Are We Watching a Planet Disintegrate?

Artist's illustration of the possible disintegrating planet in the nearby planetary system DMPP-1.

Credit: Mark A. Garlick/Haswell/ Barnes/Staab/Open University

Artist’s illustration of another DMPP-discovered planetary system, DMPP-2.

Credit: Mark A. Garlick/Haswell/ Barnes/Staab/Open University

Among the wealth of exoplanets we’ve discovered beyond our solar system, some are temperate, some less so. New observations have now revealed what may be a particularly inhospitable environment: a planet literally disintegrating as it orbits its host.

Peering Through the Shroud

With initial observations in 2015, the Dispersed Matter Planet Project (DMPP) promised an innovative approach to hunting for exoplanets closely orbiting their hosts. Using high-cadence, high-precision radial velocity measurements, the project targets bright nearby stars that shows signatures of being shrouded in hot circumstellar gas. By looking for tiny radial-velocity wiggles in the star’s signal, the DMPP team hopes to detect small planets that are losing mass as they orbit close to their hot hosts.

In December 2019, DMPP announced its first discoveries: six planets orbiting around three different target stars. Now, in a new publication led by scientist Mark Jones (The Open University, UK), the team has revisited the first of these systems, DMPP-1, with follow-up photometry from the Transiting Exoplanet Survey Satellite (TESS).

Intriguingly, the radial-velocity-detected planets are not the only signals from this system.

Phase-folded TESS light curve for DMPP-1, identifying a weak transit signal with a period of P = 3.2854 days.

Credit: Jones et al. 2020

Missing the Expected, but Finding the Unexpected

DMPP-1 is a 2-billion-year-old star located just over 200 light-years away. The radial-velocity observations of this system revealed the gravitational tugs of four planets all orbiting with periods of less than 19 days. The radial-velocity data suggest that this system is probably near edge-on and contains three super-Earths and one Neptune-like planet.

Jones and collaborators began their photometric follow-up by searching TESS data for evidence of these four planets transiting across the host star’s face. Interestingly, they found no sign of transits at the predicted periods — indicating that the four radial-velocity planets are either smaller than expected, or that the system isn’t quite edge-on after all, so the planets don’t pass directly in front of the star.

The authors did, however, find a new signal: a weak transit detection with a period of just ~3.3 days. This signal doesn’t match any of the known radial-velocity planets.

The depths of the seven detected transits are highly variable, with one even consistent with a depth of zero! This variability is common for disintegrating planets, as the cloud of ablated material is the primary cause of the transits. Click to enlarge. Credit: Jones et al. 2020

A Disappearing Planet?

What might this marginal detection be? Its variable transit depths, short period, and apparent small size are all consistent with a catastrophically disintegrating exoplanet — a close-in, small, rocky planet that is so irradiated by its host that its rocky surface is being sublimated. As time goes on, such a planet will eventually disintegrate into nothing.

This transit signal still needs to be confirmed with additional follow-up photometric observations. Assuming it proves to be a true detection, however, such a disintegrating, rocky planet orbiting a bright nearby star would provide a veritable gold mine of information.

By exploring the transit signals from DMPP-1 with future technology like the James Webb Space Telescope, we will be able to examine the composition of the ablated material, potentially revealing clues as to how hot, rocky inner planets form and evolve.

Citation

“A Possible Transit of a Disintegrating Exoplanet in the Nearby Multiplanet System DMPP-1,” Mark H. Jones et al 2020 ApJL 895 L17. doi:10.3847/2041-8213/ab8f2b

Source: American Astronomical Society (AAS)

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Black Hole Blasts May Transform "Mini-Neptunes" into Rocky Worlds

This artist's impression shows the atmosphere of a Neptune-like planet (foreground) being swept backwards by powerful radiation from an outburst in the center of the Milky Way Galaxy (right). The outburst of X-rays and ultraviolet light is produced by material falling towards the supermassive black hole located there. The planet's host star is shown on the left. M. Weiss/CfA. Low Resolution (jpg)

Cambridge, MA - A team of astrophysicists and planetary scientists has predicted that Neptune-like planets located near the center of the Milky Way galaxy have been transformed into rocky planets by outbursts generated by the nearby supermassive black hole.

