Nikola Poljak, Dora Klindzic, Mateo Kruljac
At some point in the future, if mankind hopes to settle planets outside the Solar System, it will be crucial to determine the range of planetary conditions under which human beings could survive and function. In this article, we apply physical considerations to future exoplanetary biology to determine the limitations which gravity imposes on several systems governing the human body. Initially, we examine the ultimate limits at which the human skeleton breaks and muscles become unable to lift the body from the ground. We also produce a new model for the energetic expenditure of walking, by modelling the leg as an inverted pendulum. Both approaches conclude that, with rigorous training, humans could perform normal locomotion at gravity no higher than 4 gEarth.
Read more at https://arxiv.org/pdf/1808.07417.pdf
Astronomers using data from three of NASA’s space telescopes — Hubble, Spitzer and Kepler — have discovered clear skies and steamy water vapor on a gaseous planet outside our solar system. The planet is about the size of Neptune, making it the smallest planet from which molecules of any kind have been detected.
“This discovery is a significant milepost on the road to eventually analyzing the atmospheric composition of smaller, rocky planets more like Earth,” said John Grunsfeld, assistant administrator of NASA’s Science Mission Directorate in Washington. “Such achievements are only possible today with the combined capabilities of these unique and powerful observatories.”
Clouds in a planet’s atmosphere can block the view to underlying molecules that reveal information about the planet’s composition and history. Finding clear skies on a Neptune-size planet is a good sign that smaller planets might have similarly good visibility.
“When astronomers go observing at night with telescopes, they say ‘clear skies’ to mean good luck,” said Jonathan Fraine of the University of Maryland, College Park, lead author of a new study appearing in Nature. “In this case, we found clear skies on a distant planet. That’s lucky for us because it means clouds didn’t block our view of water molecules.”
The planet, HAT-P-11b, is categorized as an exo-Neptune — a Neptune-sized planet that orbits the star HAT-P-11. It is located 120 light-years away in the constellation Cygnus. This planet orbits closer to its star than does our Neptune, making one lap roughly every five days. It is a warm world thought to have a rocky core and gaseous atmosphere. Not much else was known about the composition of the planet, or other exo-Neptunes like it, until now. Continue reading NASA Telescopes Find Clear Skies and Water Vapor on Exoplanet
Astronomers are not only discovering planets around distant suns, they are also starting to measure those worlds with astonishing precision. The diameter of a super-Earth named Kepler 93B is now known to within an accuracy of 148 miles.
In August, MIT researchers identified an exoplanet with an extremely brief orbital period: The team found that Kepler 78b, a small, intensely hot planet 700 light-years from Earth, circles its star in just 8.5 hours—lightning-quick, compared with our own planet’s leisurely 365-day orbit. From starlight data gathered by the Kepler Space Telescope, the scientists also determined that the exoplanet is about 1.2 times Earth’s size—making Kepler 78b one of the smallest exoplanets ever measured.
Now this same team has found that Kepler 78b shares another characteristic with Earth: its mass. By analyzing the movement of its host star, Kepler 78, the scientists determined that the exoplanet is about 1.7 times as massive as the Earth. From the same measurements, they calculated that the planet’s density is 5.3 grams per cubic centimeter, closely resembling Earth’s density (5.5 grams per cubic centimeter).
The findings make Kepler 78b the smallest exoplanet for which the mass and size are known. These new measurements provide strong evidence that Kepler 78b is composed mostly of rock and iron, similar to Earth.
However, that’s where the similarities may end: The exoplanet, due to its extreme proximity to its star, is likely blazing at temperatures too high to support life.
“It’s Earth-like in the sense that it’s about the same size and mass, but of course it’s extremely unlike the Earth in that it’s at least 2,000 degrees hotter,” says team member Josh Winn, an associate professor of physics at MIT and a member of the Kavli Institute for Astrophysics and Space Research. “It’s a step along the way of studying truly Earth-like planets.”
Winn and his colleagues, including lead author Andrew Howard, of the University of Hawaii, publish their results this week in the journal Nature. The group’s results appear in the same issue as a paper published by a separate group in Geneva, reporting similar results—scientific agreement that Winn says adds confidence to the mass measurement.
Watching for a wobble
Planets with extremely tight orbits offer scientists a wealth of data: For instance, each week Kepler 78b circles its star about 20 times, giving researchers numerous opportunities to observe its behavior.
The team previously determined Kepler 78b’s orbit and size by analyzing the light given off by the star as the planet passes in front of it, or transits. The researchers detected a transit each time the star’s light dipped, and measured this dimming to determine the planet’s size. (The bigger an exoplanet, the more light it blocks.)
