Using NASA’s Kepler satellite, astronomers have found about 1,000 planets around stars in the Milky Way and they have also found about 3,000 other potential planets. Many of the stars have planetary systems with 2-6 planets, but the stars could very well have more planets than those observable with the Kepler satellite, which is best suited for finding large planets that orbit relatively close to their stars.
Planets that orbit close to their stars would be too scorching hot to have life, so to find out if such planetary systems might also have planets in the habitable zone with the potential for liquid water and life, a group of researchers from the Australian National University and the Niels Bohr Institute at the University of Copenhagen made calculations based on a new version of a 250-year-old method called the Titius-Bode law.
The Titius-Bode law was formulated around 1770 and correctly calculated the position of Uranus before it was even discovered. The law states that there is a certain ratio between the orbital periods of planets in a solar system. So the ratio between the orbital period of the first and second planet is the same as the ratio between the second and the third planet and so on. Therefore, if you knew how long it takes for some of the planets to orbit around the Sun/star, you can calculate how long it takes for the other planets to orbit and can thus calculate their position in the planetary system. You can also calculate if a planet is ‘missing’ in the sequence.
“We decided to use this method to calculate the potential planetary positions in 151 planetary systems, where the Kepler satellite had found between 3 and 6 planets. In 124 of the planetary systems, the Titius-Bode law fit with the position of the planets. Using T-B’s law we tried to predict where there could be more planets further out in the planetary systems. But we only made calculations for planets where there is a good chance that you can see them with the Kepler satellite,” explains Steffen Kjær Jacobsen, PhD student in the research group Astrophysics and Planetary Science at the Niels Bohr Institute at the University of Copenhagen.
In 27 of the 151 planetary systems, the planets that had been observed did not fit the T-B law at first glance. They then tried to place planets into the ‘pattern’ for where planets should be located. Then they added the planets that seemed to be missing between the already known planets and also added one extra planet in the system beyond the outermost known planet. In this way, they predicted a total of 228 planets in the 151 planetary systems.
“We then made a priority list with 77 planets in 40 planetary systems to focus on because they have a high probability of making a transit, so you can see them with Kepler. We have encouraged other researchers to look for these. If they are found, it is an indication that the theory stands up,” explains Steffen Kjær Jacobsen.
Planets that orbit very close around a star are too scorching hot to have liquid water and life and planets that are far from the star would be too deep-frozen, but the intermediate habitable zone, where there is the potential for liquid water and life, is not a fixed distance. The habitable zone for a planetary system will be different from star to star, depending on how big and bright the star is.
The researchers evaluated the number of planets in the habitable zone based on the extra planets that were added to the 151 planetary systems according to the Titius-Bode law.
***The result was 1-3 planets in the habitable zone for each planetary system.***
Out of the 151 planetary systems, they now made an additional check on 31 planetary systems where they had already found planets in the habitable zone or where only a single extra planet was needed to meet the requirements.
“In these 31 planetary systems that were close to the habitable zone, our calculations showed that there was an average of two planets in the habitable zone. According to the statistics and the indications we have, a good share of the planets in the habitable zone will be solid planets where there might be liquid water and where life could exist,” explains Steffen Kjær Jacobsen.
If you then take the calculations further out into space, it would mean that just in our galaxy, the Milky Way, ***there could be billions of stars with planets in the habitable zone***, where there could be liquid water and where life could exist.
He explains that what they now want to do is encourage other researchers to look at the Kepler data again for the 40 planetary systems that they have predicted should be well placed to be observed with the Kepler satellite.
Planetary scientists have calculated that there are hundreds of billions of Earth-like planets in our galaxy which might support life.
They found the standard star has about two planets in the so-called Goldilocks zone, the distance from the star where liquid water, crucial for life, can exist.
“The ingredients for life are plentiful, and we now know that habitable environments are plentiful,” said Associate Professor Lineweaver, from the ANU Research School of Astronomy and Astrophysics and the Research School of Earth Sciences.
“However, the universe is not teeming with aliens with human-like intelligence that can build radio telescopes and space ships. Otherwise we would have seen or heard from them.
"It could be that there is some other bottleneck for the emergence of life that we haven’t worked out yet. Or intelligent civilisations evolve, but then self-destruct.”
There are a lot of assumptions here: For one, that alien biology will have comparable physical requirements to our own. If biotic life isn’t limited to Earth-sized planets in the habitable zone—a restriction that precludes the icy moons Europa and Titan—the number of life-harboring worlds could actually be much higher.
Using the Very Large Telescope Interferometer (VLTI) in near-infrared light, the team of astronomers observed 92 nearby stars to probe exozodiacal light from hot dust close to their habitable zones and combined the new data with earlier observations. Bright exozodiacal light, created by the glowing grains of hot exozodiacal dust, or the reflection of starlight off these grains, was observed around nine of the targeted stars.
From dark clear sites on Earth, zodiacal light looks like a faint diffuse white glow seen in the night sky after the end of twilight, or before dawn. It is created by sunlight reflected off tiny particles and appears to extend up from the vicinity of the Sun. This reflected light is not just observed from Earth but can be observed from everywhere in the Solar System.
