have heated the gas surrounding all these rapidly forming stars. The greater levels of radiation and heat blew away much of the remaining gas, leaving less raw material for planets.

When astronomers search for planets beyond our own solar system, they look at other stars. We currently know of more than 5000 such exoplanets, and the more we look, the more we find. Astronomers also find lots of rogue planets, planets that don’t orbit any star. They may have formed around a star but were ejected from their home stellar system, perhaps due to close passage of another star. The gravity of the passing star can rip a planet away from its home. Astronomers estimate that perhaps as many as four trillion rogue planets exist in our galaxy alone. That’s a huge number.

Artist's conception of a Rogue Planet

It’s likely that some of those rogue planets formed on their own, not as part of a stellar system. Perhaps some of the gas and dust blown out by the stars formed during the crowded cosmic noon eventually coalesced into rogue planets. Many of these rogue planets could have orbited a star but for the early period of rapid star growth.

Rogue planets may easily outnumber the stars in our galaxy. And some of those rogue stars may be causalities of the cosmic noon timeframe of our Milky Way galaxy.

Each month, I write an astronomy-related column for the Oklahoman newspaper. After it is published there, I post that same column to my blog page.

This is reprinted with permission from the Oklahoman and Oklahoman.com.

Are We Close to Finding Earth 2?

 

NASA and other space agencies have launched several missions to search for exoplanets, planets that orbit other stars. The Holy Grail of such programs is finding an Earth-like planet orbiting a sun-like star at the right distance to allow liquid water on the surface. Such planets seem to be the most likely candidates to search for life.

While astronomers have yet to find a perfect Earth 2, statistical analysis of NASA’s most successful planet hunter, the Kepler Mission, uncovered some promising data. A study by NASA scientists alongside collaborators from around the world who worked on the Kepler mission came to an exciting conclusion. According to the research, about half the stars similar in temperature to our Sun could have a rocky planet capable of supporting liquid water on its surface.

Image Caption/Credit: NASA’s Kepler Planet Finder telescope, credit NASA

"Kepler already told us there were billions of planets, but now we know a good chunk of those planets might be rocky and habitable," said the lead author Steve Bryson, a researcher at NASA's Ames Research. "Though this result is far from a final value, and water on a planet's surface is only one of many factors to support life, it's extremely exciting that we calculated these worlds are this common with such high confidence and precision."

Kepler detected planets by continuously staring at thousands of stars, watching for a tell-tale drop in brightness caused by an orbiting planet crossing in front of a star. Such a method couldn’t detect planetary systems seen more face on, so astronomers had to use statistical methods to extrapolate from the Kepler data to all the other stars in our galaxy. Kepler discovered so many exoplanets from its limited mission that astronomers now believe that more than half of the four billion stars in the Milky planet possess planets, typically more than one.

Using their most conservative estimate, that 7% of all sun-like stars have Earth-like planets, meaning some 300 million exist in our Milky Way alone. Their most likely estimate states that Earth-like planets orbit 50% of sun-like stars, making more than 2 billion Earth-like planets. Since we know of only one planet with life, ours, those planets are the best place to begin to search for alien forms of life.

Each month, I write an astronomy-related column piece for the Oklahoman newspaper. On the following day, I post that same column to my blog page.

 This is reprinted by permission from the Oklahoman and www.newsok.com.

Where is Everybody?

One summer day in 1950, physicists Enrico Fermi, leader of the team that built the first nuclear reactor, enjoyed lunch with several fellow physicists. Talk turned to recent UFO reports and the possibility of faster than light travel. Referring to aliens, Fermi suddenly blurted out "Don't you ever wonder where everybody is?" This question led to “The Fermi Paradox.”

Simply stated, if (as we now know) our galaxy contains billions of planets, many of them Earth-like, why haven’t we ever encountered any aliens? Earth is young, in cosmic terms. Many solar systems older than Earth exist, some billions of years older. If life existed on any of them, it would take no more than 50 million years to colonize the galaxy with the rocket technology being developed at that time, a blink of the eye in cosmic terms. So, Fermi wondered aloud, where is everybody?

Over the years, other scientists have suggested various answers.

Water is essential to life as we know it. Earth’s water exists on the surface, but for most of the other worlds in our solar system that possess oceans, four moons, the water is locked underground, below a frozen surface. That may be common for life-bearing planets or moons.

Many of the planets we’ve discovered that could have water are classified as super-Earths, planets up to ten times larger than Earth. The gravity of such planets may well make space travel impossible.

Futurist and astronomer Seth Shostak, says that all intelligent aliens may actually be intelligent machines. We ourselves are on the verge of creating such machines and, within a few thousands of years, all intelligence on Earth may be machine-based, not biological.

Some scientists suggest that alien life may be so different from us, we’d never recognize it. Or, perhaps just as we destroy ants without even realizing it when building a house, maybe all other life forms have been eradicated. Or they wiped themselves out with climate change.

Maybe we have met the aliens and they are us. The “panspermia hypothesis” says life was seeded on Earth by comets (or alien spacecraft?), making us the very aliens we search for.

Even so, we need to keep looking.

From NASA's search for extraterrestrial life. Credit NASA-HST

On or about the first Tuesday of each month, I write an astronomy-related column piece for the Oklahoman newspaper. On the following day, I post that same column to my blog page.

This is reprinted by permission from the Oklahoman and www.newsok.com.

