What Steps Are Needed To Find More Earths?

It wasn’t so long ago that we found out there is an Earth-sized planet in a habitable zone of a star. But how many others are out there, and do we know if planets like this are truly habitable?

“Looking towards the future, what we really want to do eventually is transform our knowledge from planets in the habitable zone to [characterizing] planetary environments,” said Natalie Batalha, a co-investigator on NASA’s Kepler Space Telescope, in a webcast presentation today (April 28) .

This means that astronomers will be able to, from a distance, look at “biosignatures” of life in the atmosphere. What a biosignature would be is still being characterized, but it could be something like an unusually high proportion of oxygen — as long as abiotic processes are not accounted for, of course.

Batalha identified these parameters for finding other Earths in a presentation at the “Habitable Worlds Across Time and Space” conference presented by the Space Telescope Science Institute:

Detections of planets: other telescopes (left) vs. Kepler. Credit: Natalie  Batalha / NASA (screenshot)
Detections of planets in the habitable zone: other telescopes (left) vs. the Kepler space telescope. Credit: Natalie Batalha / NASA (screenshot)

– The telescope must be sensitive to an Earth-sized planet in the habitable zone of a G, K or M-type star (which are stars that are like the sun);

– A uniform and reliable detection catalog with well-understood sizes, orbital periods and insolation fluxes (energy received from the sun);

– Knowledge of Kepler’s detection efficiency and the planetary catalog’s reliability;

– Well-documented and accessible data products for other community members to analyze.

What would also be helpful to planetary scientists is learning more about how a planet forms in the habitable region of its star.

Meet Kepler-22b, an exoplanet with an Earth-like radius in the habitable zone of its host star. Unfortunately its mass remains unknown. Image Credit: NASA
Meet Kepler-22b, an exoplanet with an Earth-like radius in the habitable zone of its host star. Unfortunately its mass remains unknown. Image Credit: NASA

In a presentation at the same conference, the University of Toronto’s Diana Valencia (an astrophysicist) pointed out there is no single predictor for how large a planet will get. It depends on how close a planetesimal disc is to its star, the rate of accretion in the area and dust opacity, among other factors.

She also gave a brief overview of processes that demonstrate how hard it is to predict habitability. Earth had at least two atmospheres in its past, presentation slides said, with the first atmosphere lost and the second built from volcanism and impacts. Valencia also pointed to complexities involving the Earth’s mantle and plate tectonics.

The University of Puerto Rico keeps a list of potentially habitable planets on its website, which as of this writing stands at 21.

The conference runs through May 1, and you can see the agenda here.

Why Doesn’t Earth Have More Water?

Water, water everywhere… Coleridge’s shipbound ancient mariners were plagued by a lack of water while surrounded by a sea of the stuff, and while 70% of Earth’s surface is indeed covered by water (of which 96% is salt water, hence not a drop to drink) there’s really not all that much — not when compared to the entire mass of the planet. Less than 1% of Earth is water, which seems odd to scientists because, based on conventional models of how the Solar System formed, there should have been a lot more water available in Earth’s neck of the woods when it was coming together. So the question has been floating around: why is Earth so dry?

According to a new study from the Space Telescope Science Institute in Baltimore, MD, the answer may lie in the snow.

The snow line, to be exact. The region within a planetary system beyond which temperatures are cold enough for water ice to exist, the snow line in our solar system is currently located in the middle of the main asteroid belt, between the orbits of Mars and Jupiter. Based on conventional models of how the Solar System developed, this boundary used to be closer in to the Sun, 4.5 billion years ago. But if that were indeed the case, then Earth should have accumulated much more ice (and therefore water) as it was forming, becoming a true “water world” with a water mass up to 40 percent… instead of a mere one.

As we can see today, that wasn’t the case.

Planets such as Uranus and Neptune that formed beyond the snow line are composed of tens of percents of water. But Earth doesn’t have much water, and that has always been a puzzle.”

– Rebecca Martin, Space Telescope Science Institute 

A study led astrophysicists Rebecca Martin and Mario Livio of the Space Telescope Science Institute took another look at how the snow line in our solar system must have evolved, and found that, in their models, Earth was never inside the line. Instead it stayed within a warmer, drier region inside of the snow line, and away from the ice.

“Unlike the standard accretion-disk model, the snow line in our analysis never migrates inside Earth’s orbit,” Livio said. “Instead, it remains farther from the Sun than the orbit of Earth, which explains why our Earth is a dry planet. In fact, our model predicts that the other innermost planets, Mercury, Venus, and Mars, are also relatively dry. ”

Read: Rethinking the Source of Earth’s Water

The standard model states that in the early days of a protoplanetary disk’s formation ionized material within it gradually falls toward the star, drawing the icy, turbulent snow line region inward. But this model depends upon the energy of an extremely hot star fully ionizing the disk — energy that a young star, like our Sun was, just didn’t have.

“We said, wait a second, disks around young stars are not fully ionized,” Livio said. “They’re not standard disks because there just isn’t enough heat and radiation to ionize the disk.”

“Astrophysicists have known for quite a while that disks around young stellar objects are NOT standard accretion disks (namely, ones that are ionized and turbulent throughout),” added Dr. Livio in an email to Universe Today. “Disk models with dead zones have been constructed by many people  for many years. For some reason, however, calculations of the evolution of the snow line largely continued to use the standard disk models.”

Without fully ionized disk, the material is not drawn inward. Instead it orbits the star, condensing gas and dust into a “dead zone”  that blocks outlying material from coming any closer. Gravity compresses the dead zone material, which heats up and dries out any ices that exist immediately outside of it. Based on the team’s research it was in this dry region that Earth formed.

The rest, as they say, is water under the bridge.

The team’s results have been accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.

Read the release on the Hubble news site here, and see the full paper here.

Lead image: Earth as seen by MESSENGER spacecraft before it left for Mercury in 2004. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington. Disk model image: NASA, ESA, and A. Feild (STScI). Earth water volume image:  Howard Perlman, USGS; globe illustration by Jack Cook, Woods Hole Oceanographic Institution (©); Adam Nieman.

Grunsfeld May Lead NASA Science Division

John Grunsfeld was one of the astronauts involved in fixing the Hubble Space Telescope. Credit: NASA

[/caption]The current buzz amongst those in the know say that astrophysicist/astronaut, John Grunsfeld, has been chosen to lead NASA’s science mission directorate. Self-confessed “Hubble Hugger” and telescope repair man may very well become NASA associate administrator in September, according to a news article in Nature. As current deputy director of the Space Telescope Science Institute in Baltimore, Maryland, Grunsfeld will be replacing the resigning Ed Weiler.

“John is a very capable guy,” Weiler was quoted by writer Eric Hand in Nature. “He knows both the human and robotic sides. He’s a very solid citizen.”

However, NASA spokesman Trent Perrotto says no appointment has yet been made official.

Nature reports that the five-time shuttle astronaut could likely be the top choice of NASA administrator Charles Bolden, also a former shuttle pilot, and may display just a bit of favoritism towards fellow astronauts. “Clearly, he’s Charlie’s pick,” says one person with knowledge of the selection.

But Nature quotes another science source that Grunsfeld might not be the right pick. Apparently he/she believes that NASA-backed scientists who aren’t part of the astronomy field shouldn’t be a prime candidate. “His entire reputation is based on fixing space telescopes,” says the scientist. “I think it will be a real tough slog for him.”

Read more in Nature News.

And we’ll keep you posted of any official announcements.

Original Story Source: Nature News.