One out of every six stars in our Milky Way galaxy has an Earth-like planet orbiting it, according to two new studies. (Photo: Steve Jurvetson via Wikimedia Commons)

There are at least 17 billion Earth-sized worlds in our Milky Way galaxy, according to two new studies.

Both groups of scientists used data from NASA’s Kepler mission to reach their conclusions, which were presented to the American Astronomical Society in Long Beach, California.

The scientists found that the closer the planets are to their stars, the easier they are to find because they transit more frequently, giving scientists more opportunities to observe them.

One group, led by Francois Fressin of the Harvard-Smithsonian Center for Astrophysics (CfA), said its studies show 50 percent of stars in our galaxy have a planet the size of Earth or larger closely orbiting them.

Add in larger planets, which have been found to be in wider orbits around its star, and the percentage of stars with planets goes up to 70 percent, according to the researchers.

Based on current ongoing observations from the Kepler mission, along with others using different detection techniques, it looks like practically all Sun-like stars have planets, according to the Harvard-Smithsonian team.

A second group of researchers, from the University of California, Berkeley and the University of Hawaii at Manoa, found smaller exoplanets to be much more plentiful than larger ones in the star systems it observed. The analysis also confirmed that the frequency of planets increased as its size decreased, which team member Andrew Howard and the Kepler team reported last year.

Perhaps one percent of stars have planets the size of Jupiter, while 10 percent have planets the size of Neptune, according to the Berkeley/ Hawaii team. The group’s research also shows the exoplanets they observed, which were two or three times the diameter of Earth, are typically more like our solar system’s Uranus and Neptune, each of which has a rocky core  surrounded by helium and hydrogen gases and, perhaps, water.

Artist’s conception of the wide variety of planets detected by NASA’s Kepler spacecraft. (C. Pulliam & D. Aguilar/CfA)

They suggest planets orbiting close to their stars may even be water worlds, with oceans hundreds of kilometers deep, surrounding a rocky core.

Although the planets between one to two times larger than Earth may not necessarily be habitable,  the Berkeley/Hawaii team  said those planets might be rocky and, if they’re located within what they call the “Goldilocks zone” –not too hot, not too cold, just right for liquid water– could support life.

The Harvard-Smithsonian researchers found that, except for the gas giants, the type of star didn’t really have much effect on the size of its planets, contradicting previous findings. Neptune-type planets, they said, can be found just as frequently orbiting around relatively cool stars, called red dwarfs, as they are around sun-like stars. The same is true for smaller worlds.

“Earths and super-Earths aren’t picky,” said Guillermo Torres of the Harvard-Smithsonian team. “We’re finding them in all kinds of neighborhoods.”

As more data is gathered, more planets in larger orbits will be revealed, according to the Harvard-Smithsonian researchers. They say when Kepler’s mission is extended, astronomers should be able to spot Earth-sized planets at greater distances, including those with Earth-like orbits within the habitable zone.

The newly discovered planet PH1 is depicted in this artist’s rendition transiting the larger of the its two host stars. Left of the planet, off in the distance, is a second pair of stars that orbit PH1. (Image: Haven Giguere/Yale)

The recent discovery of a four-star planet highlights the growing value of citizen scientists, non-professional enthusiasts – including  hobbyists, educators and amateur scientists – who participate in scientific endeavors.

The discovery of the extrasolar planet dubbed PH1, which orbits twin suns and is itself orbited by a second distant pair of stars, was made through the combined efforts of volunteer citizen scientists and  professional astronomers.

The amateur scientists behind the sighting of the phenomenon, called a “circumbinary planet in a four-star system,” were able to get involved with astronomical research through the website Planethunters.org, a collaboration between Yale University and the online citizen science project Zooniverse.

“Planet Hunters is a symbiotic project, pairing the discovery power of the people with follow-up by a team of astronomers,” says Debra Fischer, a planet expert and professor of astronomy at Yale, who helped launch the website in 2010. “This unique system might have been entirely missed if not for the sharp eyes of the public.”

Through Planethunters.org, volunteers help professional astronomers sift through mountains of data  gathered by NASA’s Kepler space mission.

That assistance includes brightness measurements, or “light curves,” for more than 150,000 stars, taken by instruments aboard Kepler  every 30 minutes.

