An artist’s conception of the exoplanet Kepler-452b (R), a possible candidate for Earth 2.0, as compared with Earth (L). Image credit: NASA/Ames/JPL-Caltech/T. Pyle.

Is Proxima Centauri’s ‘Earth-like’ planet actually like Earth at all?

By Ethan Siegel

Just 25 years ago, scientists didn’t know if any stars—other than our own sun, of course—had planets orbiting around them. Yet they knew with certainty that gravity from massive planets caused the sun to move around our solar system’s center of mass. Therefore, they reasoned that other stars would have periodic changes to their motions if they, too, had planets.

This change in motion first led to the detection of planets around pulsars in 1991, thanks to the change in pulsar timing it caused. Then, finally, in 1995 the first exoplanet around a normal star, 51 Pegasi b, was discovered via the “stellar wobble” of its parent star. Since that time, over 3000 exoplanets have been confirmed, most of which were first discovered by NASA’s Kepler mission using the transit method. These transits only work if a solar system is fortuitously aligned to our perspective; nevertheless, we now know that planets—even rocky planets at the right distance for liquid water on their surface—are quite common in the Milky Way.

On August 24, 2016, scientists announced that the stellar wobble of Proxima Centauri, the closest star to our sun, indicated the existence of an exoplanet. At just 4.24 light years away, this planet orbits its red dwarf star in just 11 days, with a lower limit to its mass of just 1.3 Earths. If verified, this would bring the number of Earth-like planets found in their star’s habitable zones up to 22, with ‘Proxima b’ being the closest one. Just based on what we’ve seen so far, if this planet is real and has 130 percent the mass of Earth, we can already infer the following:

• It receives 70 percent of the sunlight incident on Earth, giving it the right temperature for liquid water on its surface, assuming an Earth-like atmosphere.
• It should have a radius approximately 10 percent larger than our own planet’s, assuming it is made of similar elements.
• It is plausible that the planet would be tidally locked to its star, implying a permanent ‘light side’ and a permanent ‘dark side’.
• And if so, then seasons on this world are determined by the orbit’s ellipticity, not by axial tilt.

Yet the unknowns are tremendous. Proxima Centauri emits considerably less ultraviolet light than a star like the sun; can life begin without that? Solar flares and winds are much greater around this world; have they stripped away the atmosphere entirely? Is the far side permanently frozen, or do winds allow possible life there? Is the near side baked and barren, leaving only the ‘ring’ at the edge potentially habitable?

Proxima b is a vastly different world from Earth, and could range anywhere from actually inhabited to completely unsuitable for any form of life. As 30m-class telescopes and the next generation of space observatories come online, we just may find out!

Galaxy cluster Abell S1063 (left) as imaged with the Hubble Space Telescope as part of the Frontier Fields program. The distorted images of the background galaxies are a consequence of the warped space dues to Einstein’s general relativity; the parallel field (right) shows no such effects. Image credit: NASA, ESA and Jennifer Lotz (STScI)

One Incredible Galaxy Cluster Yields Two Types of Gravitational Lenses
By Ethan Siegel

There is this great idea that if you look hard enough and long enough at any region of
space, your line of sight will eventually run into a luminous object: a star, a galaxy or a
cluster of galaxies. In reality, the universe is finite in age, so this isn’t quite the case.
There are objects that emit light from the past 13.7 billion years—99 percent of the age of
the universe—but none before that. Even in theory, there are no stars or galaxies to see
beyond that time, as light is limited by the amount of time it has to travel.
But with the advent of large, powerful space telescopes that can collect data for the
equivalent of millions of seconds of observing time, in both visible light and infrared
wavelengths, we can see nearly to the edge of all that’s accessible to us.
The most massive compact, bound structures in the universe are galaxy clusters that are
hundreds or even thousands of times the mass of the Milky Way. One of them, Abell
S1063, was the target of a recent set of Hubble Space Telescope observations as part of
the Frontier Fields program. While the Advanced Camera for Surveys instrument imaged
the cluster, another instrument, the Wide Field Camera 3, used an optical trick to image a
parallel field, offset by just a few arc minutes. Then the technique was reversed, giving us
an unprecedentedly deep view of two closely aligned fields simultaneously, with
wavelengths ranging from 435 to 1600 nanometers.
With a huge, towering galaxy cluster in one field and no comparably massive objects in
the other, the effects of both weak and strong gravitational lensing are readily apparent.
The galaxy cluster—over 100 trillion times the mass of our sun—warps the fabric of
space. This causes background light to bend around it, converging on our eyes another
four billion light years away. From behind the cluster, the light from distant galaxies is
stretched, magnified, distorted, and bent into arcs and multiple images: a classic example
of strong gravitational lensing. But in a subtler fashion, the less optimally aligned
galaxies are distorted as well; they are stretched into elliptical shapes along concentric
circles surrounding the cluster.
A visual inspection yields more of these tangential alignments than radial ones in the
cluster field, while the parallel field exhibits no such shape distortion. This effect, known
as weak gravitational lensing, is a very powerful technique for obtaining galaxy cluster
masses independent of any other conditions. In this serendipitous image, both types of
lensing can be discerned by the naked eye. When the James Webb Space Telescope
NASA Space Place Astronomy Club Article September 2016
launches in 2018, gravitational lensing may well empower us to see all the way back to
the very first stars and galaxies.