These findings combine computer simulations with data from recent exoplanet findings, and X-ray and ultraviolet observations of stars and black holes.

"It's pretty wild to think of black holes shaping the evolutionary destiny of a planet, but that very well may be the case in the center of our Galaxy," said Howard Chen of Northwestern University in Evanston, IL, who led the study.

Howard Chen and collaborators from the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass., examined the environment around the closest supermassive black hole to Earth: the four-million-solar mass black hole known as Sagittarius A*.

It is well known that material falling into the black hole in occasional feeding frenzies will generate bright flares of X-ray and ultraviolet radiation. Indeed, X-ray telescopes such as NASA's Chandra X-ray Observatory and ESA's XMM-Newton have seen evidence for bright outbursts generated in the past by the black hole ranging from about 6 million years to just over a century ago.

"We wondered what these outbursts from Sagittarius A* would do to any planets in its vicinity," said John Forbes, a co-author from the CfA. "Our work shows the black hole could dramatically change a planet's life."

The authors considered the effects of this high-energy radiation on planets with masses in between Earth and Neptune that are located less than 70 light years away from the black hole.

They found that the X-ray and ultraviolet radiation would blast away a large amount of the thick, gas atmosphere of such planets near the black hole. In some cases this would leave behind a bare, rocky core. Such rocky planets would be heavier than the Earth and are what astronomers call super-Earths.

"These super-Earths are one of the most common types of planet that astronomers have discovered outside our Solar System," said co-author Avi Loeb, also of CfA, "Our work shows that in the right environment they might form in exotic ways."

The researchers think that this black hole impact may be one of the most common ways for rocky super-Earths to form close to the center of our Galaxy.

While some of these planets will be located in the habitable zone of stars like the Sun, the environment they exist within would be challenging for any life to arise. Supernova explosions and gamma ray bursts would buffet these super-Earths, which might damage the chemistry of any atmosphere remaining on these planets. Additional outbursts from the supermassive black hole could provide a knockout punch and completely erode the planet's atmosphere.

These planets would also be subjected to the gravitational disruptions of a passing star that could fling the planet away from its life-sustaining host star. Such encounters might occur frequently near the Milky Way's supermassive black hole since the region is so packed with stars. How crowded is it in the Galactic Center? Within about 70 light years of the center of the Galaxy, astronomers think the average separation between rocky worlds is between about 75 and 750 billion kilometers. By comparison the nearest star to the Solar System is 40,000 billion kilometers away.

"It is generally accepted that the innermost regions of the Milky Way is not favorable for life. Indeed, even though the deck seems stacked against life in this region, the likelihood of panspermia, where life is transmitted via interplanetary or interstellar contact, would be much more common in such a dense environment," said Loeb. "This process might give life a fighting chance to arise and survive."

There are formidable challenges required to directly detect such planets. The distance to the Galactic Center (26,000 light years from Earth), the crowded region, and the blocking of light by intervening dust and gas all make the observation of such planets very difficult.

However, these challenges may be met by the next generation of extraordinarily large ground-based telescopes. For example, searches for transits with future observatories like the European Extremely Large Telescope might detect evidence for these planets. Another possibility is searching for stars with unusual patterns of elements in their atmosphere that have migrated away from the center of the galaxy.

A paper describing these results appeared in the February 22, 2018 issue of The Astrophysical Journal Letters and is available online.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

Megan Watzke
Harvard-Smithsonian Center for Astrophysics
+1 617-496-7998
mwatzke@cfa.harvard.edu

Peter Edmonds
Harvard-Smithsonian Center for Astrophysics
+1 617-571-7279
pedmonds@cfa.harvard.edu

Source: Harvard-Smithsonian Center for Astrophysics