Exoplanets everywhere, but not another Earth

Credit: exoworld.info

Astrobiologist David Grinspoon has wisely observed that scientists will never find “another Earth” because every planet is unique. Yet the search for “another Earth” continues.

Today NASA announced that scientists have “confirmed the first near-Earth-size planet in the ‘habitable zone’ around a sun-like star,” 1400 light years away.

In a July 20 media alert about today’s announcement, NASA had said “astronomers are on the cusp of finding something people have dreamed about for thousands of years — another Earth.”

“The newly discovered Kepler-452b” – which NASA characterizes as a “bigger, older cousin to Earth” – “is the smallest planet to date discovered orbiting in the habitable zone — the area around a star where liquid water could pool on the surface of an orbiting planet — of a G2-type star, like our sun,” says the agency.

This discovery was the result of analyses of data collected NASA’s Kepler extrasolar-planet-searching space telescope and follow-up ground-based-observations.

In a media teleconference today about the discovery of Kepler-452b, NASA Associate Administrator for Science John Grunsfeld described this planet as the closest object to Earth 2.0 found thus far in the Kepler dataset.

According to today’s NASA press release, “Kepler-452b is 60 percent larger in diameter than Earth and is considered a super-Earth-size planet. While its mass and composition are not yet determined, previous research suggests that planets the size of Kepler-452b have a good chance of being rocky [emphasis added]…. While Kepler-452b is larger than Earth, its 385-day orbit is only 5 percent longer. The planet is 5 percent farther from its parent star Kepler-452b than Earth is from the Sun. Kepler-452b is 6 billion years old, 1.5 billion years older than our sun, has the same temperature, and is 20 percent brighter and has a diameter 10 percent larger…. This discovery and the introduction of 11 other new small habitable zone candidate planets mark another milestone in the journey to finding another ‘Earth’.””

During Q&A with reporters, Kepler scientists Jon Jenkins of NASA and Jeff Coughlin of the SETI Institute added a lot of qualifiers and caveats to their report on Kepler-452b. Jenkins said the discoverers of Kepler-452b worked with a planetary geologist to come up with a reasonable description of what the planet might be like. They think it’s probably rocky, but they can’t say for sure. (See above.) One reporter asked how they came up with a description of the planet “given that you have so little information about it.” Jenkins said there’s a “slightly better than even chance” it’s rocky.

Another reporter asked how the six billion year age of the host star and its planet is known. Jenkins said the age is an estimate based on models of stellar evolution. For stars like Kepler-452b’s hot star, age is less certain than other details about them. For this planetary system, the age estimate is six billion plus or minus two billion years.

Another reporter asked the scientists to explain “why this one [planet] in particular today stands out as the closest cousin yet” to Earth. Another asked if this planet is “the most likely to support life” discovered thus far. Jenkins replied that it’s the one most like Earth discovered thus far. Another asked how scientists know the mass of the planet. Jenkins said, “I don’t expect we’ll ever have a mass measurement” of Kepler-452b. The planet’s too small to enable direct measurements of its mass. We have an estimate of five Earth masses, based on the masses and distribution of other known exoplanets.

(According to a press release about Kepler-452b from the McDonald Observatory at the University of Texas, “Kepler data provide the ratio of a potential planet’s size to the star’s size, but not the actual size of either.” It also notes that, “at around 1.5 times the Earth’s radius there seems to be a transition going on from predominantly rocky planets to planets that contain more volatiles – ices…which would make [Kepler-452b] a mini-ice giant…we don’t know if it’s a big rocky planet or if it’s a mini-Neptune.”)

Coughlin noted that scientists are just starting to make estimates of the types of planets in our galaxy. Right now it’s estimated that 15-25 percent of sunlike stars have planets. “The most common planets are small rocky planets like Earth…. Planets like Earth do appear to be quite common.”

Questions to NASA on Twitter were all over the place – for example:

• Is there life on this planet?
  • (“Planetary habitability is complicated,” according to exoplanet scientist Rory Barnes. See my blog post of July 1 for some thoughts from the experts on looking for signs of life on other planets.)
• What is its atmosphere like? Is there methane?
• Does the planet have plate tectonics?
• Does it have moons?
• Does it have water?
• How long would it take to travel to this planet?
  • (Kepler-452b is 1400 light years from Earth. Light travels at 300,000 kilometers (186,000 miles) per second. One light year=approximately 9 trillion kilometers (about 6 trillion miles). According To Wikipedia, NASA’s New Horizons spacecraft to Pluto traveled to its target at “58,536 km/h (36,373 mph), making it the fastest spacecraft to ever leave Earth orbit.” How long would it take a spacecraft traveling at 58,536 km an hour to travel 1400 light years? Do the math.)
• Is there a good possibility of finding a habitable planet that is within traveling distance using current technology? (See above.)
• Are SETI scientists pointing their telescopes to this planet to listen for radio signals of technological origin?

(I don’t know if anybody at NASA actually answers all of these questions.)

I came away from today’s Kepler teleconference feeling as though scientists were tantalizing people with “news” of a “near-twin” of Earth, only to reveal that most of what’s “known” about this planet is educated guesses. Questions on Twitter indicated to me that people don’t have a very good grasp of cosmic scales or exoplanet detection methods or the space community’s current scientific and technical capabilities, among other things. I feel sad about that. I know that most scientists work hard to explain the work they do, and it’s difficult to avoid talking down or talking over non-experts. We science communication people have a lot of work to do.