Our Ever-Growing Menagerie of Exoplanets

With so many exoplanets already detected, with the pace of discovery continuing to be so fast, and with efforts to find more distant worlds so constant and global,  it’s easy to become somewhat blase´ about new discoveries.  After so many “firsts,” and so many different kinds of planets found in very different ways, it certainly seems that some of the thrill may be gone.

Surely the detection of a clearly “Earth-like planet” would cause new excitement — one that is not only orbiting in the habitable zone of its host star but also has signs of a potentially nurturing atmosphere in a generally supportive cosmic neighborhood.

But while many an exoplanet has been described as somewhat “Earth-like” and potentially habitable, further observation has consistently reduced the possibility of the planets actually hosting some form of biology.  The technology and knowledge base needed to find distant life is surely advancing, but it may well still have a long way to go.

In just the last few days, however, a slew of discoveries have been reported that highlight the allure and science of our new Exoplanet Era.  They may not be blockbusters by themselves, but they are together part of an immense scientific exploration under way, one that is re-shaping our understanding of the cosmos and preparing us for bigger discoveries and insights to come.

 

What I have in mind are these discoveries:

• The first Earth-sized planet detected by NASA’s year-old orbiting telescope TESS (Transiting Exoplanet Survey Satellite.)  TESS is designed to find planets orbiting massive stars in our near neighborhood, and it has already made 10 confirmed discoveries.  But finding a small exoplanet — 85 percent the size of Earth — is a promising result for a mission designed to not only locate as many as 20,000 new exoplanets, but to find 500 to 1,000 the rough size of Earth or SuperEarth.  This first discovery  (HD 21749-c) is no candidate for supporting life because it’s so close to its star, but it shows how small TESS can go.
• Five new hard-to-find planets that are far from their host stars — rather like our Jupiter or Neptune.  These planets orbit in decades rather than in months or years, and so they  take enormous amounts of work to detect and confirm.  The five join 26 other “long-period” planets identified so far and provide tempting targets for what astronomers generally see as the long-term future of exoplanet hunting — the direct imaging of planets rather than finding them via their transits in front of a star or how they change the motion of their host star.
• A “Tatooine” circumbinary system with three planets orbiting two suns.  Using archived data from NASA’s Kepler mission, researchers found a third planet in the Kepler-47 system, the only circumbinary known to have more than one planet in orbit.

 

 

These are all discoveries significant in themselves, but maybe most interesting as indications of what’s to come.

For the TESS Earth-sized planet, the message is twofold:  That the orbiting telescope is operating as hoped and will likely provide many more detections than even the super-productive Kepler mission.

And unlike Kepler — which looked at a small portion of the sky and at very distant stars — TESS is looking for nearby planets that can later be more extensively studied.

This is the 10th confirmed planet discovered by TESS, and hundreds of additional candidates are now being studied.  The result was published in the The Astrophysical Journal Letters.

TESS focuses stars that are 30 to 100 times brighter than those the Kepler mission surveyed, which will allow  for far easier follow-up observations with both ground-based and space-based telescopes. TESS will also cover a sky area 400 times larger than that monitored by Kepler.

A key goal is to identify for the astronomy community at least 50 small, rocky Earth-like planets  along with estimates of their masses.

As it circles our own planet, TESS focuses its four cameras outward to monitor stars for for any periodic dips in starlight that could indicate the presence of an exoplanet as it passes in front of its host star.

The TESS mission was proposed and led by MIT, and the first small-planet detection came from an MIT team.

Putting TESS into the larger exoplanet hunting picture, MIT professor and TESS deputy science director Sara Seager said that “TESS is almost the culmination of a couple of decades of hard work, trying to iron out the wrinkles of how to find planets by the transiting method. So TESS isn’t changing the way we look for planets, it’s more like it’s riding on the wave of success of how we’ve done it already.”

Once the transits are detected, the data are shared with ground-based astronomers who — using a different method of studying an exoplanet — measure the planet’s mass with greater precision.  In the case most recent detection, the follow-up work was done by a Carnegie Institution for  Science team using the Giant Magellan Telescope in Chile. They studied the  planet via the radial velocity method, which detects and characterizes exoplanets based on the gravitational pull they exert on their host stars.  Many similar confirmations of TESS candidate planets are underway at observatories across the globe.

 

 

The detection of the five new long-period exoplanets — with orbits of 15 years to more than 40 years — was the result of more than twenty years of observations at the La Silla Observatory in Chile.

While these planets are huge, they are so far from their host stars that they are a challenge to detect.  Of the almost 4,000 exoplanets detected so far, only 31 have orbits greater than 15 years.

“It can be extremely difficult to detect planets with such long orbital periods because you need so many years of data to confirm them,” said Emily Richman of the University of Geneva, and first author of the paper in Astronomy & Astrophysics.  That is “why it’s so important and special that we have over 20 years of continuous observations to find such planets.”

Since the technique used to collect that data was pioneered only three years before the La Silla long-orbit observations began in 1998, their data is definitely unique and important.  As Richman described,  once the formal planetary monitoring program was set up it was implemented continuously by innumerable University of Geneva astronomers who flew to Chile and, for 20 years, took two week observing turns at La Silla.

But while two decades of data on one planet is a lot, how can planet hunters determine that a particular planet is on a 30 or 40 year orbit?  Richman explained in an email:  “It’s not necessarily about the number of detections but rather the continuous observations over a long time span.  When we have enough of a curve over time caused by the radial velocities of a planet around a star then we can be confident that we have detected a planet.”

And once that happens with a long-period planet, the discovery “provides us with new targets for direct imaging,”  Richman said with enthusiasm.

The direct-imaging method resembles photography, whether via visible or infrared light. But photographing a planet is tough, especially because they are often lost in the much-brighter light of the parent star.

Scientists use an instrument known as a coronagraph to block the light from the star, revealing the dimmer light reflected by a potential planet.  But even this technique works only when the planet is huge and its orbit is very long.

 

The discovery of a third planet orbiting the two suns of Kepler-47 was teased out from data collected by NASA’s now-defunct Kepler space telescope.  A team of researchers, led by astronomers at San Diego State University, detected the new Neptune-to-Saturn-size planet orbiting between two previously known planets.

With its three planets orbiting two suns, Kepler-47 is the only known multi-planet circumbinary system.

The planets in the Kepler-47 system were detected via the transit method, and the third was not found earlier because of weak transit signals as it crossed in front of one of its star.

As is common with circumbinary planets, the alignment of the orbital planes of the planets change with time. In this case, the middle planet’s orbit became more aligned, leading to a stronger transit signal. The transit depth went from undetectable at the beginning of the Kepler mission to having the strongest transit signal of the three planets over the span of just four years.

“We saw a hint of a third planet back in 2012, but with only one transit we needed more data to be sure,” said SDSU astronomer Jerome Orosz, lead author of the paper in Astronomical Journal. “With an additional transit, the planet’s orbital period could be determined, and we were then able to uncover more transits that were hidden in the noise in the earlier data.”

Just as the characteristics of exoplanets are endlessly different, so too, it seems, are the paths to finding them.