The glow being observed in this new study is a much more extreme version of the same phenomenon. While this exozodiacal light—zodiacal light around other star systems—had been previously detected, this is the first large systematic study of this phenomenon around nearby stars.
In contrast to earlier observations the team did not observe dust that will later form into planets, but dust created in collisions between small planets of a few kilometres in size—objects called planetesimals that are similar to the asteroids and comets of the Solar System. Dust of this kind is also the origin of the zodiacal light in the Solar System.
“If we want to study the evolution of Earth-like planets close to the habitable zone, we need to observe the zodiacal dust in this region around other stars,” said Steve Ertel, lead author of the paper, from ESO and the University of Grenoble in France. “Detecting and characterising this kind of dust around other stars is a way to study the architecture and evolution of planetary systems.”
By analysing the properties of the stars surrounded by a disc of exozodiacal dust, the team found that most of the dust was detected around older stars. This result was very surprising and raises some questions for our understanding of planetary systems. Any known dust production caused by collisions of planetesimals should diminish over time, as the number of planetesimals is reduced as they are destroyed.
The sample of observed objects also included 14 stars for which the detection of exoplanets has been reported. All of these planets are in the same region of the system as the dust in the systems showing exozodiacal light. The presence of exozodiacal light in systems with planets may create a problem for further astronomical studies of exoplanets.
Exozodiacal dust emission, even at low levels, makes it significantly harder to detect Earth-like planets with direct imaging. The exozodiacal light detected in this survey is a factor of 1000 times brighter than the zodiacal light seen around the Sun. The number of stars containing zodiacal light at the level of the Solar System is most likely much higher than the numbers found in the survey. These observations are therefore only a first step towards more detailed studies of exozodiacal light.
“The high detection rate found at this bright level suggests that there must be a significant number of systems containing fainter dust, undetectable in our survey, but still much brighter than the Solar System’s zodiacal dust,” explains Olivier Absil, co-author of the paper, from the University of Liège. “The presence of such dust in so many systems could therefore become an obstacle for future observations, which aim to make direct images of Earth-like exoplanets.”
Planet OGLE-2013-BLG-0341LBb Orbits in Binary Star System
Although the planet orbits one of the two stars at almost the same distance that the Earth orbits the Sun, that particular star is much dimmer, meaning that the planet itself is probably colder than Jupiter’s moon Europa.
That said, the researchers say the dynamics of the system show that it is possible for the planet to maintain a stable orbit if that parent star was of the same mass and energy as the Sun. If that were the case, the planet could be considered habitable.
“This greatly expands the potential locations to discover habitable planets in the future,” said Scott Gaudi, an astronomer researcher at Ohio State University who participated in the research. “Half the stars in the galaxy are in binary systems. We had no idea if Earth-like planets in Earth-like orbits could even form in these systems.”
Are ‘Super-Earths’ and ‘Habitable Zones’ Misleading Terms?
The “burdensome” problem for the scientific community in describing a habitable zone is that scientists know little about where life forms, Marcy said.
“There’s a split brain that we scientists have right now. Half of our brain says there’s a habitable zone and it lies between a region inward of where the Earth is and a region outside the Earth’s orbit [for stars our size],” he said. “The other half of our brain knows perfectly well that excellent destinations for our search for life lie elsewhere in the Solar System.”
Are ‘Super-Earths’ and ‘Habitable Zones’ Misleading Terms?Read more "Are ‘Super-Earths’ and ‘Habitable Zones’ Misleading Terms?"
Behold the Superhabitable World
Superhabitability describes a perfect storm of life-friendly factors. In their recent paper, astrobiologists René Heller and John Armstrong describe no fewer than 18 of them. First and foremost, superhabitability arises on terrestrial planets with masses two to three times that of Earth. Planets that size have a number of things working for them, including: Long […]Read more "Behold the Superhabitable World"
Study: seemingly cool planets could be warm enough to host life underground (Wired UK)
To calculate habitable zones across the galaxy we take into account a host star’s luminosity, along with the planet’s distance from it and that planet’s size relative to the star.
The well-established Goldilocks theory, however, fails to take life beneath the surface into account, where temperatures dramatically change.
“As you get deeper below a planet’s surface, the temperature increases, and once you get down to a temperature where liquid water can exist – life can exist there too,” PhD student at the University of Aberdeen Sean McMahon said. “The deepest known life on Earth is 5.3km below the surface, but there may well be life even 10km deep in places on Earth that haven’t yet been drilled.”
The computer model was used to estimate what the temperature beneath the surface would be of any given planet it had the necessary parameters for. It found that the habitable zone would be around three times bigger than previously thought if it included the first 5km beneath an Earth-like planet’s surface. When depths of up to 10km below the Earth’s surface were included, the model found the habitable zone was 14 times wider. Applied to our own Solar System, it means the habitable zone extends beyond Saturn.
“The results suggest life may occur much more commonly deep within planets and moons than on their surfaces.”
“Rocky planets a few times larger than the Earth could support liquid water at about 5km below the surface even in interstellar space (i.e. very far away from a star), even if they have no atmosphere because the larger the planet, the more heat they generate internally.”
Study: seemingly cool planets could be warm enough to host life underground (Wired UK)Read more "Study: seemingly cool planets could be warm enough to host life underground (Wired UK)"