Our Nearest Potentially Habitable Cosmic Neighbor

According to NASA’s Exoplanet web page (exoplanetarchive.ipac.caltech.edu/index.html), we know of 3,779 planets orbiting stars other than our sun, with 2,737 more candidates awaiting confirmation. And that is just the tip of the exoplanet iceberg.

The large majority of confirmed and suspected exoplanets are discovered by the transit method. A telescope in space or on Earth stares at a star and watches for small drops in the light output that indicate a planet is passing in front of, or transiting, the star. There are many ways a star’s light may vary, but each has a specific signature as to how the brightness varies. Planetary transits cause a unique alteration in the star’s light.

This method can only detect planets whose orbit lies along our line of sight, and that’s quite unlikely. That astronomers have found so many exoplanets when they can only detect such a tiny fraction of potential candidates implies a huge number of exoplanets exist. In fact, astronomers estimate based on the known sample that the 400 billion stars in the Milky Way average 1.3 planets each.

That’s a lot of planets.

It turns that that one exoplanet is literally right next door. The closest star system to us is Alpha Centauri. It actually consists of three stars. Two of them, Alpha Centauri A and B, both roughly the size of our sun, orbit each other rather closely. The third member, named Proxima Centauri, orbits those two in a wide, 550,000-year orbit. Proxima comes closer to us than any other star, 4.2 light years at its closest.

Proxima is known to possess a planet only slightly larger than earth. And the planet lies in the star’s habitable zone, where the star provides enough heat to allow liquid water, as on Earth. Since Proxima is a red dwarf star, much smaller than our sun, the planet must orbit close to the star to be warm enough. The planet’s orbit takes only 11 days. But it is exactly in the middle of Proxima’s habitable zone. Being so close to the parent star, it is probably tidally locked. One side constantly faces the star, just like only one side of the Moon always faces Earth. This means one side is in constant daylight, the other perpetual night.

Artist conception of Proxima Centauri b - credit NASA

Anthony Del Genio, a planetary scientist at the NASA Goddard Institute for Space Studies, led a group of scientists doing computer simulations on Proxima b. They wanted to know if the planet could support life. They made the reasonable assumptions that the planet had an atmosphere as thick as Earth’s and enough water to form an ocean. Using computer models like those used by researches to study climate change on Earth, they found that under a broad range of conditions, the planet can sustain liquid water even on the night side. On Earth, where there’s water, there’s life. “The major message from our simulations is that there’s a decent chance that the planet would be habitable,” said Del Genio.

Our nearest habitable neighbor may literally orbit our nearest stellar neighbor.

On the first Tuesday of each month, I write an astronomy-related column piece for the Oklahoman newspaper. On the following day, I post that same column to my blog page.

This is reprinted by permission form the Oklahoman and www.newsok.com.

How common Are Earth-Like Planets? At Least They're Made of the Same Stuff.

On the first Tuesday of each month, I write an astronomy-related column piece for the Oklahoman newspaper. On the following day, I post that same column to my blog page.

This is reprinted by permission form the Oklahoman and www.newsok.com.

With nearly 4,000 known planets orbiting other stars (exoplanets), a few questions inevitably come up from both scientists and lay people alike: How similar are they to Earth? Do they have a composition similar to our planet? Can they support life? The problem with answering these questions is that exoplanets are tiny and extremely faint compared to the stars they orbit. Any signal from them that might help answer these questions is drowned out by the parent star.

Now, scientists have figured out a way to answer one of those questions, that of the composition of other planetary systems. We can’t directly measure the composition of the planets, but as parent stars age and evolve, they present a way to determine planetary composition.

When a sun-like star evolves to its final state, a white dwarf, it contains almost nothing but hydrogen and helium. As Dr. Siyi Xu of the Gemini Observatory in Hawaii and one of the authors of the new study explained, “White dwarfs’ atmospheres are composed of either hydrogen or helium, which give out a pretty clear and clean spectroscopic signal. However, as the star cools, it begins to pull in material from the planets, asteroids, comets and so on which had been orbiting it, with some forming a dust disk, a little like the rings of Saturn. As this material approaches the star, it changes how we see the star.”

Gemini South Observatory. Credit NSF

The star’s light shines through the dusty rings allowing astronomers here on Earth to determine the composition of the dust. It turns out, as Dr. Xu explains, “Most of the building blocks we have looked at in other planetary systems have a composition broadly similar to that of the Earth.” Such studies don’t yet tell us if the planets have water, believed to be a prime ingredient necessary for life. But they reveal that Earth’s overall composition is rather common. And since water is one of the most abundant compounds in the universe, it seems likely that if other factors are similar to our own solar system then water exits in those planetary systems as well.

Dr. Robert Jedicke of the University of Hawaii studies our moons. That’s right, plural. We are all quite familiar with our big, bright Moon in the sky. But our solar system occasionally picks up hitchhikers in the form of small asteroids that pass near us. These mini-moons, as Dr. Jedicke calls them, allows us to study wandering asteroids to get a better look at them than we can from their distance in the Asteroid Belt. Not only will we learn more about them, Dr. Jedicke tells us they offer an even more exciting possibility. "Mini-moons are perfect targets for bringing back significant chunks of asteroid material, shielded by a spacecraft, which could then be studied in detail back on Earth." Such access to asteroids opens up both scientific and, possibly, financial opportunities, as asteroids contain significant amounts of precious metals and, perhaps more importantly, rare-earth metals, essential for our computer technology.