Artist’s rendition of the newly discovered circumbinary planet PH1 that orbits two suns (upper left) and is 5,000 light years from Earth. (Image: Haven Giguere/Yale)

The Planet Hunters pour through the data looking for possible transit events, a brief dip in brightness that occurs when a planet or other celestial body passes in front of the star.

“The thousands of people who are involved with Planet Hunters are performing a valuable service,” says Jerome Orosz, associate professor of astronomy at San Diego State University and co-author of the paper outlining PH1’s discovery. “Many of the automated techniques used to find interesting features in the Kepler data don’t always work as efficiently as we would like. The hard work of the Planet Hunters helps ensure that important discoveries are not falling through the cracks.”

PH1, some 5000 light years from Earth, has been described by astronomers as a gas giant planet slightly bigger than Neptune.  It takes the extrasolar planet 138 days to complete an orbit around its two host stars, which have masses about 1.5 and 0.41 times that of the sun and also circle each other once every 20 days.  The two distant stars that orbit the new-found planet are about 144,840,960,000 km away, roughly 1000 times the distance between the Earth and the Sun.

Yale’s Meg Schwamb of Yale, the lead author of the paper said that the circumbinary planets that are being discovered are the extremes of planet formation.

“The discovery of these systems is forcing us to go back to the drawing board to understand how such planets can assemble and evolve in these dynamically challenging environments.”

This June 8, 2004 photo shows the transit of Venus, which occurs when the planet Venus passes between the Earth and the sun, pictured in Hong Kong. (AP Photo/Vincent Yu,File)

On Tuesday,  June 5, people in most parts of the world have a good chance of witnessing a once-in-a-lifetime astronomical event called the transit of Venus.

It’s a planetary alignment which occurs when the planet Venus passes directly between the Earth and sun.

For those of us here on Earth, it’ll look like a little black dot beginning on the edge of the sun and gliding across the face of it over a couple of hours.

The transit of Venus is expected to occur at around 2209 UTC.  Of course,  clouds could obscure the view and people in Portugal, southern Spain, western Africa, and the portions of South America are unlikely to see the occurrence because that part of the world will be in total darkness the entire time of the transit.

The striking event occurs about once a century, in pairs, with around eight years separating each twin event.  This upcoming occurrence is the second in a recent pair of events, the first of which took place in 2004.

Tomorrow’s transit will mark only the seventh time the phenomenon has been seen since it was first observed in 1639 by Jeremiah Horrocks and William Crabtree of England.

In 1627, astronomer Johannes Kepler became the first person to successfully forecast a transit of Venus when he predicted the 1631 event.

According to NASA, the 18th century astronomer, Edmund Halley – of Halley’s Comet fame – came up with a way of using the transit of Venus to determine the distance between Earth and the sun. He suggested  measuring the start and end times of the transit from different points on Earth, using methods of triangulation to make the calculation.

The method was first tried in 1761, but the international team’s efforts were unsuccessful due to poor weather and other factors.

If you want to watch the transit of Venus be sure to take precautionary measures. These people, for example are using solar viewing glasses to safely view the Sun. (Photo: NASA)

Another effort, in 1769,  with observation points all over the world, proved more successful, providing statistical data that led scientists to calculate the mean distance from the Earth to the sun was around 150,838,824 kilometers. Using more modern, technologically-advanced equipment, scientists now say the mean distance from the Earth to sun is 149,600,000 kilometers. Seems that, with Sir Edmund Halley’s help, the 18th century calculations were pretty darn close.

Speaking of NASA, the space agency announced that Don Pettit, a flight engineer on the International Space Station, will photograph the event. It’s the first time the transit of Venus has been observed from space.  The astronauts aboard the ISS in 2004 weren’t able to see the transit because they didn’t have any solar filters which would allow them to safely observe it.

Observatories and other various science-oriented institutions from around the world will be opening their doors to host transit watching parties and events.

World map show visibility of transit of Venus on June 5-6, 2012. (Map: Michael Zeiler)

If you’re planning on watching the transit of Venus, please make sure to take precautions, since looking at the sun directly can cause serious damage to your eyes.  Experts recommend you use protective glasses or telescope lenses with special solar filters, or better yet, watch it on TV or the Web.  Good luck with your observations because the next set of transits won’t take place until 105 years from now, in 2117.