Measuring the planet’s mass was a somewhat trickier endeavor. Instead of tracking the planet’s motion, the researchers tracked the motion of the star itself. Depending on its mass, a planet can exert a gravitational tug on its star. This stellar motion can be detected as a very slight wobble, known as a Doppler shift.
Winn and his colleagues looked to measure Kepler 78’s Doppler shift by analyzing observations from the Keck Observatory in Hawaii—one of the largest telescopes in the world. The team analyzed starlight data taken over a period of eight days. Despite the telescope’s strength, the signal from the star was incredibly faint, making a daunting task for the scientists.
“Each of the eight nights along the way, we were agonizing over it, whether it was worth continuing or not,” Winn recalls.
Out, damn starspot
In addition to the challenge of picking out such tiny signals, the researchers had to contend with an effect that initially muddled the data: starspots, dark patches on the surface of stars. Graduate student Roberto Sanchis-Ojeda, who has studied the effect of starspots on exoplanet detection, says the troublesome patches can make a star’s Doppler shift appear larger, dramatically complicating scientists’ calculations of a planet’s mass.
Sanchis-Ojeda was able to solve this puzzle by taking into account Kepler 78’s rotational period. By tracking the frequency at which certain starspots reappeared, Sanchis-Ojeda determined that the star completes a full rotation every 12.5 days—considerably longer than the planet’s orbital period of 8.5 hours. From these measurements, Sanchis-Ojeda was able to calculate the star’s true Doppler shift.
From his calculations, Sanchis-Ojeda found that the star rotates relatively slowly, at 1.5 meters per second—about the speed of a jog, or a brisk walk.
“The star is moving at the same speed as when we walk to school or go grocery shopping,” Sanchis-Ojeda notes. “The difference is that this star is 700 light-years away, so imagine how complicated it is to measure such speeds from so far away.”
From the star’s Doppler shift, the team determined that Kepler 78b’s mass is 1.7 times that of Earth—a measurement that suggests the planet is made mostly of rock and iron. Such a composition, Winn says, is not surprising, given the planet’s extremely close proximity to its star. A less massive planet, such as one made entirely of gas, would not be able to hold together in such a tight orbit.
While its similarities to Earth likely end with Kepler 78b’s size and mass, Winn says there is still more to learn about the planet, such as its surface and atmospheric composition—a goal that the group plans to pursue next.
Astronomers have created the first map of the clouds on a planet outside our Solar System.
The planet in question is Kepler-7b, a large gaseous world like Jupiter, roughly 1,000 light-years away.
The researchers used data from Nasa’s Spitzer and Kepler space telescopes to study the exoplanet, which orbits close to its parent star.
Their results suggest the hot giant is marked by high clouds in the west and clear skies in the east.
The findings have been accepted for publication in the journal Astrophysical Journal Letters.
“By observing this planet with Spitzer and Kepler for more than three years, we were able to produce a very low-resolution ‘map’ of this giant, gaseous planet,” said co-author Brice-Olivier Demory of Massachusetts Institute of Technology (MIT) in Cambridge, US.
“We wouldn’t expect to see oceans or continents on this type of world, but we detected a clear, reflective signature that we interpreted as clouds.”
Astronomers have previously been able to make temperature maps of planets orbiting other stars, but this is the first look at cloud structures on a distant world. Kepler-7b is something of an oddity – bigger than Jupiter, but lower in mass – with a density about the same as polystyrene.
The Kepler telescope’s visible-light observations of this distant world’s moon-like phases led to a rough map of the planet that showed a bright spot on its western hemisphere. But these data were not enough on their own to decipher whether the bright spot was coming from clouds or heat.
So the team used Spitzer to gather further clues about the planet’s atmosphere. They determined that light from the planet’s star was bouncing off cloud tops located on the west side of Kepler-7b.
“Kepler-7b reflects much more light than most giant planets we’ve found, which we attribute to clouds in the upper atmosphere,” said Thomas Barclay from Nasa’s Ames Research Center in Moffett Field, California, US, who works on the Kepler telescope team.
“Unlike those on Earth, the cloud patterns on this planet do not seem to change much over time – it has a remarkably stable climate.”
Nasa says the findings are an early step towards using similar techniques to study the atmospheres of exoplanets that are more like Earth in composition and size.
Paul Hertz, director of Nasa’s astrophysics division in Washington DC commented: “We’re at a point now in exoplanet science where we are moving beyond just detecting [them], and into the exciting science of understanding them.”
The Kepler mission has now ended because of problems with the spacecraft’s reaction wheels – the spinning components that aid fine-pointing of the satellite. But astronomers are still studying the data it gathered; the mission has so far discovered more than 150 bona fide exoplanets and thousands of other candidate worlds.
Read more at http://www.bbc.co.uk/news/science-environment-24348024